What determines the climate? Explanations proliferated — models for climate built out of little more than basic physics, a few equations aided by hand-waving. All began with a traditional picture of a stable system, self-regulated by natural feedbacks. A few nineteenth-century scientists suggested that a change in the level of carbon dioxide gas might cause an ice age or global warming, but most scientists thought the gas could not possibly have such effects. Yet climate did change, as proven by past ice ages. Some pointed out that feedbacks did not necessarily bring stability: in particular, changes in snow cover might amplify rather than dampen a climate shift. In the 1950s, an ingenious (although faulty) model involving changes in the Arctic Ocean suggested a disturbing possibility of arbitrary shifts. Experiments with fluids made that more plausible. Apparently the interlinked system of atmosphere, ice sheets, and oceans could swing in regular cycles or even in random jerks. Worse, around 1970 highly simplified computer models raised the specter of a catastrophic climate runaway. In the 1980s, the center of research shifted to large and complex computer models. These did not show a runaway, but reinforced what many simpler models had been suggesting: the next century would probably see significant greenhouse warming. Meanwhile the simple models remained useful for exploring questions that the large models could not handle. ( Basic general greenhouse effect ideas and observations are covered in the core essay on The Carbon Dioxide Greenhouse Effect. Technical calculations on how radiation and heat move through levels of the atmosphere are described in a supplementary essay on Basic Radiation Calculations and for the large-scale computer work, see the essay on General Circulation Models of the Atmosphere.) climate change, global warming, greenhouse effect, CO2
| "This is a difficult subject:
by long tradition the happy hunting ground for robust speculation,
it suffers much because so few can separate fact from fancy." — G.S. Callendar(1) |
| Subsections: Basic Physics (19th century), Many Sorts of Models (1900-1960), Feedbacks Causing Cycles or Catastrophe (1960s-1970s), From Small Models to Big Computers (1980s), After 1988 |
| "Meteorology is a branch of physics," a weather expert remarked in 1939, "and physics makes use of two powerful tools: experiment and mathematics. The first of these tools is denied to the meteorologist and the second does not prove of much use to him in climatological problems." So many interrelated factors affected climate, he explained, that you couldn't write it all down mathematically without making so many simplifying assumptions that the result would never match reality. It wasn't even possible to calculate from first principles the average temperature of a place, let alone how the temperature might change in future years. And "without numerical values our deductions are only opinions."(2) | - LINKS - |
| That didn't stop people from putting forth explanations of climate change. A scientist would come up with an idea about how certain factors worked and explain it all in a page or two, helped along by some waving of hands. Some scientists went on to build a few equations and calculate a few numbers. At best they could show only that the factors they invoked could have effects of roughly the right magnitude. There was no way to prove that some other explanation, perhaps not yet thought of, would not work better. These mostly qualitative "theories" (in fact, merely plausible stories) were all anyone had to offer until digital computers came into their own, late in the 20th century. Until then, the climate community had good reason to keep theory at arms' length. Even those who tried to think in general physical terms hesitated to call themselves "theorists," an almost pejorative term in meteorology. |
More discussion in |
| The science did have a foundation, at least potentially, in simple ideas based on undeniable physical principles. The structures that scientists tried to build on these principles were often called "models" rather than "theories." Sometimes that was just an attempt to hide uncertainty (a paleontologist complained that "'model'... is just a word for people who cannot spell 'hypothesis'").(3) But calling a structure of ideas a "model" did emphasize the scientist's desire to deal with a simplified system that one could almost physically construct on a workbench — something that embodied a hands-on feeling for processes. The great trick of science is that you don't have to understand everything at once. Scientists are not like the people who have to make decisions in, say, business or politics. Scientists can pare down a system into something so simple that they have a chance to understand it. | |
| The first job of a model was to explain, however crudely, the world's climates as presently seen in all their variety. After all, the main business of climatologists until the mid-20th century was the simple drudgery of compiling statistics. Knowledge of average and extreme temperatures and rainfall and the like was important to farmers, civil engineers, and others in their practical affairs — never mind guessing at explanations. But people could not resist trying to explain the numbers. A textbook would start off with the main factor, the way sunlight and thus warmth varies with latitude (perhaps with some calculations and charts). There would follow sections on the prevailing winds that brought rain, and how mountain ranges and ocean currents could affect the winds, and so forth. It was all soundly based on elementary physics. It was a dry exercise, however, not so much a theory of climate as a static regional description.(4) |
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| Asked about changes in climate, most climatologists at mid-century would think of the extremes that people should plan for — the worst heat wave to be expected or the "hundred-year flood." If there was any pattern to such changes, experts believed it would be cyclical. Rather than try to build a physical theory, those who took any interest in the question mostly looked to numerical studies. Perhaps eventually someone would find correlations that pointed to a simple physical explanation. The varying number of sunspots, for example, might signal changes in the Sun that correlated with climate cycles. |
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| The simplest and most widely accepted model of climate change was self-regulation, which meant that changes were only temporary excursions from some natural equilibrium. Through the first half of the 20th century, textbooks of climatology treated climate in a basically static fashion. The word "climate" itself was defined as the long-term average weather conditions, the stable point around which annual temperature and rainfall fluctuated.(5*) After all, in their records of reliable observations the meteorologists found only minor fluctuations from decade to decade. These records went back less than a century, but they supposed that one century was much like the next (aside from changes that took place over many thousands of years, like the ice ages, which were themselves seen as excursions from the very long-term average). Climatologists expanded this idea into a "doctrine," as one critic called it, "that the present causes of climatic instability are not competent to produce anything more than temporary variations, which disappear within a few years."(6) A leading climatologist put it straightforwardly in 1946: "We can safely accept the past performance as an adequate guide for the future."(7) |
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| Almost everyone believed in the natural world's propensity to automatically compensate for change in a self-sustaining "balance." If climate ever diverged toward an extreme, before long it would restore itself to its "normal" state. As evidence, the atmosphere had not changed — or at least not extremely radically — over the past half-billion years.(8) And scientists came up with plausible regulating mechanisms (some of them are described below). The approach expressed a generally sound intuition about the nature of climate as a process governed by a complex set of interactions, all feeding back on one another. But romantic views that stability was guaranteed by the supra human, benevolent power of Nature gave a false confidence that every feature of our environment would stay within limits suitable for human civilization. Issues of complexity and stability in the social structure of climate science are explored in a supplementary essay on Climatology as a Profession. |
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| Certainly people knew that climate did change. There was abundant historical evidence of variations lasting a few decades or centuries, random swings or (as some thought) regular cycles. Perhaps periods of drought like the American Dust Bowl of the 1930s recurred on some schedule, or perhaps not. Most of these changes were poorly measured, but some climatologists sought to understand them, more or less as a sideline. Far more impressive were the ice ages of the past few million years, undeniable proof that climate could change enormously. Looking farther back, geologists found evidence of much earlier ice ages, as well as periods when most of the Earth had basked in tropical warmth. Understanding these great alterations of the far past posed a fascinating scientific puzzle, with no apparent practical value whatsoever. |
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| Basic Physics (19th century) TOP OF PAGE | |
| "As a dam built across a river causes a local deepening of the stream, so our atmosphere, thrown as a barrier across the terrestrial rays, produces a local heightening of the temperature at the Earth's surface." Thus in 1862 John Tyndall described the key to climate change. He had discovered in his laboratory that certain gases, including water vapor and carbon dioxide ( CO2), are opaque to heat rays. He understood that such gases high in the air help keep our planet warm by interfering with escaping radiation.(9) | |
| This kind of intuitive physical reasoning had already appeared in the earliest speculations on how atmospheric composition could affect climate. It was in the 1820s that Joseph Fourier first explained that the Earth's atmosphere retains heat radiation. He had asked himself a deceptively simple question, of a sort that physics theory was just then beginning to learn how to attack: what determines the average temperature of a planet like the Earth? When light from the Sun strikes the Earth's surface and warms it up, why doesn't the planet keep heating up until it is as hot as the Sun itself? Fourier's answer was that the heated surface emits invisible infrared radiation, which carries the heat energy away into space. But when he calculated the effect with his new theoretical tools, he got a temperature well below freezing, much colder than the actual Earth. |
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| The difference, Fourier recognized, was due to the Earth's atmosphere. Somehow it kept part of the heat radiation in. He tried to explain this by comparing the Earth with its covering of air to a box with a glass cover. That was a well-known experiment — the box's interior warms up when sunlight enters while the heat cannot escape.(10) This was an over simple explanation, for it is quite different physics that keeps heat inside an actual glass box, or similarly in a greenhouse. (The main effect of the glass is to keep the air, heated by contact with sun-warmed surfaces, from wafting away, although the glass does also keep heat radiation from escaping.) Nevertheless, trapping of heat by the atmosphere eventually came to be called "the greenhouse effect."(11*) |
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| Not until the mid-20th century would scientists fully grasp, and calculate with some precision, just how the effect works. A rough explanation goes like this. Visible sunlight penetrates easily through the air and warms the Earth’s surface. When the surface emits invisible infrared heat radiation, this radiation too easily penetrates the main gases of the air. But as Tyndall found, even a trace of CO2, no more than it took to fill a bottle in his laboratory, is almost opaque to heat radiation. Thus a good part of the radiation that rises from the surface is absorbed by CO2 in the middle levels of the atmosphere. Its energy transfers into the air itself rather than escaping directly into space. Not only is the air thus warmed, but also some of the energy trapped there is radiated back to the surface, warming it further. |
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| That’s a shorthand way of explaining the greenhouse effect — seeing it from below, from "inside" the atmosphere. Unfortunately, shorthand arguments can be misleading if you push them too far. Fourier, Tyndall and most other scientists for nearly a century used this approach, looking at warming from ground level, so to speak, asking about the radiation that reaches and leaves the surface of the Earth. So they tended to think of the atmosphere overhead as a unit, as if it were a single sheet of glass. (Thus the "greenhouse" analogy.) But this is not how global warming actually works, if you look at the process in detail. | |
What happens to infrared radiation emitted by the Earth's surface? As it moves up layer by layer through the atmosphere, some is stopped in each layer. (To be specific: a molecule of carbon dioxide, water vapor or some other greenhouse gas absorbs a bit of energy from the radiation. The molecule may radiate the energy back out again in a random direction. Or it may transfer the energy into velocity in collisions with other air molecules, so that the layer of air where it sits gets warmer.) The layer of air radiates some of the energy it has absorbed back toward the ground, and some upwards to higher layers. As you go higher, the atmosphere gets thinner and colder. Eventually the energy reaches a layer so thin that radiation can escape into space. |
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| What happens if we add more carbon dioxide? In the layers so high and thin that much of the heat radiation from lower down slips through, adding more greenhouse gas means the layer will absorb more of the rays. So the place from which most of the heat energy finally leaves the Earth will shift to higher layers. Those are colder layers, so they do not radiate heat as well. The planet as a whole is now taking in more energy than it radiates (which is in fact our current situation). As the higher levels radiate some of the excess downwards, all the lower levels down to the surface warm up. The imbalance must continue until the high levels get warmer and radiate out more energy. As in Tyndall's analogy of a dam on a river, the barrier thrown across the outgoing radiation forces the level of temperature everywhere beneath it to rise until there is enough radiation pushing out to balance what the Sun sends in. | |
| While that may sound fairly simple once it is explained, the process is not obvious if you have started by thinking of the atmosphere from below as a single slab. The correct way of thinking eluded neary all scientists for more than a century after Fourier. Physicists learned only gradually how to describe the greenhouse effect. To do so, they had to make detailed calculations of a variety of processes in each layer of the atmosphere. (For more on absorption of infrared by gas molecules, see this discussion in the essay on Basic Radiation Calculations.) |
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| Despite Fourier's exceptional prowess in mathematics and physics, he lacked the knowledge to make even the simplest numerical calculation of how radiation is absorbed in the atmosphere.(12*) A few other 19th-century scientists attempted crude calculations and confirmed that at the Earth’s distance from the Sun, our planet would be frozen and lifeless without its blanket of air .(13) Tyndall followed with rich Victorian prose, arguing that water vapor "is a blanket more necessary to the vegetable life of England than clothing is to man. Remove for a single summer-night the aqueous vapour from the air... and the sun would rise upon an island held fast in the iron grip of frost."(14) Tyndall needed no equations, but only simple logic, to see what many since him overlooked: it is at night that the gases are most important in blocking heat radiation from escape, so it is night-time temperatures that the greenhouse effect raises the most. |
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| These elementary ideas were developed much further by the Swedish physical chemist Svante Arrhenius, in his pioneering 1896 study of how changes in the amount of CO2 may affect climate. Following the same line of reasoning as Tyndall, Arrhenius pointed out that an increase in the blocking of heat radiation would make for a smaller temperature difference between summer and winter and between the tropics and the poles. (Link from below) | |
| Arrhenius's model used an "energy budget," getting temperatures by adding up how much solar energy was received, absorbed, and reflected. This resembled what his predecessors had done with less precise physics.(15) But Arrhenius's equations went well beyond that by taking into account another physical concept, elementary but subtle, and essential for modeling real climate change. This was what one turn-of-the-century textbook called "the mutual reaction of the physical conditions" — today we would call it "feedback." | |
An early example had been worked out by James Croll. This self-taught British scientist had worked as a janitor and clerk in institutions where he could be near the books he needed to develop his influential theory of the ice ages. Croll noted how the ice sheets themselves would influence climate. When snow and ice had covered a region, they would reflect most of the sunlight back into space. The Sun would warm bare, dark soil and trees, but a snowy region would tend to remain cool. If India were somehow covered with ice, its summers would be colder than England's. Croll further argued that when a region became cooler, the pattern of winds would change, which would in turn change ocean currents, perhaps removing more heat from the region. Once something started an ice age, the pattern could become self-sustaining.(16) Arrhenius stripped this down to the simple idea that a drop of temperature in an Arctic region could mean that some of the ground that had been bare in summer would become covered with snow year-round. With less of the dark tundra exposed, the region would have a higher "albedo" or reflectivity, that is, the ground would reflect more sunlight away from the Earth. That would lower the temperature still more, leaving more snow on the ground, which would reflect more sunlight, and so on. This kind of self-reinforcing cycle would today be called "positive feedback" (in contrast to "negative feedback," a reaction that acts to hold back a change). Such a cycle, Arrhenius suggested, could turn minor cooling into an ice age. Such gradual processes were far beyond his power to calculate, however, and it would be a big enough job to find the immediate effect of a change in CO2. |
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| Arrhenius showed his physical insight at its best when he realized that he could not set aside another simple feedback, one that would immediately and crucially exaggerate the influence of any change. Warmer air would hold more moisture. Since water vapor is itself a greenhouse gas, the increase of water vapor in the atmosphere would augment the temperature rise. Arrhenius therefore built into his model an assumption that the amount of water vapor contained in the air would rise or fall with temperature. He supposed this would happen in such a way that relative humidity would remain constant. That oversimplified the actual changes in water vapor, but made it possible for Arrhenius to roughly incorporate the feedback into his calculations. The basic idea was sound. The consequences of adding CO2 and warming the planet a bit would indeed be amplified because warmer air held more water vapor. In a sense, raising or lowering CO2 acted mainly as a throttle to raise or lower the really important greenhouse gas, H2O. | |
| The numerical computations cost Arrhenius month after month of laborious pencil work as he estimated the energy balance for each zone of latitude. It seems he undertook the massive task partly as an escape from melancholy: he had just been through a divorce, losing not only his wife but custody of their little boy. The countless computations could hardly be justified scientifically, given the large uncertainties in the available data — for example, the details of how the atmosphere absorbs radiation in different regions of the infrared spectrum were largely unknown. Moreover his model was crude, neglecting crucial effects such as possible changes in cloudiness as the moisture in the air changed with warming or cooling. Nevertheless he came up with numbers that he published with some confidence.(17) | |
| "I should certainly not have undertaken these tedious calculations," Arrhenius wrote, "if an extraordinary interest had not been connected with them."(18) The prize sought by Arrhenius was the solution to the riddle of the ice ages. He focused on a decrease in CO2 as a possible cause of cooling. But he also took the trouble to estimate what might happen if the amount of gas in the atmosphere, at some distant time in the past or future, was double its present value. He computed that would bring roughly 5 or 6 °C of global warming. |
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| This result is not far from the range that scientists would compute a century later using vastly better models — the current estimate is that a doubling of CO2 will bring some 3 degrees of warming, give or take a degree or two. Did Arrhenius end up in the same range by sheer luck? Partly, but not entirely. In the sort of simple physics and chemistry calculations where Arrhenius had made his name, you can expect to come out roughly right if you address a powerful physical effect in a straightforward way, starting with decent data. The data Arrhenius fed into his calculations (based on Samuel P. Langley's measurements of solar radiation reaching the Earth's surface) were mostly in the right range. And Arrhenius included all the obvious physics theory. | |
| But climate is not a simple physical system. A true calculation of greenhouse effect warming requires measurements far more accurate and far more complete than Langley's. The details of exactly what bands of radiation are absorbed by CO2 and water molecules might have happened to be arranged so as to produce a markedly higher or lower amount of warming. As for theory, Arrhenius's model planet was mostly static. He deliberately left aside factors he could not calculate, such as the way cloudiness might change over the real Earth when the temperature rose. He left aside the huge quantities of heat carried from the tropics to the poles by atmospheric movements and ocean currents, which also might well change when the climate changed. Most important, he left aside the way updrafts would carry heat from a warmer surface into the upper atmosphere. In 1963, when a scientist made a calculation using roughly comparable assumptions, but with the aid of improved data on the absorption of radiation and an electronic computer, he found a far greater greenhouse warming — indeed impossibly greater. The assumptions left out too much that was necessary to get a valid answer.(19*) (See below) | |
| Yet Arrhenius could have felt some confidence that he had not overlooked any terribly potent effect. Calculations aside, since the atmosphere keeps the surface of the Earth warm — in fact, roughly 40°C warmer than a bare rock at the same distance from the Sun — a few degrees sounds like about the right effect for a change in the atmosphere that modestly altered the balance of radiation. Arrhenius also knew that in past geological ages the Earth’s climate had in fact undergone changes of a few degrees up or down, not many tens of degrees nor mere tenths of a degree. While neither Arrhenius nor anyone for the next half-century had the tools to show what an increase of CO2 would really do to climate, he had given a strong hint of what it could possibly do. |
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| A crude idea of how the amount of CO2 could affect radiation was only the first half of a calculation of global warming. The other half would be a model for figuring how the amount of CO2 itself might change. A colleague of Arrhenius, Arvid Högbom, had already published some preliminary ideas. That stimulated an American geologist and bold thinker, Thomas C. Chamberlin, to ponder the role of the gas in climate change. Arrhenius's 1896 paper spurred Chamberlin to publish "a paper which, I am painfully aware, is very speculative..." The speculations revolved around the great puzzle of the ice ages. Chamberlin later remarked how ice ages were "intimately associated with a long chain of other phenomena to which at first they appeared to have no relationship." He was the first to demonstrate that the only way to understand climate was to understand almost everything about the planet together — not just the air but the oceans, the volcanoes bringing gases from the deep interior, the chemistry of weathered minerals, and more. | |
| Chamberlin's novel hypothesis was that ice ages might follow a self-oscillating cycle driven by feedbacks involving CO2. The gas was originally injected into the atmosphere in spates of volcanic activity. It was steadily withdrawn as it combined with minerals during the weathering of rocks and soil. If the volcanic activity faltered, then as minerals leached the gas out of the atmosphere, the planet would cool. Feedbacks could make a temporary dip spiral into a self-reinforcing decline. For one thing, as the land cooled, bogs and the like would decompose more slowly, which meant they would lock up carbon in frozen peat, further lowering the amount of CO2 in the air. Moreover, as the oceans cooled, they too would take up the gas — warm water evaporates a gas out, cold water absorbs it. The process would stop by itself once ice sheets spread across the land, for there would then be less exposed rock and bogs taking up CO2. Reversing the process could bring a warming cycle.(20) |
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| Chamberlin seemed only to be adding to the tall pile of speculations about ice ages, but along the way he had pioneered the modeling of global movements of carbon. He made rough calculations of how much carbon was stored up in rocks, oceans, and organic reservoirs such as forests. He went on to point out that compared with these stockpiles, the atmosphere contained only a minor fraction — and most of that CO2 cycled in and out of the atmosphere every few thousand years. It was a delicate balance, he warned. Climate conditions "congenial to life" might be short-lived on geological time scales.(21) |
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| Chamberlin quickly added that "This threat of disaster is not, however, a scientific argument..." He was offering the idea more for its value "in awakening interest and neutralizing inherited prejudice," namely, the assumption that the atmosphere is stable. Other scientists were not awakened. While some admitted that geological processes could alter the CO2 concentration, on any time scale less than millions of years the atmosphere seemed to be unchanging and unchangeable. | |
| The CO2 model, "recommended to us by the brilliant advocacy and high authority of Prof. T.C. Chamberlin," briefly became a popular theory to explain the very slow climate changes of the past. Within a few years, however, scientists dismissed the theory for what seemed insuperable problems.(22) It appeared that there was already enough CO2 in the air so that its effect on infrared radiation was "saturated" — meaning that all the radiation that the gas could block was already being absorbed, so that adding more gas could make little difference. Moreover, water vapor also absorbed heat rays, and water was enormously more abundant in the atmosphere than CO2. How could adding CO2 affect radiation in parts of the spectrum that H2O (not to mention the CO2 itself) already entirely blocked? These studies with the crude techniques of the early 20th century were inaccurate. Modern data show that even in the parts of the infrared spectrum where water vapor and CO2 are effective, only a fraction of the heat radiation emitted from the surface of the Earth is blocked before it escapes into space. And that is beside the point anyway. The greenhouse process works regardless of whether the passage of radiation is saturated in lower layers. As explained above, the energy received at the Earth’s surface must eventually work its way back up to the higher layers where radiation does slip out easily. Adding some greenhouse gas to those high, thin layers must warm the planet no matter what happens lower down. Through the first half of the 20th century, however, hardly any of the few scientists who took an interest in the topic thought in this fashion. They were convinced by the subtly flawed viewpoint that looked at the atmosphere as a single slab. Even Chamberlin concluded that Arrhenius had failed to get his physics right. After all, was it reasonable to imagine that humans could alter something as grand as the world's climate by changing a tiny fraction of the atmosphere’s content? The notion clashed with common ideas that everyone found reasonable. Scientists were confident they could dismiss changes like the global warming foreseen by Arrhenius, because the climate was known to be self-regulating. |
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| Many Sorts of Models (1900-1960) TOP OF PAGE | |
| While most people thought it was obvious from everyday observation that the climate was self-regulating, scientists had not identified the mechanisms of regulation. They had several to choose from. | |
| Through the first half of the 20th century, one common objection to the idea of a future global warming was that only a little of the CO2 on the planet's surface was in the air. Vastly more was locked up in sea water, in equilibrium with the gas in the atmosphere. The oceans would absorb any excess from the atmosphere, or evaporate gas to fill out any deficiency. "The sea acts as a vast equalizer," as one scientist wrote, making sure all fluctuations "are ironed out and moderated."(23) | <=The oceans |
| If the oceans somehow failed to stabilize the system, there was another large reservoir of carbon stored up in organic matter such as forests and peat bogs. That too seemed likely to provide what one scientist called "homeostatic regulation."(24) For if more CO2 entered the atmosphere, it would act as fertilizer to help plants grow more lushly, and this would lock up the excess carbon in soil and other organic reservoirs. | |
| Beginning in the 1950s, a few scientists attempted to work out real numbers to check the idea. They constructed primitive models representing the total carbon contained in an ocean layer, in the air, in vegetation, and so forth, with elementary equations for the fluxes of carbon between these reservoirs. These were only one of a number of "bookkeeping" studies, begun early in the century and increasingly common by the 1950s, that added up the entire atmosphere's stock of heat, energy, and various chemicals. The implicit aim was to balance each budget in an assumed equilibrium.(25) There was little solid data for any of these things, least of all the biological effects. Scientists could easily adjust numbers until their models showed self-stabilization by way of CO2 fertilization, as expected. |
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| Regardless of the CO2 budget, scientists expected other feedbacks would regulate the world's temperature. In particular, any increase of temperature would allow the air to hold more moisture, where it would create more clouds, which would reflect sunlight away, moderating the heat and doubtless restoring the equilibrium. Such was the view of no less an authority than the President of the Royal Meteorological Society, Sir George Simpson, K.C.B., F.R.S. In 1939 he explained that "the change in the cloud amount is the predominating factor in the regulation of the temperature of the atmosphere. The atmosphere appears to act as a great thermostat, keeping the temperature nearly constant by changing the amount of cloud."(26*) That was about as simple as a physical model could get. | |
| Climate changes had undoubtedly happened in the past, and slightly more complicated models were needed to explain these. The most widely accepted style of explanation invoked altered "weather patterns." The atmosphere could shift to a different arrangement of winds, lasting decades or perhaps centuries, with different storm tracks and precipitation. Such changes could plausibly be caused by slow geological movements. The raising or lowering of a mountain range would obviously alter winds and temperatures, and opening or closing a strait would of course redirect ocean currents.(27) Perhaps changes of geography were all that geologists needed to explain the major climate changes in the Earth's history. |
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| These changes would be mostly regional, not global, but many experts thought of climate changes as mostly local affairs in any case. This view was in line with the traditional climatology that explained the current distribution of deserts, rain forests, and ice caps in terms of the location of mountain ranges and warm or cold ocean currents. It was only necessary to take the reasoning about prevailing winds, the tracks followed by storms, and so forth, and apply it to a different geography. The result was what one expert described as "a large amount of literature which is both geological and meteorological."(28) |
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| Through the first half of the 20th century, scientific theories on climate change continued to revolve mainly around attempts to explain the ice ages. The explanations by geological rearrangements remained the favorite type of theory, "never seriously challenged," as one authority said in 1922.(29) On the other hand, nobody ever made these explanations precise, and they remained more a kind of story-telling than useful science. | |
| An important example of work on the topic was an idea developed by the meteorologist Alfred Wegener in the 1920s. It happened that Wegener loved geology as much as meteorology (he was also dedicated to studies in Greenland, where he disappeared on an expedition in his fiftieth year). In collaboration with another meteorologist, Wladimir Köppen, Wegener worked through the geological evidence of radical climate change. Traces of ancient ice caps were found in rock beds near the equator, and fossils of tropical plants in rocks near the poles. Wegener hoped to resolve the puzzle with his controversial claim that continents drifted about from tropics to Arctic and back. Along the way the two meteorologists worked out a climate change theory. | |
| They started off from Arrhenius's idea that the key variable, albedo, depended on whether snow melted or persisted through the summer. The great sheets of ice that reflected away sunlight could persist only if they rested on land, not ocean. So the authors figured that the recent epoch of ice ages had begun when the North Pole wandered over Greenland, and ice ages had ceased once it moved on into the Arctic Ocean. | |
| Wegener and Köppen went into further detail using a theory that had been hanging around since the 19th century. Croll had suggested that ice ages could be linked with regular cycles in the Earth's orbit, the kind of thing astronomers computed. Over many centuries these shifts caused minor variations in the amount of sunlight that reached a given latitude on the Earth. The variations gave rise to ice ages, Croll argued, whenever enfeebled sunlight allowed excess snow accumulation. In the 1920s, Milutin Milankovitch began to develop these astronomical calculations and plug them into equations that simulated the global climate. His energy budget model was like Arrhenius's, but paid closer attention to how much sunlight was received at each latitude in each season, and what that would mean for ice and snow. Milankovitch found that it was summers with weaker sunlight, in other words colder summers, that counted for keeping the reflective snow in place — not cold winters, as Croll had supposed. Wegener and Köppen took up these ideas, insisting that they were "nearly self-evident, and yet contested by some authors!"(30) |
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| From then on, everyone who worked on climate change took into account possible changes in albedo due to ice and snow. For example, when G.S. Callendar took up the question of greenhouse warming in 1938, in a discussion at a meeting of the Royal Meteorological Society he noted that in recent decades temperatures had been rising noticeably in the Arctic. That led him to suggest cryptically that an increase of CO2 might be acting "as a promoter to start a series of imminent changes in the northern ice conditions."(31) |
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| Some experts offered more specific elaboration, backed up by a few primitive calculations. The most striking came from a respected British scientist, C.E.P. Brooks. He argued that once an Arctic ice cap formed it would chill the overlying air, which would flow down upon the surrounding regions. Behind these frigid winds the snows would swiftly advance to lower latitudes. Wind patterns would thus redouble the impact of the familiar cooling feedback caused by increasing reflection of sunlight. (Link from below) Only two stable states of the polar climate were possible, Brooks asserted — one with little ice, the other with a vast white cap on the planet. A shift from one state to the other might be caused by a comparatively slight perturbation, say, a change of ocean currents that put a little extra heat into the Arctic Ocean. Such a shift, he warned, might be shockingly abrupt.(32) | |
| Scientists were beginning to recognize that feedback might grossly magnify the smallest change. The meteorologist W. J. Humphreys, for one, wrote in Atlantic magazine in 1932 that the current situation was close to the conditions where ice sheets had ruled. Thus "we must be just teetering on an ice age which some relatively mild geologic action would be sufficient to start going." As an example, he suggested that if a very wide sea-level canal were built across Panama, currents flowing through it might shut off the Gulf Stream, bringing "utterly destructive glaciation" to Northern Europe. Or dust thrown into the air by a series of volcanic eruptions like the famous Krakatau explosion might block enough sunlight to allow the formation of ice sheets. This ice, scientists now understood, might reflect enough sunlight to sustain the cold. Humphreys also mentioned (following Chamberlin and others) that additional feedbacks could reduce the main greenhouse gases. Colder oceans would evaporate less water vapor into the air, and the colder water would also tend to take up more of the "Earth's blanket" of CO2. However, like nearly all the scientists of his time, Humphreys did not consider changes in CO2 particularly important. Believing that adding or subtracting the gas could have little effect on radiation, when they thought about climate change they concentrated on volcanic dust, reflective ice sheets and the like.(33) |
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| These models evidently left much room for chance. Some pointed out that ice sheets should be self-sustaining only in certain geological periods, when gross geographical changes such as uplifting of mountain ranges had created a suitable configuration. Even then, Brooks pointed out, "if the Arctic ice could once be swept away, it might find some difficulty in re-establishing itself."(34) He told a Life magazine reporter in 1950 that the Arctic ice had declined to a "critical size" and might no longer be able to chill the air enough to maintain itself. Melting might increase, and over centuries the seas might rise by tens of meters.(35*) |
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| Even when experts worked these ideas up in a few equations, the results were scarcely quantitative, but only qualitative and indeed speculative. Overall, theory remained in much the same state that Simpson, as Director of the British Meteorological Office, had criticized back in 1922. Writers on climate, he had said, each pushed their own individual theory, and biased the evidence in their own favor. "There are so many theories and radically different points of view," he complained, "And new theories are always being propounded."(36) | |
| Simpson himself did not resist the temptation to propound a personal theory, which can serve as an example of the general style of argument of the times. In 1937, he pointed out that, paradoxically, an increase of solar radiation might bring on an ice age. The logic was straightforward. A rise in the Sun's radiation would warm the equator more than the poles. More water would evaporate from the tropics and the rate of the general circulation of the atmosphere would increase. This would bring more snowfall in the higher latitudes, snow that would accumulate into ice sheets. The albedo of the ice sheets would cool the poles. Furthermore, "the ice which enters the sea from these regions will have a large effect on remote regions," cooling the entire hemisphere. Of course, if the Sun grew brighter still, the ice sheets would melt. Simpson worked out a complicated model of double-peaked glacial cycles, driven by a supposed long-term cycling in the level of solar radiation.(37) It was no more convincing than anyone else's ideas. At a time when scientists were unable to explain the observed general circulation of the atmosphere, not even the trade winds, theories about climate change could be little more than an amusement. |
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| To wrestle with complex systems, for centuries scientists had imagined mechanical models, and some had physically constructed actual models. If you put a fluid in a rotating pan, you might learn something about the circulation of fluids in any rotating system — like the ocean currents or trade winds of the rotating Earth. You might even heat the edge of the pan to mimic the temperature gradient from equator to pole. Various scientists had tried their hand at this from time to time since the turn of the century.(38) The results seemed encouraging to the leading meteorologist Carl-Gustav Rossby, who invited young Athelstan Spilhaus to join him in such an experiment at the Woods Hole Oceanographic Institution. In their pan they produced a miniature current with eddies. If this represented an ocean, the current would have looked like the Gulf Stream, if an atmosphere, like a jet stream (a phenomenon not understood at that time). But they could not make a significant connection with the real world.(39) | |
| Rossby persevered after he moved to the University of Chicago in 1942 and built up an important school of meteorologists. His group was the pioneer in developing simple mathematical fluid-dynamics models for climate, taking climate as an average of the weather seen in the daily circulation of the atmosphere. They averaged weather charts over periods of 5 to 30 days to extract the general features, and sought to analyze these using basic hydrodynamic principles. The group had to make radical simplifying assumptions, ignoring essential but transient weather effects like the movements of water vapor and the dissipation of wind energy. Still, they began to get a feeling for how large-scale features of the general circulation might arise from simple dynamical principles.(40) In the 1950s, Rossby's students and others moved this work onto computers. |
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| Meanwhile, to get another peephole into the physics, Rossby encouraged Dave Fultz and others to experiment with rotating mechanical systems. Funding came from the Geophysics Research Directorate of the U.S. Air Force, always keen to get a handle on weather patterns. The Chicago group started with a layer of water trapped between hemispheres (made by sawing down two glass flasks). They were delighted to see flow patterns that strongly resembled the Earth's pattern of trade winds, and even, what was wholly unexpected, miniature cyclonic storms. The group moved on to rotate a simple aluminum dishpan. They heated the dishpan at the outer rim (and later also cooled it in the middle), injecting dye to reveal the flow patterns. The results, as another meteorologist recalled, were "exciting and often mystifying."(41) The crude, physical model showed something rather like the wavering polar fronts that dominate much of the real world's weather.(42) |
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| Meanwhile a group at Cambridge University carried out experiments with water held between two concentric cylinders, one of which they heated, rotating on a turntable. Their original idea had been to mimic the dynamics of the Earth's fluid core in hopes of learning about terrestrial magnetism. But the features that turned up looked more like meteorology. "The similarity between these motions and some of the main features of the general atmospheric circulation is striking," reported the experimenter. The water had something like a little jet stream and a pattern of circulation that vacillated among different states, sometimes interrupted by "intense cyclones."(43) It seemed reminiscent of certain changing wind patterns at middle latitudes that Rossby had earlier observed in the atmosphere (the "Rossby waves" seen in the meanderings of the jet stream and elsewhere). He had explained these patterns theoretically with a simple two-dimensional mathematical model. | |
| Following up with his own apparatus, Fultz reported in 1959 the most interesting result of all. His rotating fluid sometimes showed a symmetric circulation regime, resembling the real world's "Hadley" cells that bring the regular mid-latitude westerly winds. But at other times the pattern looked more like a "Rossby" regime with a regular set of wiggles. This pattern was somewhat like the standing waves that form in swift water downstream from a rock (in the real Earth, the Rocky Mountains act as the rock). Perturb the rotating fluid by stirring it with a pencil, and when it settled down again it might have flipped from one regime to the other. It could also flip between a Rossby system with four standing waves and one with five. In short, different configurations were equally stable under the given external conditions.(44) This was realistic, for the circulation of the actual atmosphere shifts among quite different states (the great trade winds in particular come and go with the seasons). Larger shifts in the circulation pattern might represent long-term climate changes. |
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| Fultz hoped that this kind of work would lead meteorologists to "the type of close and fruitful interaction between theory and experiment, mostly lacking in the past, that is characteristic of the older sciences."(45) But in fact, fluid theory was wretchedly incapable of calculating the behavior of even this extremely simplified model system. Anyway the model was only a crude cartoon of the atmosphere — interesting to be sure, but unable to lead to anything definite about our actual planet. The real contribution of the "dishpan" experiments was to show plainly the tricky instability of any such system. | |
| The physical models reinforced a growing suspicion that it was futile to attempt to model a pattern of global winds on a page of equations, in the way a physicist might represent the orbits of planets. This mathematical research plan, pursued ever since the 19th century, aimed to deduce from first principles the general pattern of atmospheric circulation. But nobody managed to derive a set of mathematical functions whose behavior approximated that of the real atmosphere.(46) The huge ignorance of scientists was nakedly visible to the public, which looked with bemusement on the farrago of simplistic theories that science reporters dug out and displayed in magazines and newspapers. |
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| The press gave special attention to a qualitative argument about the puzzle of the ice ages from two scientists at the Lamont Geological Observatory in New York. Maurice Ewing and William Donn had been interested for some time in natural catastrophes such as hurricanes and tsunamis.(47) Provoked by recent observations of a surprisingly abrupt end to the last ice age, they sought a mechanism that could produce rapid change. Also influencing them was recent work in geology — indications that over millions of years the Earth's poles had wandered, just as Wegener had claimed. Probably Ewing and Donn had also heard about speculations by Russian scientists on how diverting rivers that flowed into the Arctic Ocean might change the climate of Siberia. In 1956, all these strands came together in a radically new idea.(48*) |
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| Our current epoch of ice ages, Ewing and Donn argued, had begun when the North Pole wandered into the Arctic Ocean basin. The ocean, cooling but still free of ice, had evaporated moisture and promoted a pattern of severe weather. Heavy snows fell all around the Arctic, building continental ice sheets. That withdrew water from the world's oceans, and the sea level dropped. This blocked the shallow channels through which warm currents flowed into the Arctic Ocean, so the ocean froze over. That meant the continental ice sheets were deprived of storms bringing moisture evaporated from the Arctic Ocean, so the sheets began to dwindle. The seas rose, warm currents spilled back into the Arctic Ocean, and its ice cover melted. And so, in a great tangle of feedbacks, a new cycle began.(49*) (Link from below) |
=>Sea flooding
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| This theory was especially interesting in view of reports that northern regions had been noticeably warming and ice was retreating. Ewing and Donn suggested that the polar ocean might become ice-free, and launch us into a new ice age, within the next few thousand years — or even the next few hundred years. | <=Modern temp's |
| The theory was provocative, to say the least. "You will probably enjoy some criticism," a colleague wrote Ewing, and indeed scientists promptly contested what struck many as a far-fetched scheme. "The ingenuity of this argument cannot be denied," as one textbook author wrote, "but it involves such a bewildering array of assumptions that one scarcely knows where to begin."(50) Talk about a swift onset of glaciation seemed only too likely to reinforce popular misconceptions about apocalyptic catastrophes, and contradicted everything known about the pace of climate change. Critics pointed out specific scientific problems (for example, the straits are in fact deep enough so that the Arctic and Atlantic Oceans would exchange water even in the midst of an ice age). Ewing and Donn worked to patch up the holes in their theory by invoking additional phenomena, and for a while many scientists found the idea intriguing, even partly plausible. But ultimately the scheme won no more credence than most other theories of the ice ages.(51) "Your initial idea was truly a great one," a colleague wrote Ewing years later, "...a beautiful idea which just didn't stand the test of time."(52) |
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| Ewing and Donn's theory was nevertheless important. Published in 1956, and picked up by journalists who warned that ice sheets might advance within the next few hundred years, the theory gave the public for the first time a respectable scientific backing for images of disastrous climate change.(53) The discussions also pushed scientists to inspect data for new kinds of information. For example, the theory stimulated studies to find out whether, as Ewing and Donn claimed, the Arctic Ocean had ever been ice-free during the past hundred thousand years (evidently not). These studies included work on ancient ice that would eventually provide crucial clues about climate change. Above all, the daring Ewing-Donn theory rejuvenated speculation about the ice ages, provoking scientists to think broadly about possible mechanisms for climate change in general. As another oceanographer recalled, Donn would "go around and give lectures that made everybody mad. But in making them angry, they really started getting into it."(54*) |
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| Feedbacks Causing Cycles or Catastrophe
(1960s-1970s) TOP OF PAGE |
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| Norbert Wiener, a mathematical prodigy, had interests ranging from electronic computers to the organization of animals' nervous systems. It was while working on automatic control systems for antiaircraft guns during the Second World War that he had his most famous insights. The result was a theory, and a popular book published in 1948, on something he called "cybernetics."(55) Wiener's book drew attention to feedbacks and the stability or collapse of systems. These were timely topics in an era when electronics opened possibilities ranging from automated factories to novel modes of social communication and control. Through the 1950s, the educated public got used to thinking in cybernetic terms. Climate scientists were swimming with the tide when they directed their attention to feedback mechanisms, whereby a small and gradual change might trigger a big and sudden transition. |
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| At the start of the 1960s, a few scientists began to think about transitions between different states of the oceans. Study of cores drilled from the seabed showed that water temperatures could shift more quickly than expected. A rudimentary model of ocean circulation constructed by Henry Stommel suggested that under some conditions only a small perturbation might shift the entire pattern of deep currents from one state to another. It was reminiscent of the shifts in the dishpan fluid models.(56) All this was reinforced by the now familiar concept that fluctuations in ice sheets and snow cover might set off a rapid change in the Earth's surface conditions.(57) |
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| Similar ideas had been alive in the Soviet Union since the 1950s, connected to fabulous speculations about deliberate climate modification — making Siberia bloom by damming the Bering Straits, or by spreading soot across the Arctic snows to absorb sunlight. According to the usual ideas invoking snow albedo, if you just gave a push at the right point, feedback would do the rest. These speculations led the Leningrad climatologist Mikhail Budyko to privately advance doubts about how feedbacks might amplify human influences. His entry-point was a study on a global scale. Computing the balance of incoming and outgoing radiation energy according to latitude, Budyko found the heat balance worked very differently in the snowy high latitudes as compared with more temperate zones. It took him some time, Budyko later recalled, to understand the importance of this simple calculation.(58) It led him to wonder, before almost any other scientist, about the potentially huge consequences of fossil fuel burning as well as more deliberate human interventions. |
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| In 1961, Budyko published a generalized warning that the exponential growth of humanity's use of energy will inevitably heat the planet. The next year he followed up with more specific, if still quite simple, calculations of the Earth's energy budget . His equations suggested that climate changes could be extreme. In the nearer term, he advised that the Arctic icepack might disappear quickly if something temporarily perturbed the heat balance. Budyko did not see an ice-free Arctic as a problem so much as a grand opportunity for the Soviet Union, allowing it to become a maritime power (although he admitted the longer-term consequences might be less beneficial).(59) |
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| Even setting aside ice-albedo effects, interest in feedbacks was growing. Improvements in digital computers were the main driving force. Now it was possible to compute feedback interactions of radiation and temperature along the lines Arrhenius had attempted, but without spending months grinding away at the arithmetic. |
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| Meanwhile, new data on CO2 absorption made such calculations desirable. A few scientists took a new look at the old ideas about the greenhouse effect. Nobody fully grasped that the arguments about “saturation” of absorption of radiation were irrelevent, since adding more gas would make a difference in the crucial thin layers from which radiation did escape into space. But the way radiation traversed the high, thin layers was attracting increasing scientific attention. New data showed that there was not enough water vapor and CO2 in the atmosphere to saturate the gases’ absorption of radiation in the upper atmosphere (actually it was not even saturated at lower levels, although that didn’t matter). A few physicists decided that it was worth their time to calculate what happened to the radiation in detail, layer by layer up through the atmosphere. (The details are discussed in the essay on Basic Radiation Calculations, follow link at right.) |
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In 1963, building on pioneering work by Gilbert Plass, Fritz Möller produced a model for what happens in a column of typical air (that is, a "one-dimensional global-average" model). His key assumption was that the humidity of the atmosphere should increase with increasing temperature. To put this into the calculations he held the relative humidity constant, which was just what Arrhenius had done long ago. (The simplest alternative would have been to assume that the water vapor concentration, the actual amount of water held in the world's air, remained unchanged as climate changed. That was roughly equivalent to the old assumption that a little global heating would make more clouds, which would promptly cool things back to equilibrium.)(60) Möller got the same amplifying feedback that Arrhenius had found. As the temperature rose more water vapor would remain in the air, adding its share to the greenhouse effect. |
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| When he finished his calculation, Möller was astounded by the result. Under some reasonable assumptions, doubling the CO2 could bring a temperature rise of 10°C — or perhaps even higher, for the mathematics would allow an arbitrarily high rise. More and more water would evaporate from the oceans until the atmosphere filled with steam! Möller himself found this result so implausible that he doubted the whole theory (in fact it failed to include essential features of the atmosphere). Yet others thought his calculation was worth noticing. The model, as one expert noted, "served to increase confusion as to the real effect of varying the CO2 concentrations."(61) (Link from above) | <=Radiation math |
| Increased confusion is valuable when it pushes scientists to get a better answer. Möller's disturbing calculation was one stimulus for taking up the challenging job of building full-scale computer models that would take better account of key processes. By 1967 o By 1967 a team in Princeton had removed the runaway by adding more realism to a one-dimensional model (they allowed heated air to rise, included some effects of clouds, etc.). But it would take another decade or two of hard work before computer models would offer a reasonably convincing simulacrum of the present global climate, let alone a changing one.(62) |
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| Crude models of climate change became common during the 1960s, and some of them showed uncomfortably plausible possibilities for disaster. One reason these drew attention was that climate scientists were beginning to admit that there was no such thing as a "normal" climate. By now they had many good long-term weather records, and analysis showed that weather patterns did not always swing back and forth around a stable average. The traditional model of a self-regulating balance of nature was gradually yielding to a picture in which climate continually changed. Feedbacks were no longer seen as invariably helpful, ever restoring an equilibrium. Rather, they might push the system into a fatal runaway. | <=Climatologists |
| The scientists were not causing a change of attitude so much as reflecting one that was sweeping through the world public. Many people were taking up the idea that humanity was liable to bring down global disaster on itself, one way or another. Crude calculations pointed to ruinous consequences from the spread of pesticides, radioactive materials, and above all nuclear war. People no longer saw all this as mere science fiction for teenagers, but as plain scientific possibility. | <=Public opinion |
| Alongside the occasional models of spectacular climate catastrophes, scientists continued to develop more workaday studies of how this or that force or feedback might influence climate. The subject remained a minor out-of-the-way field, salted with individualists who dreamed of winning honor by discovering the key to the ice ages or a way to predict droughts. As the Director of Research of the United Kingdom Meteorological Office remarked in 1963, nobody had yet produced a quantitative model that could show even "that the climate of the Earth should be distributed as it is." Without such a model for the present state of climate, so much the worse for understanding climate change — any discussion "is necessarily conjectural and inconclusive." That was no wonder, he pointed out, when even the most basic data, like the Earth's budget of incoming and outgoing radiation energy, were known only approximately. "With theory so rudimentary and the data so incomplete... the subject has largely been left as a topic for armchair speculation."(63) |
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| Another expert tallied significant theories about causes of climate change extant in 1960 and came up with 54 distinct hypotheses. When a colleague looked again in 1968, he found the total had mounted to 60. "There is nothing to suggest that an end to the speculation on climatic change is in sight," he sighed. "It seems that we have a long way to go before the correct answer can be affirmed."(65) The few and scattered scientists who tried to do scientific work on climate change usually distrusted all the primitive models, including their own. Hardly anyone pursued a given idea except the author, who usually just presented a paper or two before moving on to more productive work. |
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| As the 1960s proceeded, scientists found it harder to get any respect at all for a physical model unless it incorporated at least a few equations and numerical results. Such calculations, involving ice sheets or CO2 or whatever, became increasingly common, even if the product was often little better than hand-waving dressed up with graphs. As the power of computers rose, people began to think about building models that would work out the whole three-dimensional general circulation of the atmosphere. The main impetus was to predict daily weather, but some hoped eventually to learn something about climate. The early models did give a recognizable climate, but it was more qualitative than quantitative, no close reproduction of the Earth's actual climate. Such models were not easily built, however. One problem was that computers were too slow to handle millions of numbers in a reasonable time. But a worse problem was pure ignorance of how to build a general circulation model. An infinitely fast computer would be no use unless it began with the correct equations for complex effects like the way moisture in the air became raindrops or snowflakes. | |
| Many people preferred to keep on developing simple models of climate instability. Such models were easy and satisfying to grasp, and however qualitative and speculative they might be, they offered genuine insights. The best of these insights would eventually be incorporated into the gigantic computer models. Meanwhile some climate scientists took advantage of computers in a less expensive and arduous way, putting them to work on simple models and working out the numbers in minutes instead of weeks. |
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| Among various simplified models that were written down in a few equations and run through a calculation, the most important was built in the late 1960s by Budyko. He continued to worry about the climate modification proposals that had concerned Soviet climatologists since the 1950s, the grand schemes to divert rivers from Siberia or spread soot over the icepack. Budyko and his colleagues recognized that existing models were far too primitive to predict how such activities might alter climate. At first, they tried instead to make predictions using the simplest sort of empirical model. They would study past climates, compiling statistics on what had happened during years when the icepack was a bit smaller, the temperatures a bit warmer, the atmosphere a bit dustier. The way weather patterns had shifted in the past might well indicate how they would shift in response to future interventions. This resembled the traditional weather prediction method of "modeling" tomorrow's weather by comparing today's weather with similar maps from the past. The approach was also a natural extension of traditional climatology, with its piles of statistics and its idea of climate change as a simple question of changed weather patterns. |
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| In service of this program, Budyko's institute in Leningrad had been laboriously compiling old temperature figures from around the world. He noticed an apparent correlation over the past century between fluctuations in global temperature and variations in atmospheric transparency, due to dust from occasional volcanic eruptions. Other climatologists reported similar findings in the late 1960s. Apparently temperature was sensitive to any haze of particles that lingered in the atmosphere. Budyko was well aware of vigorous ongoing debates over the general warming trend that had been reported for some regions, and he already expected that human industry would cause an accelerated warming. Moreover, studying new satellite data on the albedo of different parts of the Earth, he found dramatic differences depending on snow cover. Combining these separate concerns, he worried that a change in sea ice, or a similar feedback mechanism, "can multiply a comparatively small initial change in air temperature created by men's activities."(66) | <=Aerosols <=Modern temp's |
| To pin down the idea, in the mid 1960s Budyko constructed a highly simplified mathematical model. It was a "zero-dimensional" model that looked at the heat balance of the Earth as a whole, summing up radiation and albedo over all latitudes. When he plugged plausible numbers into his equations, Budyko found that for a planet under given conditions — that is, a particular atmosphere and a particular amount of radiation from the Sun — more than one state of glaciation was possible. If the planet had arrived at the present after cooling down from a warmer climate, the albedo of sea and soil would be relatively low, and the planet could remain entirely free of ice. (In particular, as Donn was continuing to insist, once the Arctic Ocean was free of its ice pack it would be less likely to freeze over in winter).(67) But the Earth had come to the present by warming up from an ice age, keeping some snow and ice that reflected sunlight, and so it could retain its chilly ice caps. |
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| Under present conditions, the Earth's climate looked stable in Budyko's model. But not too far above the present temperatures and snow cover, the equations reached a "critical point." The global temperature would shoot up as the ice melted away entirely. That would give a uniformly and enduringly warm planet with high ocean levels, as seen in the time of the dinosaurs. And if the temperature dropped not too far below present conditions, the equations hit another critical point. Here temperature could drop precipitously as more and more water froze, until the Earth reached a stable state of total glaciation — the oceans entirely frozen over, the Earth transformed permanently into a gleaming ball of ice! Budyko thought it possible that our era was one of "coming climatic catastrophe... higher forms of organic life on our planet may be exterminated."(68) (Link from below) |
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| Others were on the same trail, independently of Budyko's work in Leningrad — communications were sporadic across the Cold War frontiers. Already in 1964, a New Zealand ice expert, Alex Wilson, had offered some thought-provoking if schematic calculations. Antarctic ice sheets might be unstable enough to collapse so that icebergs would spread swiftly across vast tracts of the southern oceans, then melt away, raising and then lowering the Earth's albedo. He proposed that this "provides the 'flip-flop' mechanism to drive the Earth into and out of an ice age."(69*) The following year Erik Eriksson in Stockholm wrote a set of differential equations involving temperature and ice cover. The mathematics revealed instabilities that might lead to either "an explosive growth" or "a very rapid retreat of ice." As Eriksson explained in a 1965 conference on climate change, the system had a "'flip-flop' mechanism."(70*) | |
| That was an extreme example of what the American meteorologist Edward Lorenz had begun to call "intransitive" effects. Under given external conditions, the atmospheric system could get itself locked into one persistent state or into another and quite different state. The choice might depend on only minor variations in the starting-point. These ideas were no doubt provocative, but so blatantly primitive and speculative that few scientists spent much time thinking about them. |
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| What did at last catch attention was the drastic outcome of an energy-budget model published in 1969. The author, William Sellers at the University of Arizona, built on Budyko's and Eriksson's ideas. Rather than attempt another grand but rudimentary global model, Sellers computed possible variations from the average state of the actual atmosphere, separately for each latitude zone. The model was still "relatively crude," as Sellers admitted (adding that this was unfortunately "true of all present models"), but it was straightforward and elegant. Climatologists were impressed to see that although Sellers used equations different from Budyko's, his model too could approximately reproduce the present climate — and that it too showed a cataclysmic sensitivity to small changes. If the energy received from the Sun declined by 2% or so, whether because of solar variations or increased dust in the atmosphere, it might bring on another ice age. Beyond that, Budyko's nightmare of a totally ice-covered Earth seemed truly possible. At the other extreme, Sellers suggested, "man's increasing industrial activities may eventually lead to a global climate much warmer than today."(71) |
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| The striking results published by Budyko and Sellers kindled increased interest in simple models. While some scientists gave them no credence, others felt that such models were valuable "educational toys" — a helpful starting point for testing assumptions, and for identifying spots where future work could be fruitful.(72) But did the Budyko-Sellers catastrophes reflect real properties of the global climate system? That was a matter of brisk debate.(73) |
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| In the early 1970s, some scientists did find it plausible that feedbacks could build up a continental ice sheet more rapidly than had been supposed. The ice's albedo was not the only feedback that might contribute. Ewing continued to defend his theory that a North American ice sheet could build up over only a few thousand years because of the increase in snowfall if the Arctic Ocean became ice-free. Other climate experts consistently rejected the idea. Aside from specific details, many continued to doubt the basic picture of a climate sensitive to small perturbations. For example, a 1971 climatology textbook pointed out that the Arctic Ocean occupied less than 5% of the globe's surface, and asked, "Is it not inherently improbable that the freezing and thawing of this surface should have major repercussions over the whole globe?"(74) Whether such magnified consequences were truly improbable got different answers from different scientists. Some went so far as to take seriously the idea offered by C.E.P. Brooks back in the 1920s, that thanks to feedback, frigid winds sweeping down from snow fields could move the snowline rapidly southward year by year.(75) (See above: Ewing and Brooks.) Such a runaway freeze might possibly be triggered soon, according to some, as smog and smoke emitted by human industry increasingly shaded the Earth.(76) |
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| The opposite extreme — a self-sustaining heating of the planet — might be even more catastrophic, according to another set of calculations from a few equations. In the early 1960s, telescope measurements had revealed that the planet Venus was at a temperature far above the boiling point of water. It stayed so hot because a dense blanket of water vapor and CO2 maintained a ferociously strong greenhouse effect. The furnace-like conditions not only kept water vaporized in the atmosphere but also kept the CO2 there, for the surface minerals, red-hot, would not absorb the gas. The system was thus self-perpetuating. Perhaps Venus had originally been similar to the Earth, only just enough warmer to begin evaporating gases into its atmosphere — greenhouse gases, which produced further warming, and so forth. If so, the end had been a "runaway greenhouse." According to one calculation, the Earth would need to be only a little warmer for enough water to evaporate to tilt the balance here as well. If our planet had been formed only 6% closer to the Sun, the authors announced, "it may also have become a hot and sterile planet." This was published in 1969, the same time as the work of Budyko and Sellers.(77) |
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| By 1971, the risks to climate were under vigorous discussion in the small community of climate scientists. When Budyko presided over a large meeting in Leningrad, a rare occasion when most of the leading American, Western European and Soviet experts all met together, he put the issue to them forcefully. At the conclusion of the conference, where the organizer would traditionally sum up with some bland remarks, "Instead of general words," Budyko recalled, "I presented in short form an idea which proved to be absolutely unacceptable to everybody: the idea that global warming is unavoidable... The result was a sensation. Everybody had very strong feelings, and extremely unfavorable... A few very prominent men said, first, that it was absolutely impossible to have any [effect] of man's activity on the climate... And absolutely impossible to predict any climate change."(78) It was not pleasant, Budyko later recalled, to present unconventional ideas and provoke negative feelings, but the risk to the planet seemed so grave that it was important to provoke scientists to study the question and find whether the ideas were valid.(79) | |
| Budyko was not in fact alone in his concerns. They were taken up in an influential report (the "SMIC report") as the consensus of a major scientific meeting held in Stockholm that same year, 1971. The experts concluded that there was a possibility that a mere 2% increase or decrease of solar radiation, helped by albedo feedback, could leave the planet either totally ice-free or totally frozen.(80*) Budyko, Sellers, and others pressed ahead, finding that under a variety of simple assumptions, any model that gave a good representation of the Earth's present climate looked unstable and could just as easily produce a radically different climate.(81*) In 1972, Budyko calculated that a mere few tenths of a percent increase in solar radiation input could melt the icecaps. More important still, changing the level of greenhouse gases in the atmosphere would have an effect similar to changing the Sun's radiation. His model indicated that a 50% increase in CO2 would melt all the polar ice, whereas reduction of the gas by half "can lead to a complete glaciation of the Earth." Budyko went on to note that any changes in CO2 caused by natural geological processes had been overtaken by human activity. Sometime "comparatively soon (probably not later than a hundred years)... a substantial rise in air temperature will take place." As early as 2050, he calculated, the Arctic Ocean's ice cover could be melted away entirely.(82) |
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| A mathematical model like those of Budyko and Sellers, built out of only a few simple equations, is quite likely to predict sharp changes. The more complex processes of the real world, however, might become saturated at some point, or react so as to counter any big shift. Scientists tended to be skeptical about this entire genre of models. As one expert later remarked, many in the 1970s thought the Budyko-Sellers instability was a nuisance — "an artifact of the idealized models, and the usual approach was to dismiss it or introduce additional ad hoc mechanisms that would remove it."(83) The few who pursued the calculations found no easy way to avoid the catastrophic instability, but they understood that it would take a much larger and more complete computer model to produce credible results.(84) Sellers himself developed a somewhat more elaborate model (although it still took only 18 seconds on the computer to work out a year of climate change), and again he got a planet that was highly sensitive to perturbations. But he admitted that resolving the question must wait for some future "super-computer."(85) Besides, in the early 1970s the public had become agitated about possible climate shifts, and it could seem irresponsible to talk too loudly about world doom predicted by patently deficient models. |
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| Some senior climatologists, attacking "the glibly pessimistic pronouncements about the imminent collapse of our terrestrial environment," stuck by their traditional model of climate as a self-regulating system. They continued to expect, for example, that a negative feedback from cloudiness would stabilize global temperature. But others were taking a new view of their field. Not only theoretical studies, but a flood of data on past climate changes were hard to reconcile with the old definition of “climate” as a long-term average of weather. An average made sense only if you calculated it over a period where things were roughly the same during the first half as during the second half. But was there ever such a period? As one prominent climatologist explained, "it cannot be ruled out... that [climate] varies on all scales of time." He admitted that "it can be argued that the very concept of climate is sterile," unless you gave up "the classical concept of something static."(85a) |
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In 1973, studies in wholly different fields brought new credence to the idea that positive feedbacks could defeat stability, with drastic results. A spacecraft reached Mars and sent back images with dramatic evidence that although the planet was now in a deep freeze, in the past there had been floods of water. Carl Sagan and his collaborators calculated that the planet had two stable states, and ice albedo feedback helped to drive the shift between them. Enormous flips of climate were apparently not a mere theoretical possibility but something that had actually befallen our neighboring planet.(86) For more on the way studies of other planets supported ideas of radical climate change, see the supplementary essay on Mars and Venus. |
<=Venus & Mars |
| Another field of study produced even more telling news. By the mid 1970s, analysis of layers of clay extracted from the seabed gave unassailable evidence that ice ages had come and gone in a 100,000-year cycle, closely matching Milankovitch's astronomical computations of periodic shifts in the Earth's orbit.(87) Yet the subtle orbital changes in the amount of sunlight that reached the Earth were far too small to have a direct effect on climate. The only reasonable explanation was that there were other natural cycles that resonated at roughly the same timescale. The minor variations of external sunlight evidently served as a "pacemaker" that pinned down the exact timing of internally-driven feedback cycles. |
