A planet is not an object in the laboratory that scientists can subject to different pressures and radiations, comparing how it reacts to this or that. We have only one Earth, and that makes climate science difficult. We can learn much by studying how past climates were different from that of today, and observing how the climate changes in reaction to humanity's "large scale geophysical experiment" of emitting greenhouse gases may teach us a great deal. But these are limited comparisons—different breeds of cat, but still cats. Fortunately, our solar system contains planets with radically different atmospheres.
By the mid 1950s, scientists knew that the atmosphere of Mars was unbreathable—composed mainly of carbon dioxide (CO2), very tenuous and cold, and occasionally stirred up with yellowish dust storms. If Mars had features resembling "canals", as some suspected, they were not full of water for water could not exist as a liquid on the planet's surface. The Mariner 4 spacecraft of 1965, sending back blurry pictures that showed a surface scarred with craters like the Moon, confirmed that the planet was an unlikely abode of life. As for Venus, radio observations published in 1958 showed an amazingly hot climate, with temperatures upwards of 600° Kelvin (K), around the melting point of lead. "It was very disappointing to many people," one of the discoverers recalled, "who were reluctant to give up the idea of a sister planet and perhaps even the possibility of life." Some astronomers worked up arguments that the radio measurements were misleading, representing something in the upper atmosphere, so that life might still exist on Venus. The matter was settled in 1962 when the spacecraft Mariner 2 flew past the planet and detected an unquestionably hot surface.
Already back in 1940, Rupert Wildt had made a rough calculation of the greenhouse effect, caused by the great amount of CO2 others had found in telescope studies of Venus, predicting the effect could raise the surface temperature above the boiling point of water. But raising it as high as 600°K seemed impossible, and nobody mounted a serious attack on the problem; after all, there were very few people in the field of planetary astronomy in those decades. Finally in 1960 a young doctoral student, Carl Sagan, took up the problem and found a solution that made his name known among astronomers. Using what he later recalled as "embarrassingly crude" methods, taking data from tables designed for steam boiler engineering, he confirmed that Venus could indeed be a greenhouse effect furnace. The atmosphere would have to be almost totally opaque, and this "very efficient greenhouse effect" couldn't all be due to CO2—he pointed to absorption by water vapor as the likely culprit.
Sagan concluded that "Venus is a hot, dry, sandy... and probably lifeless planet." He proposed, most significantly, that the situation was self-perpetuating—the surface of the planet was so hot that whatever water the planet possessed remained in the atmosphere as vapor, helping maintain the extreme greenhouse effect condition. However, it was later found that Sagan was mistaken, for Venus's atmosphere has little water. Today's explanation of the planet's strong greenhouse effect is that Venus has a much denser CO2 atmosphere than assumed by astronomers in the past. But mistakes in science can be as useful as valid results when they stimulate further work and ultimately point in the right direction.
A few researchers tried putting a feedback between temperature and water vapor into a simple system of equations; the results were strange. In 1969, Andrew Ingersoll reported "singularities"—mathematical points where the numbers went out of bounds. This signaled "a profound change in the physical system which the model represents," and Ingersoll pointed to CO2 as the key ingredient in the effect. (The Soviet Venera 4 had penetrated Venus’s atmosphere in 1967 and showed it was mostly CO2, and in 1978 the American Pioneer spacecraft found it was almost entirely CO2.) Sagan had estimated that Venus had started out with roughly the same amount of CO2 as the Earth. On our planet, most of the carbon is locked up in minerals and buried in sediments. The surface of Venus, by contrast, was so hot and dry that carbon-bearing compounds evaporated rather than remaining in the rocks, releasing great amounts of the greenhouse gas into the atmosphere. Perhaps Venus had once enjoyed a climate of the sort hospitable to life, but as water had gradually evaporated into the warming atmosphere, followed by CO2, the planet had fallen into its present hellish state? In a 1971 paper, James Pollack argued that Venus might once have had oceans like Earth's. It seemed that such a "runaway greenhouse" could have turned the Earth too into a furnace, if the starting conditions had been only a little different.
In the late 1970s Michael Hart pursued the idea with a more complex computer model, and concluded that the balance was exceedingly delicate. Hardly any planets in the universe, he said, orbited in the narrow "habitable zone" around a star where life could flourish. For our solar system, the orbits in which a planet would be too close to the Sun—so that at some point the planet would suffer a runaway greenhouse effect from which it could never recover—were separated by only a 5% gap from orbits in which the planet would be so far away that runaway glaciation would freeze any ocean solid. The Earth, then, was a lucky place. However, Hart's calculations were riddled with untested assumptions, and many scientists denied that our situation was so extremely precarious. Nonetheless, Hart defended his ideas energetically among his colleagues, and to the public, including an appearance on television in "Walter Cronkite's Universe." Later calculations disproved Hart's conclusions—a Venus-type runaway on our planet is scarcely possible, even if we burn all available fossil fuels.)
The atmosphere of Venus was filled not only with CO2 but also with an opaque haze. Its nature was unknown, and in the 1960s scientists could only say that the haze was probably caused by some kind of tiny particles. "The clouds on Venus had long been a mystery," as one expert recalled, "in which stratospheric aerosols now appeared to play a key role. The unraveling of the precise role of aerosols in the Venus atmosphere would certainly benefit studies of chemical contamination of Earth's atmosphere." In the early 1970s, ground-based telescope observations produced extraordinarily precise data on the optical properties of these aerosols, and at last they were identified—the haze was made up of sulfur compounds.
The greenhouse effect of the sulfates could be calculated, and by the late 1970s, NASA climate modeler James Hansen stated confidently that the sulfates, together with CO2, "are responsible for the basic climatic state on Venus." (CO2 was by far the largest factor, and the exact effects of sulfates would be debated in many subsequent studies.) Hansen had originally become interested in the greenhouse effect when, in response to Sagan's primitive calculations, he tried to derive a better explanation of why the planet's atmosphere was so hot. Hansen's findings about sulfate aerosols strengthened his belief that these particles could have a significant effect on Earth's climate as well. Sulfates were emitted by volcanoes and, increasingly, by human industry, so perhaps Venus had things to tell us about climate change at home.
And then there was Mars—the Red Planet—which also inspired important new thinking about the Earth's atmosphere, even before spacecraft observations. In the 1960s, NASA asked a group of scientists to derive a means of detecting life on Mars. A few noticed that life on Earth makes its presence blatantly evident by driving the atmosphere far from chemical equilibrium. In particular, the abundant oxygen in the air would swiftly drain away, by combining with surface minerals, but the oxygen is renewed by daily emissions from plantlife. Telescopic studies found practically no oxygen in the Red Planet's atmosphere, and, overall, the Martian atmosphere showed no signs of any chemical disequilibrium. Biochemist James Lovelock dismayed his peers by arguing that this showed any search for life on the planet would be fruitless. The sterile atmosphere of Mars, so strikingly different from the Earth's, helped Lovelock, and eventually others, to recognize that life plays a central role in determining the nature of our own planet's atmosphere.
In 1971 the spacecraft Mariner 9, a marvelous jewel of engineering, settled into orbit around Mars and saw... nothing. A great dust storm was shrouding the entire planet. While such storms are rare for Mars, this one was no misfortune, but great luck. Observers immediately saw that the dust had profoundly altered the Martian climate, warming the planet by tens of degrees. The dust settled after a few months, but its lesson was clear—haze could warm an atmosphere. More generally, one studying the climate of any planet would have to take dust very seriously. In particular, it seemed that on Mars the temporary warming had reinforced a pattern of winds, which kept the dust stirred up. It was a striking demonstration that feedbacks in a planet's atmospheric system could flip weather patterns into a drastically different state. This was no longer speculation but an actual event in full view of scientists—"the only global climatic change whose cause is known that man has ever scientifically observed."
Crude speculations about such radical instabilities in the Earth’s own climate had been published independently by two scientists a few years before, about the same time as Ingersoll’s calculation of the Venus runaway greenhouse effect. Pursuing such thoughts before Mariner arrived at Mars, Carl Sagan had made a bold prediction. He suggested that the Red Planet's atmosphere could settle in either of two stable climate states. Besides the current "ice age" there was another possible state, more clement, which might even support life. The prediction seemed to be validated by crisp images of the surface that Mariner beamed home after the dust cleared. The canals some astronomers had imagined were nowhere to be seen, but geologists did see strong signs that vast water floods had ripped the planet in the far past. It was a deadly blow to the old, comforting belief that planets had naturally stable climates, and reinforced the “runaway greenhouse” speculations about Venus that were emerging around the same time.
Calculations by Sagan and his collaborators now suggested that the planet's climate system was balanced so that it could have been flipped from a state with oceans to the present frigid desert, or even back and forth between the two states, by relatively minor changes—changes in its orbit, in the strength of the Sun's radiation, or in the reflection of sunlight off the polar ice cap. These were also, as the authors remarked, "fashionable variables in theories of climatic change on Earth." (Not until 2004 did direct observations on the surface of Mars prove that the planet had indeed once carried standing water, although much farther in the past than Sagan had guessed.)
Speculations published recently about the Earth’s climate suggested possibilities as spectacular as anything proposed for Mars. A small change could start a warming in which the Earth's polar ice caps would shrink, lowering the planet's reflectivity and pushing the warming further into a self-sustaining climate shift. Much the same thing could perhaps happen on Mars, releasing the CO2 frozen at its poles, starting a greenhouse effect process that would melt the ice buried in the soil. In fact, some kind of drastic climate shift had happened on Mars, if evidence of ancient floods is correct. In these arguments, dust stood at the fore, since storms that deposited dust on the polar caps and darkened them seemed the most likely mechanism for pushing the planet into its warm phase.
In later years, spacecraft observed the exotic weather of Venus and Mars in great detail. Meanwhile, as computer models of atmospheres improved, the Earth's two neighbors, plus more exotic planets such as Jupiter, occasionally served as testbeds to probe the limits of the modelers' methods. If a set of equations gave plausible results for such utterly different atmospheres, it created more confidence for their applications on Earth. But the main lesson was a larger one. The idea that feedbacks involving the greenhouse effect could have huge consequences for a planet's climate was no longer mere speculative theory—it was an observation of real events.
- Benestad, Rasmus and Pierrehumbert, Ray. 2006. Lessons from Venus. RealClimate.
- Math Program Cracks Cause of Venus Climate Change, Jet Propulsion Lab, National Aeronautics and Space Administration (NASA).
- Mark A. Bullock and David H. Grinspoon, Global Climate Change on Venus, March 1999, Scientific American Magazine.
- Bruce M. Jakosky1,2 and Roger J. Phillips, Mars' volatile and climate history, Nature 412, 237-244 (12 July 2001).
- Climate History of Mars, California Space Institute, University of California Office of the President.