In the 1960s and 1970s, observations of Mars and Venus showed that
planets that seemed much like the Earth could have frightfully different
atmospheres. The greenhouse effect had made Venus a furnace, while lack
of atmosphere had locked Mars in a deep freeze. This was visible evidence
that climate can be delicately balanced, so that a planet's atmosphere
could flip from a livable state to a deadly one.
|A planet is not a lump
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. To be sure, we
can learn a lot by studying how past climates were different from
the present one. 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 wholly other species, planets with radically different
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|Until the 1950s, science fiction writers gave about as accurate
a picture of the nearest planets as scientists themselves held.
The writers, after all, read up on the science. Mars seemed much
like the Earth, although its atmosphere was certainly far colder
and dryer. Strange life might crawl over the Red Planet's sands
under an indigo sky brushed with wispy clouds. Perhaps it had once
been a bit warmer and wetter, but everyone assumed that planetary
atmospheres (including our Earth’s) were naturally stable,
their composition held fixed within rough limits by simple geochemical
processes. As for Venus, visitors might expect something even more
Earth-like, a steam bath under perpetual clouds. Some scientists
suspected that the atmosphere was largely carbon dioxide gas (CO2),
but even so abundant life was not ruled out.
|Could study of these strange atmospheres provide, by comparison,
insights into the Earth's weather and climate? With this ambitious
hope Harry Wexler, head of the U.S. Weather Bureau, instigated a
"Project on Planetary Atmospheres" in 1948. Several leading scientists
joined the interdisciplinary effort. But the other planets were
so unlike the Earth, and information about their atmospheres
was so minimal, that the scientists could reach no general conclusions
about climate. The project was mostly canceled in 1952.(1)
| Telescopic observations continued, benefitting
from improved photographic techniques. By the mid 1950s, scientists,
if not science fiction writers, knew that the atmosphere of Mars was
unbreathable mainly CO2, very tenuous
and very cold, occasionally stirred up with yellowish dust storms.
If Mars had features resembling "canals" they were not full of water,
for water could not exist as a liquid on the planet's surface.(2) 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 temperature,
upwards of 600°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 II flew
past and showed unequivocally that the heat radiation came from the hot surface.(3)
| Already back in 1940, Rupert Wildt had made a rough calculation
of the greenhouse effect from the large amount of CO2
that others had found in telescope studies of Venus; he predicted the
effect could raise the surface temperature above the boiling point
of water. But raising it as high as 600°K seemed impossible.(4*) Nobody mounted a serious attack on the problem (after all,
very few people were doing any kind of planetary astronomy in those
decades). Finally in 1960 a young doctoral student, Carl Sagan, took
up the problem and got 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.(5)
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 of radiation by water vapor as the likely culprit.
| Sagan, a science fiction fan from his early years, was among those
who had dreamed of a living sister planet of swamp and ocean, but
now he had to admit that "Venus is a hot, dry, sandy... and probably
lifeless planet." Most significantly, the situation was self-perpetuating.
The surface was so hot that whatever water the planet possessed remained
in the atmosphere as vapor, helping maintain the extreme greenhouse
effect condition. It was later found that Sagan was mistaken, for
Venus's atmosphere has little water. In 1967 the Soviet Union's spacecraft Venera 4 penetrated Venus's
atmosphere and found it was mostly CO2; in 1978 the American Pioneer spacecraft proved it was almost almost
entirely CO2. If the greenhouse effect there is strong
anyway, that is because Venus has a much denser CO2
atmosphere than astronomers of the time thought. But mistakes in science
can be as useful as valid results, when they stimulate further work
and point in the right direction.(6)
that Venus's atmosphere had a composition, and probably a history,
very different from Earth's prompted scientists to abandon their old
assumption that planetary atmospheres were fixed forever by simple
chemistry. 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. That signaled "a profound
change in the physical system which the model represents."(7) Ingersoll pointed at CO2
as the key ingredient. 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 has always been 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. Thus its atmosphere was filled with a
huge quantity of the greenhouse gas.
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|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 eroded from the rocks,
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. (Into the 21st century the question of whether Venus was in fact once a watery planet remained unresolved.) (8*)
|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 was a lucky place, then. Hart's calculations were
riddled with untested assumptions, and many scientists denied that
our situation was so extremely precarious. (Later calculations showed
they were right — a Venus-type runaway on our planet is scarcely
possible, even if we burn all available fossil fuels.) Hart defended
his ideas energetically among his colleagues, and also in public,
including an appearance on television in "Walter Cronkite's Universe."(9*)
| 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.(10)
"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." (11*) 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 from sulfur compounds.(12*)
|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." 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. Now Hansen's findings about
sulfate aerosols strengthened his belief that these particles could
make a serious difference for the Earth's climate as well. Sulfates
were emitted by volcanoes and, increasingly, by human industry, so
Venus had things to tell us about climate change at home.(13*) (CO2 was by
far the largest factor in warming Venus, and the effect of sulfates
would be debated for decades. It turned out that the greenhouse effect
of sulfate clouds reflecting heat back to the surface of Venus was
outweighed by cooling due to their reflection of incoming sunlight.)
| And then there was Mars. That planet too
inspired important new thinking about the Earth's atmosphere, even
before spacecraft paid a visit. In the 1960s, NASA asked some scientists
to think about ways to detect life on Mars. A few of them 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, except the oxygen is renewed by a daily emission from plants.
Telescopic studies found practically no oxygen in the Red Planet's
atmosphere. 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 there 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. Such storms are rare for Mars, and this
one was no misfortune for the observers, but great good luck. They
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, anyone 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 that
had 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. That was no longer speculation
but an actual event in full view of scientists as of 1975 it was "the only global
climatic change whose cause is known that man has ever scientifically
|Crude speculations about just 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 greenhouse runaway.(16)
(See above) 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, reinforcing 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, or in the strength of the
Sun's radiation, or in the reflection of sunlight off the polar
icecap. 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 only much farther in the past
than Sagan guessed.)
|The speculations recently
published about the Earth's climate had 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 water
ice buried in the soil. In fact some kind of drastic climate shift
had happened on Mars, if the evidence of ancient floods and lakes was correct. Perhaps storms that deposited dust on the polar caps and darkened them had pushed the planet into its warm phase. Or perhaps (according to an idea published much later) clouds of CO2 ice had exerted a different kind of greenhouse effect. In any case even this supposedly "dead" planet gave evidence of climate instability.(17)
|Studies of Mars stimulated interest in a puzzle concerning a third planet, like yet unlike our own world, that gave an additional reason to believe that CO2 plays a crucial role in temperature regulation. That planet was the Earth itself as it was several billion years ago. Sagan and a colleague pointed out in 1972 what became known as the "faint early Sun paradox." Astrophysical calculations show that the Sun changes as it burns its hydrogen fuel, gradually getting brighter. Three billion years ago, if the atmosphere was like the present, the seas should have been permanently frozen. But geological evidence shows there was liquid water. Sagan suggested this was thanks to the greenhouse effect of ammonia gas, but further geological studies showed that such an atmosphere was unlikely. In 1979 another group produced a model whereby a very high concentration of CO2 provided the necessary warmth. (More recent measurements don't support such a high early CO2 level, however, so discussion of the paradox continues; besides CO2, extra methane and/or changes in clouds seem needed, and perhaps other factors, for example N2O.)(17a)
| In later years, spacecraft probed the exotic
weather of Venus and Mars in great detail. These observations left
open the possibility that in their younger days both planets had been
more Earth-like, perhaps with oceans and even primordial life. (Mars
also played a bit part in the drama of climate change denial in the
early 21st century. An Internet myth asserted that Mars was getting
warmer, which supposedly implied that the Earth's warming was part
of a natural solar-system cycle. The actual scientific literature,
however, contained no claim that any planet except our own was getting
steadily warmer.) Meanwhile computer models of atmospheres were improving,
and 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, that gave more confidence in their applications
on Earth. For example, computer models and observations converged to confirm that wide variations in temperature on Mars were connected with feedback effects from occasional huge dust storms.(18) 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 not a mere speculative
theory. It was an observation of real events.
Simple Models of Climate
1. Doel (1996), pp. 57-67.
2. Kuiper and Middlehurst
(1961), pp. 386-88, 438.
3. The first observation was in 1956 by Mayer, McCullough, and
Sloanaker. Mayer (1983), p. 271.
4. The actual temperature is around 750°K. Wildt predicted
roughly 400, "higher than the terrestrial boiling-point." Wildt
(1940); something other than CO2 would be necessary
to get to 600 according to Kuiper in Kuiper (1952), ch. 12. BACK
5. Sagan, interview by Ron Doel, Aug. 27, 1991, AIP, tape 4 side
6. Sagan (1960b); Sagan (1960a), "efficient" p. vii, "lifeless" p. 20; see also Sagan (1961); arguments against a waterless Venus were
developed by Gold (1964); see Davidson (1999), pp. 101-106.
7. Ingersoll (1969), p.1191.
8. Rasool and de Bergh
(1970) calculated that water would always have boiled on Venus, but
Pollack (1971) was the first to
deploy enough computer power to calculate a reasonable "runaway greenhouse"
atmosphere. The ever prescient Tommy Gold had already speculated in a
1963 symposium about a "runaway process" when water boiled away, Gold
(1964), p. 250. Others continued to speculate about a Venus that had
once had a "clement" climate, e.g., Wang
et al. (1976). BACK
9. Hart (1979). More accurate
calculations in the 1990 found that for our Sun, a considerably larger zone should be habitable
despite the gradually increasing brightness of the Sun itself.
10. Mayer (1961), p. 458.
11. Newell (1980), ch. 20.
Newell also says that "study of the role of halogens in the atmosphere of Venus... led to the
suspicion that chlorine produced in Earth's stratosphere from the exhausts of Space Shuttle
launches or from Freon used at the ground in aerosol sprays might dangerously deplete the ozone
12. The first publication was Sill
(1972). The idea that sulfuric acid was "the most probable constituent
of the Venus clouds" was independently suggested by Louise Young,
whose husband credited her in his publication, Young
(1973), p. 564. Young relied especially on measurements by Hansen,
who had also identified the acid but was dissuaded from publishing the
idea. Hansen, interview by Weart, Oct. 2000, AIP; Hansen’s contribution
to the identification was noted by Prather (2002).
Confirmation from an infrared telescope carried in an aircraft was reported
by Pollack et al. (1975).
13. He thought "a great deal stands to be gained"
by studying other planets' climates alongside the Earth's. Hansen
et al. (1978), p. 1067. For these matters and NASA contributions in
general see Conway (2008). BACK
14. Hitchcock and Lovelock
(1967); see Lovelock (2000), pp. 228ff.; the absence of
detectable oxygen on Mars was long considered no definitive argument against the presence of
vegetation, and in the early 1960s, NASA's ideas on detecting life through atmospheric analysis
centered on a search for complex organic molecules. Dick
(1998), pp. 48, 175.
15. Observations: Hanel et al.
(1972); Feedbacks: Leovy et al. (1973); "observed": Toon et al. (1975), p. 495.
16. Budyko (1968); Sellers (1969).
17. Prediction (hoping that Martian life was only hibernating
through the winter of a 50,000-year cycle): Sagan (1971); Sagan et al. (1973), quotes pp. 1045, 1048. CO2 ice: Forget and Pierrehumbert (1997).
17a. Also called the "faint young Sun" problem. Sagan and Mullen (1972); Owen et al. (1979); Feulner (2012); on self-stabilization of such a system see Walker et al. (1981). BACK
18. According to O.B. Toon, a radiative transfer model
developed by Sagan and J. Pollack was influential for atmospheric studies in general. Davidson (1999), p. 244; Martian storms: Fenton (2009).
© 2003-2019 Spencer Weart & American Institute of Physics