A Comparative Planetary Perspective: 1990

Comparative Temperatures on Venus and Earth

We can begin by examining Earth and its nearest planetary neighbor, Venus. Our beliefs about Venusian temperatures underwent striking revisions. Being somewhat closer to the Sun than Earth (7/10 the distance), Venus receives more intense sunlight and should have been a bit warmer than Earth. But the presence of a thick cover of clouds on Venus led early astronomers to wrong conclusions about this planet's temperatures. These clouds, it was believed, would shield Venus from excess sunlight, thus keeping its temperatures closer to ours. The fact that Venus is just about the same size as Earth, and that its clouds are made of water, led some astronomers to suppose that it had oceans and perhaps even lush life. Unfortunately, space travel disclosed harsher realities.

The surface of the entire planet turned out to have kiln-like temperatures. At 860F (460C), lead and tin would melt, mercury boil, animals and plants sizzle. Near the ground, water can't exist on the Venusian surface in liquid form. Life as we know it was but a dream in Venus, except perhaps as an afterlife inferno. Venus is a sizzling cauldron, not the idyllic water planet it was believed to be.


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Venus' atmosphere as see by the Galileo spacecraft.

Thus, temperatures on Venus and Earth differ markedly. For instance, water on Venus' surface can only exist as steam, while on Earth it can exist as liquid, vapor, and ice. Numerous calculations convincingly show that this vast temperature difference is not for the most part traceable to Earth's greater distance from the Sun. Hence, something other than proximity to the sun must account for the observed temperatures. This additional factor, as we shall now see, is the presence and chemical composition of planetary atmospheres.

Why is Venus so Hot?

The rather different atmospheres of Venus and Earth play a decisive role in determining their climates. Venus' atmosphere is about 90 times denser than Earth's. That is, if we took two identical cups and filled one with Venus' air, the other with Earth's, and weighed the contents of both on Earth, the contents of the Venusian cup would be 90 times heavier. Another way of putting this: in Venus, the mercury column of a barometer would be more than 80 times taller than it is on Earth (it is not exactly 90 times because objects on Venus weigh less than they do on Earth). This is equal to the pressure exerted in the ocean at a depth of one-half a mile.

Measurements showed that Venus' atmosphere is approximately 97% carbon dioxide (CO2) and 3% nitrogen. Its thick clouds are made of water and highly corrosive sulfuric acid. Owing to the incredible density of its air, Venus' slow-moving ground winds are more powerful than Earthly hurricanes. Venus' thick clouds indeed block most of the sunlight, as early astronomers believed. Venus is closer to the Sun than Earth and gets more intense sunlight, but only little of this light reaches the ground. Consequently, Venus' surface receives only one-sixth the sunlight Earth receives. Its surface is not as sunny as Earth's, although it is sufficiently lit to make distant objects visible.

But what is it about the atmospheres of the two planets which keep Earth warm enough to support life? Why is life as we know it impossible on Venus? Why is all the water of Venus found in the clouds above it, whereas the bulk of Earth's water is in its oceans?

To answer these questions, we need to understand the nature of radiation. Each type has its own characteristic wavelength—what we call red, for instance, has longer wavelength than what we call yellow. Under certain conditions, one type of radiation changes into another. Sunlight can go through the atmosphere—that's why we see the sun. Sunlight then warms the ground, which then emits different forms of radiation than sunlight. Although we can't see this radiation, we can feel it—it is heat radiation. In physical terms, heat radiation belongs to a different part of the spectrum. Since it is beyond visible red, it is called infrared (below red) radiation. This radiation has a longer wavelength than visible light.

Carbon dioxide, water vapor, and a few other gases have the wonderful property of letting sunshine in, but blocking some of the infrared radiation. They serve as a blanket, keeping the ground—the source of heat radiation—warmer by not letting this heat escape. This heat, having no place to go, is trapped. (The greenhouse effect received its name from the belief that glass had similar properties to CO2, and that a similar process keeps a greenhouse warmer; it is now believed however that a different principle operates behind a greenhouse. It was too late, by then, to change the name).

The greenhouse theory was first propounded by Rupert Wildt in the 1940s to explain the unexpectedly high temperatures of Venus. By the early 1960s, it was revived and championed by an American astronomer who is a household name in this country—Carl Sagan.

This fabulous ability of greenhouse gases such as CO2 to trap heat explains why Earth is warm enough to support life. Without its greenhouses gases, Earth may have been, on average, 86 F colder than it is now, and hence, lifeless. This is why it makes perfect sense to say that the atmosphere is Earth's blanket. Sunlight goes through the lower atmosphere. At the surface, some of this light is converted into heat. The ground reflects this heat, which is then trapped by the lower atmosphere. Sunshine goes through CO2 readily, but heat radiation bounces back. Our planet is thus a comfortably sleeping giant, with the atmospheric greenhouses gases serving as its blanket.

As we have seen, Venus' atmosphere has much more CO2 than Earth. In the first place, CO2 exists only in small amounts in our atmosphere, but it comprises some 97% of the Venusian atmosphere. In the second place, Venus' atmosphere is far denser than Earth's. So, for every CO2 molecule in earth's atmosphere there are over 60,000 molecules of CO2 molecules in Venus'. Most likely, this is why Venus is so much hotter than Earth, even though much of the sunshine Venus receives is permanently blocked by its cloud cover.


On Earth, then, CO2 is a critically important compound. For one thing, it keeps our planet just warm enough to sustain life. For another, it is the raw material from which plants are made through the process known as photosynthesis (which literally means "light production). Here, plants absorb CO2 from the atmosphere through their leaves and water through their roots. Then, through a quantum step that makes virtually all life possible, the light energy is captured to begin the process by which CO2 and water are transformed into glucose and oxygen. The plants use glucose as a source of energy for life and growth. Animals eat plants to live and grow, and they breathe the oxygen these plants produce.


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Can there be too much of as good a thing as CO2? Venus tells us that there can be, and that we ought to be careful. Industrial civilization burns enormous quantities of fossil fuels—coal, gas, and oil, thereby steadily raising the concentration of CO2 in the atmosphere. Our civilization is also burning massive tracts of forests, thereby releasing more CO2. Moreover, since living plants consume CO2, deforestation adds insult to injury by turning trees from long term consumers of CO2 to short term producers. One recent estimate suggests that levels went up in the last 300 years by 25%.

Moreover, CO2 is not the only greenhouse gas. Water vapor can trap heat too. That is one reason why cloudy nights in summer are the hottest—clouds trap heat, not allowing the Earth to cool as much it would have in the absence of clouds. That is one reason why deserts, which are often cloudless, have more extreme day/night temperature shifts. Methane—the gas we use to heat our homes and which is given off in enormous quantities in cattle feedlots and garbage landfills—has similar properties. Some manmade chemicals released to the atmosphere will stay there for decades or centuries, and they too block some heat radiation. Chief among these manmade scoundrels is a group of substances chemists call chlorofluorocarbons (CFC's for short), more commonly known as freons. CFC's are used as propellants in aerosol sprays, in styrofoam cups, packing materials, refrigerators, and air conditioners.

Besides the rising levels of CO2 and other greenhouse gases in the atmosphere, and besides the global warming of the last decade, there are other indications that at least a mild form of global warming is slowly overtaking our planet. Since 1935, some glaciers in the Himalayas retreated considerably. In the last century, warm-climate fish and trees have been migrating northward. Sea levels have been slightly rising every year.

It is impossible to predict with certainty anything as complex as major climate shifts; we can only make educated guesses. Still, the most reasonable guess leads most scientists to worry about the long term effects of greenhouse gases on Earth's climate. In fifty years, if we continue the unbridled release of greenhouse gases into the atmosphere, temperatures might be high enough to melt the polar ice caps, raise sea levels, and submerge low lying areas such as Louisiana. Indeed, if all the icecaps completely melted, sea level might rise by as much as 200 feet, drowning the coastal areas of all continents, including many of the world's largest cities. Climates may shift dramatically, perhaps turning once prosperous agricultural areas into deserts.

If the process continues unchecked, it could reach a point of no return, as it apparently had on Venus. As the Earth heats up, water would convert into steam, which is a greenhouse gas. The Earth would get hotter and hotter. As the atmosphere heats up, more CO2 might escape from its present location in ocean rocks and shells. Beyond a certain point, the process may be self-sustaining. Venus tells us how far such a process can go—hellish temperatures, a cloud cover that lets less sunlight reach the earth but helps trap the heat, enormous pressures at ground levels weighing heavily upon everything and distorting the landscape. Venus provides us a timely warning. If we pay no attention, in some 800 years life on Earth might perish.

The uncertainties involving the greenhouse effect cannot be overemphasized. Scientists can predict some things extremely well, e.g., the return of Halley's comet, but are poor at studying and forecasting other things, e.g., tomorrow's stock prices. The greenhouse effect belongs to the second category. Some scientists are saying that the few unusually hot recent summers are a natural phenomenon. Others are convinced that the oceans will be able to absorb the excess amounts of CO2, with no consequent warming and disasters. Still others argue that the warming effects of greenhouse gases will be neatly counterbalanced by the chilling effects of increasing quantities of dust and smog, or by the next ice age. Naturally, you can expect scientists in the employ of the powerful oil industry, CFC's industry, cattle industry, packing industry, power companies, car companies, fast food industries—whose employers benefit over the short term from the release of greenhouse gases—to be somewhat more skeptical about the greenhouse effect than their independent counterparts in the academic world. But, besides all this, the subject is surrounded by much uncertainty—we can only make educated guesses. This gives our policy makers the excuse they need to bury their heads in the sand and hope that humanity will be lucky and that the greenhouse effect will turn out to be a mere chimera.

Time magazine, not particularly known for its radical views, felt forced to put the situation in the following terms:

Unfortunately, scientists cannot agree on how much global warming has occurred, how much more is on the way and what the climatic consequences will be, giving policy makers an excuse for delay. But no one disputes the fact that the amount of CO2 in the atmosphere has risen and continues to increase rapidly and that the human race is thus conducting a dangerous experiment on an unprecedented scale. The possible consequences are so scary that it is only prudent for governments to slow the buildup of CO2 through preventive measures, from encouraging energy conservation to developing alternatives to fossil fuels.

Time goes on to suggest that we could reduce somewhat the severity of the greenhouse effect, if we wanted. The good news is that Time and all other mass circulation media outlets in the U.S. are far too pessimistic about what we could and could not do to save our species from a possible cataclysm. In a rational world, we could do away with the greenhouse effect on Earth, if we cared to, especially since doing so would not adversely affect our material quality of life. We could, in principle, stop the burning of forests. We could make sure that a forest always remains a forest by replanting trees. We could recycle paper, thereby considerably reducing deforestation. Rich countries could give sufficient economic incentives to Third World countries such as Brazil to cease and desist from the permanent destruction of forests. We could achieve zero population growth or roll back our numbers to lessen humanity's impact on the biosphere. We could ban CFC's and replace them with benign substitutes. We could cut our energy use by 3/4 without suffering any reduction in our material standard of living (see your Living in the Environment Reader). In the long-term we could switch to safe forms of energy which don't generate carbon dioxide and which don't poison the earth, e.g., the type of energy used now by solar calculators.

The bad news is that so far humanity is doing nothing about the greenhouse effect except pontificate. The bad news is that if we keep waiting, it might just be too late to do anything. We seem to be on a suicidal path. The answer, if there is one, is in the political arena. And, since the U.S. is the world's most powerful nation and, at the same time, by far the world's greatest polluter, the key to humanity's future can be found in the American political process. The people who launched this country's nationhood called our political system an experiment in self-government. Humankind's environmental woes (of which the greenhouse effect is just one) and the potential for nuclear holocaust place this experiment's (and all others') existence at risk. The ordinary citizen's steadfast refusal to become informed, to safeguard his and his children's interests; his insistence on life in the never-never land of celebrities and fads, television and commercials, political illiteracy and ignorance of humankind's peril; his willingness to make money his supreme goal in life; his appetite for infinite distractions and aversion to live and think in the here and now about the things that really matter; put this experiment to its most severe test in its 200 years history. One can only hope that we shall pass this test, and that freedom, civilization, and life will survive and flourish long after grass has grown over our cheeks.

Source:  Moti Nissan, 1990.  Permission for the free use of this material is hereby granted.  Additional, more up-to-date (but lacking a comparative astonomical  perspective) greenhouse writings and references can be found at:

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