* At the present time there is still a little uncertainty about the abundance of water vapor on Venus. The gas chromatograph on the Pioneer Venus entry probes gave an abundance of water in the lower atmosphere of a few tenths of a percent. On the other hand, infrared measurements by the Soviet entry vehicles, Veneras 11 and 12, gave an abundance of about a hundredth of a percent. If the former value applies, then carbon dioxide and water vapor alone are adequate to seal in almost all the heat radiation from the surface and keep the Venus ground temperature at about 480°C. If the latter number applies – and my guess is that it is the more reliable estimate – then carbon dioxide and water vapor alone are adequate to keep the surface temperature only at about 380°C, and some other atmospheric constituent is necessary to close the remaining infrared frequency windows in the atmospheric greenhouse. However, the small quantities of SO2, CO and HCl, all of which have been detected in the Venus atmosphere, seem adequate for this purpose. Thus recent American and Soviet missions to Venus seem to have provided verification that the greenhouse effect is indeed the reason for the high surface temperature.
Our neighboring world turns out to be a dismally unpleasant place. But we will go back to Venus. It is fascinating in its own right. Many mythic heroes in Greek and Norse mythology, after all, made celebrated efforts to visit Hell. There is also much to be learned about our planet, a comparative Heaven, by comparing it with Hell.
The Sphinx, half human, half lion, was constructed more than 5,500 years ago. Its face was once crisp and cleanly rendered. It is now softened and blurred by thousands of years of Egyptian desert sandblasting and by occasional rains. In New York City there is an obelisk called Cleopatra’s Needle, which came from Egypt. In only about a hundred years in that city’s Central Park, its inscriptions have been almost totally obliterated, because of smog and industrial pollution – chemical erosion like that in the atmosphere of Venus. Erosion on Earth slowly wipes out information, but because they are gradual – the patter of a raindrop, the sting of a sand grain – those processes can be missed. Big structures, such as mountain ranges, survive tens of millions of years; smaller impact craters, perhaps a hundred thousand*; and large-scale human artifacts only some thousands. In addition to such slow and uniform erosion, destruction also occurs through catastrophes large and small. The Sphinx is missing a nose. Someone shot it off in a moment of idle desecration – some say it was Mameluke Turks, others, Napoleonic soldiers.
* More precisely, an impact crater 10 kilometers in diameter is produced on the Earth about once every 500,000 years; it would survive erosion for about 300 million years in areas that are geologically stable, such as Europe and North America. Smaller craters are produced more frequently and destroyed more rapidly, especially in geologically active regions.
On Venus, on Earth and elsewhere in the solar system, there is evidence for catastrophic destruction, tempered or overwhelmed by slower, more uniform processes: on the Earth, for example, rainfall, coursing into rivulets, streams and rivers of running water, creating huge alluvial basins; on Mars, the remnants of ancient rivers, perhaps arising from beneath the ground; on Io, a moon of Jupiter, what seem to be broad channels made by flowing liquid sulfur. There are mighty weather systems on the Earth – and in the high atmosphere of Venus and on Jupiter. There are sandstorms on the Earth and on Mars; lightning on Jupiter and Venus and Earth. Volcanoes inject debris into the atmospheres of the Earth and Io. Internal geological processes slowly deform the surfaces of Venus, Mars, Ganymede and Europa, as well as Earth. Glaciers, proverbial for their slowness, produce major reworkings of landscapes on the Earth and probably also on Mars. These processes need not be constant in time. Most of Europe was once covered with ice. A few million years ago, the present site of the city of Chicago was buried under three kilometers of frost. On Mars, and elsewhere in the solar system, we see features that could not be produced today, landscapes carved hundreds of millions or billions of years ago when the planetary climate was probably very different.
There is an additional factor that can alter the landscape and the climate of Earth: intelligent life, able to make major environmental changes. Like Venus, the Earth also has a greenhouse effect due to its carbon dioxide and water vapor. The global temperature of the Earth would be below the freezing point of water if not for the greenhouse effect. It keeps the oceans liquid and life possible. A little greenhouse is a good thing. Like Venus, the Earth also has about 90 atmospheres of carbon dioxide; but it resides in the crust as limestone and other carbonates, not in the atmosphere. If the Earth were moved only a little closer to the Sun, the temperature would increase slightly. This would drive some of the CO2 out of the surface rocks, generating a stronger greenhouse effect, which would in turn incrementally heat the surface further. A hotter surface would vaporize still more carbonates into CO2, and there would be the possibility of a runaway greenhouse effect to very high temperatures. This is just what we think happened in the early history of Venus, because of Venus’s proximity to the Sun. The surface environment of Venus is a warning: something disastrous can happen to a planet rather like our own.
The principal energy sources of our present industrial civilization are the so-called fossil fuels. We burn wood and oil, coal and natural gas, and, in the process, release waste gases, principally CO2, into the air. Consequently, the carbon dioxide content of the Earth’s atmosphere is increasing dramatically. The possibility of a runaway greenhouse effect suggests that we have to be careful: Even a one- or two-degree rise in the global temperature can have catastrophic consequences. In the burning of coal and oil and gasoline, we are also putting sulfuric acid into the atmosphere. Like Venus, our stratosphere even now has a substantial mist of tiny sulfuric acid droplets. Our major cities are polluted with noxious molecules. We do not understand the long-term effects of our course of action.
But we have also been perturbing the climate in the opposite sense. For hundreds of thousands of years human beings have been burning and cutting down forests and encouraging domestic animals to graze on and destroy grasslands. Slash-and-burn agriculture, industrial tropical deforestation and overgrazing are rampant today. But forests are darker than grasslands, and grasslands are darker than deserts. As a consequence, the amount of sunlight that is absorbed by the ground has been declining, and by changes in the land use we are lowering the surface temperature of our planet. Might this cooling increase the size of the polar ice cap, which, because it is bright, will reflect still more sunlight from the Earth, further cooling the planet, driving a runaway albedo* effect?
* The albedo is the fraction of the sunlight striking a planet that is reflected back to space. The albedo of the Earth is some 30 to 35 percent. The rest of the sunlight is absorbed by the ground and is responsible for the average surface temperature.
Our lovely blue planet, the Earth, is the only home we know. Venus is too hot. Mars is too cold. But the Earth is just right, a heaven for humans. After all, we evolved here. But our congenial climate may be unstable. We are perturbing our poor planet in serious and contradictory ways. Is there any danger of driving the environment of the Earth toward the planetary Hell of Venus or the global ice age of Mars? The simple answer is that nobody knows. The study of the global climate, the comparison of the Earth with other worlds, are subjects in their earliest stages of development. They are fields that are poorly and grudgingly funded. In our ignorance, we continue to push and pull, to pollute the atmosphere and brighten the land, oblivious of the fact that the long-term consequences are largely unknown.
A few million years ago, when human beings first evolved on Earth, it was already a middle-aged world, 4.6 billion years along from the catastrophes and impetuosities of its youth. But we humans now represent a new and perhaps decisive factor. Our intelligence and our technology have given us the power to affect the climate. How will we use this power? Are we willing to tolerate ignorance and complacency in matters that affect the entire human family? Do we value short-term advantages above the welfare of the Earth? Or will we think on longer time scales, with concern for our children and our grandchildren, to understand and protect the complex life-support systems of our planet? The Earth is a tiny and fragile world. It needs to be cherished.