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and what happens when we have too much of one thing or too little of another。 We learn how a planet can go wrong。 We gain a new understanding； foreseen by the spaceflight pioneer Robert Goddard； called parative planetology。
The exploration of other worlds has opened our eyes in the study of volcanos； earthquakes； and weather。 It may one day have profound implications for biology； because all life on Earth is built on a mon biochemical master plan。 The discovery of a single extraterrestrial organism—even something as humble as a bacterium—would revolutionize our understanding of living things。 But the connection between exploring other worlds and protecting this one is most evident in the study of Earth's climate and the burgeoning threat to that climate that our technology poses。 Other worlds provide vital insights about what dumb things not to do on Earth。
Three potential environmental catastrophes—all operating on a global scale—have recently been uncovered： ozone layer depletion； greenhouse warming； and nuclear winter。 All three discoveries； it turns out； have strong ties to the exploration of the planets。
（1） It was disturbing to find that an inert material with all sorts of practical applications—it serves as the working fluid in refrigerators and air conditioners； as aerosol propellant for deodorants and other products； as lightweight foamy packaging for fast foods； and as a cleaning agent in microelectronics； to name only a few…can pose a danger to life on Earth。 Who would have figured？
The molecules in question are called chlorofluorocarbons （CFCs）。 Chemically； they're extremely inert； which means they're invulnerable—until they find themselves up in the ozone layer； where they're broken apart by ultraviolet light from the Sun。 The chlorine atoms thus liberated attack and break down the protective ozone； letting more ultraviolet light reach the ground。 This increased ultraviolet intensity ushers in a ghastly procession of potential consequences involving not just skin cancer and cataracts； but weakening of the human immune system and； most dangerous of all； possible harm to agriculture and to photosynthetic organisms at the base of the food chain on which most life on Earth depends。
Who discovered that CFCs posed a threat to the ozone layer？ Was it the principal manufacturer； the DuPont Corporation； exercising corporate responsibility？ Was it the Environmental Protection Agency protecting us？ Was it the Department of Defense defending us？ No； it was two Ivory…tower； white…coated university scientists working on something else—Sherwood Rowland and Mario Molina of the University of California； Irvine。 Not even an Ivy League university。 No one instructed them to look for dangers to the environment。 They were pursuing fundamental research。 They were scientists following their own interests。 Their names should be known to every schoolchild。
In their original calculations； Rowland and Molina used rate constants of chemical reactions involving chlorine and other halogens that had been measured in part with NASA support。 Why NASA？ Because Venus has chlorine and fluorine molecules in its atmosphere； and planetary aeronomers had wanted to understand what's happening there。
Confirming theoretical work on the role of CFCs in ozone depletion was soon done by a group led by Michael McElroy at Harvard How is it they had all these branching networks of halogen chemical kinetics in their puter ready to go？ Because they were working on the chlorine and fluorine chemistry of the atmosphere of Venus。 Venus helped make and helped confirm the discovery that the Earth's ozone layer is in danger。 An entirely unexpected connection was found between the atmospheric photochemistries of the two planets。 A result of importance to everyone on Earth emerged from what might well have seemed the most blue…sky； abstract； impractical kind of work； understanding the chemistry of minor constituents in the upper atmosphere of another world。
There's also a Mars connection。 With Viking we found the surface of Mars to be apparently lifeless and remarkably deficient even in simple organic molecules。 But simple organic molecules ought to be there； because of the impact of organic…rich meteorites from the nearby asteroid belt。 This deficiency is widely attributed to the lack of ozone on Mars。 The Viking microbiology experiments found that organic matter carried from Earth to Mars and sprinkled on Martian surface dust is quickly oxidized arid destroyed。 The materials in the dust that do the destruction are molecules something like hydrogen peroxide—which we use as an antiseptic because it kills microbes by oxidizing there； Ultraviolet light from the Sun strikes the surface of Mars unimpeded by an ozone layer； if any organic matter were there； it would be quickly destroyed by the ultraviolet light itself and its oxidation products。 Thus part of the reason the topmost layers of Martian soil are antiseptic is that Mars has an ozone hole of planetary dimensions—by itself a useful cautionary tale for us； who are busily thinning and puncturing our ozone layer。
（2） Global warming is predicted to follow from the increasing greenhouse effect caused largely by carbon dioxide generated in the burning of fossil fuels—but also from the buildup of other infrared…absorbing gases （oxides of nitrogen； methane； those same CFCs； and other molecules）。
Suppose that we have a three…dimensional general circulation puter model of the Earth's climate。 Its programmers claim it's able to predict what the Earth will be like if there's more of one atmospheric constituent or less of another。 The model does very well at 〃predicting〃 the present climate。 But there is a nagging worry： The model has been 〃tuned〃 so it will e out right—that is； certain adjustable parameters are chosen； not from first principles of physics； but to get the right answer。 This is not exactly cheating； but if we apply the same puter model to rather different climatic regimes—deep global warming； for instance—the tuning might then be inappropriate。 The model might be valid for today's climate； but not extrapolatable to others。
One way to test this program is to apply it to the very different climates of other planets。 Can it predict the structure of the atmosphere on Mars and the climate there？ The weather？ What about Venus？ If it were to fail these test cases； we would be right in mistrusting it when it makes predictions for our own planet。 In fact； climate models now in use do very well in predicting from first principles of physics the climates on Venus and Mars。
On Earth； huge upwellings of molten lava are known and attributed to superplumes convecting up from the deep mantle and generating vast plateaus of frozen basalt。 A spectacular example occurred about a hundred million years ago； and added perhaps ten times the present carbon dioxide content to the atmosphere； inducing substantial global warming。 These plumes； it is thought； occur episodically throughout Earth's history。 Similar mantle upwelling seem to have occurred on Mars and Venus。 There are sound practical reasons for us to want to understand how a major change to the Earth's surface and climate could suddenly arrive unannounced from hundreds of kilometers beneath our feet。
Some of the most important recent work on global warming has been done by James Hansen and his colleagues at the Goddard Institute for Space Sciences； a NASA facility in New York City。 Hansen developed one of the major puter climate models and employed it to predict what will happen to our climate as the greenhouse gases continue to build up。 He has been in the forefront of testing these models against ancient climates of the Earth。 （During the last ice ages； it is of interest to note； more carbon dioxide and methane are strikingly correlated with higher temperatures。） Hansen collected a wide range of weather data from this century and last； to see what actually happened to the global temperature； and then pared it to the puter model's predictions of what should have happened。 The two agree to within the errors of measurement and calculation； respectively。 He courageously testified before Congress in the face of a politically generated order from the White House Office of Management and Budget （this was in the Reagan years） to exaggerate the uncertainties and minimize the dangers。 His calculation on the explosion of the Philippine volcano Mt。 Pinatubo and his prediction of the resulting temporary decline in the Earth's temperature （about half a degree Celsius） were right on the money。 He has been a force in convincing governments worldwide that global warming is something to be taken seriously。
How did Hansen get interested in the greenhouse effect in the first place？ His doctoral thesis （at the University of Iowa in 1967） was about Venus。 He agreed that the high radio brightness of Venus is due to a very hot surface； agreed that greenhouse gases keep the heat in； but proposed that heat from the interior rather than sunlight was the principal energy source。 The Pioneer 12 mission to Venus in 1978 dropped entry probes into the atmosphere； they showed directly that the ordinary greenhouse effect—the surface heated by the Sun and the heat retained by the blanket of air—was the operative cause。 But it's Venus that got Hansen thinking about the greenhouse effect。
Radio astronomers； you note； find Venus to be an intense source of radio waves。 Other explanations of the radio emission fail。 You conclude that the surface must be ridiculously hot。 You try to understand where the high temperatures e from and are led inexorably to one or another kind of greenhouse effect。 Decades later you find that this training has prepared you to understand and help predict an unexpected threat to our global civilization。 I know many other instances where scientists who first tried to puzzle out the atmospheres of other worlds are making important and highly practical discoveries about this one。 The other planets are a superb training ground for students of the Earth。 They require both breadth and depth of knowledge； and they challenge the imagination。
Those who are skeptical about carbon dioxide greenhouse warming might profitably note the massive greenhouse effect on Venus。 No one proposes that Venus's greenhouse effect derives froth imprudent Venusians who burned too much coal； drove fuel…inefficient autos； and cut down their forests。 My point is different。 The climatological history of our planetary neighbor； an otherwise Earthlike planet on which the surface became hot enough to melt tin or lead； is worth considering—especially by those who say that the increasing greenhouse effect on Earth will be self…correcting； that we don't really have to worry about it； or （you can see this in the publications of some groups that call themselves conservative） that the greenhouse effect itself is a 〃hoax。〃
（3） Nuclear winter is the predicted darkening and cooling of the Earth—mainly from fine smoke particles injected into the atmosphere from the burning of cities and petroleum facilities—that is predicted to follow a global thermonuclear war。 A vigorous scientific debate ensued on just how serious nuclear winter might be。 The various opinions have now converged。 All three…dimensional general circulation puter models predict that the global temperatures resulting from a worldwide thermonuclear war would be colder than those in the Pleistocene ice ages。 The implications for our planetary civilization—especially through the collapse of agriculture—are very dire。 It is a consequence of nuclear war that was somehow overlooked by the civil and military authorities of the United States； the Soviet Union； Britain； France； and China when they decided to accumulate well over 60；000 nuclear weapons。 Although it's hard to be certain about such things； a case can be made that nuclear Winter played a constructive role （there were other causes； of course） in convincing the nuclear…armed nations； especially the Soviet Union； of the futility of nuclear war。
Nuclear winter was first calculated and named in 1982/83 by a group of five scientists； to which I'm proud to belong。 This team was given the acronym TTAPS （for Richard P。 Turco； （even B。 Toon； Thomas Ackerman； James Pollack； and m