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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 from 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 is a "hoax".
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p. 227

 

Carl Sagan

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The greenhouse effect is a more apt name than those who coined it imagined. The carbon dioxide and trace gases act like the panes of glass on a greenhouse--the analogy is accurate. But it's more than that. We have built a greenhouse, 'a human creation' where once there bloomed a sweet and wild garden.

 

Bill McKibben

 

One thing I try to point out to people, and Dick Lindzen has pointed this out in the past, too, is that, as most of you probably know, about ninety percent of the earth’s greenhouse effect is water vapor, then carbon dioxide, methane and some trace gases. The water vapor con-centration in the atmosphere goes up and down quite a bit, but it is self-regulating. Things don’t get too warm before precipitation systems suck the water vapor out again. I don’t understand how we can know how much warming there will be with global warming until we understand how precipitation systems change with warming. We don’t under-stand that, and I have papers from modelers who have said the same thing. Modelers don’t like to talk about what they don’t know. So until we understand these natural proc-esses that remove the earth’s primary greenhouse gas, water vapor, from the atmosphere – which is a self-regulating part of the earth keeping a constant temperature – I don’t think we can predict how much warming there will be due to increasing CO2. It is a mat-ter of faith again, I think. We are not saying that we don’t believe that there can be sig-nificant global warming. As John said, if you add CO2, something has to change. But things are changing all the time anyway. The big question is: So what? How much is it going to change, compared to other things? And what can you do about it?

 

Roy Spencer

 

The danger is that global warming may become self-sustaining, if it has not done so already. The melting of the Arctic and Antarctic ice caps reduces the fraction of solar energy reflected back into space, and so increases the temperature further. Climate change may kill off the Amazon and other rain forests, and so eliminate once one of the main ways in which carbon dioxide is removed from the atmosphere. The rise in sea temperature may trigger the release of large quantities of carbon dioxide, trapped as hydrides on the ocean floor. Both these phenomena would increase the greenhouse effect, and so global warming further. We have to reverse global warming urgently, if we still can.

 

Stephen Hawking

 

Mars has global warming, but without a greenhouse and without the participation of Martians. These parallel global warmings -- observed simultaneously on Mars and on Earth -- can only be a straightline consequence of the effect of the one same factor: a long-time change in solar irradiance.

 

Khabibullo Abdusamatov

 

Since this series' maiden voyage, the impossible has come to pass: Mighty walls that maintained insuperable ideological differences have come tumbling down; deadly enemies have embraced and begun to work together. The imperative to cherish the Earth and protect the global environment that sustains all of us has become widely accepted, and we've begun, finally, the process of reducing the obscene number of weapons of mass destruction. Perhaps we have, after all, decided to choose life. But we still have light years to go to ensure that choice. Even after the summits and the ceremonies and the treaties, there are still some 50,000 nuclear weapons in the world — and it would require the detonation of only a tiny fraction of them to produce a nuclear winter, the predicted global climatic catastrophe that would result from the smoke and the dust lifted into the atmosphere by burning cities and petroleum facilities.
The world scientific community has begun to sound the alarm about the grave dangers posed by depleting the protective ozone shield and by greenhouse warming, and again we're taking some mitigating steps, but again those steps are too small and too slow. The discovery that such a thing as nuclear winter was really possible evolved out of the studies of Martian dust storms. The surface of Mars, fried by ultraviolet light, is also a reminder of why it's important to keep our ozone layer intact. The runaway greenhouse effect on Venus is a valuable reminder that we must take the increasing greenhouse effect on Earth seriously.
Important lessons about our environment have come from spacecraft missions to the planets. By exploring other worlds we safeguard this one. By itself, I think this fact more than justifies the money our species has spent in sending ships to other worlds. It is our fate to live during one of the most perilous and, at the same time, one of the most hopeful chapters in human history.
Our science and our technology have posed us a profound question. Will we learn to use these tools with wisdom and foresight before it's too late? Will we see our species safely through this difficult passage so that our children and grandchildren will continue the great journey of discovery still deeper into the mysteries of the Cosmos? That same rocket and nuclear and computer technology that sends our ships past the farthest known planet can also be used to destroy our global civilization. Exactly the same technology can be used for good and for evil. It is as if there were a God who said to us, “I set before you two ways: You can use your technology to destroy yourselves or to carry you to the planets and the stars. It's up to you.”

 

Carl Sagan