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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.
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ABC News interview (16 August 2006)

 

Stephen Hawking

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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

 

Many scientists are now warning that we are moving closer to several "tipping points" that could — within as little as 10 years — make it impossible for us to avoid irretrievable damage to the planet's habitability for human civilization. In this regard, just a few weeks ago, another group of scientists reported on the unexpectedly rapid increases in the release of carbon and methane emissions from frozen tundra in Siberia, now beginning to thaw because of human caused increases in global temperature. The scientists tell us that the tundra in danger of thawing contains an amount of additional global warming pollution that is equal to the total amount that is already in the earth's atmosphere.

 

Al Gore

 

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".

 

Carl Sagan

 

Humans are significantly altering the global climate, but in a variety of diverse ways beyond the radiative effect of carbon dioxide. The IPCC assessments have been too conservative in recognizing the importance of these human climate forcings as they alter regional and global climate. These assessments have also not communicated the inability of the models to accurately forecast the spread of possibilities of future climate. The forecasts, therefore, do not provide any skill in quantifying the impact of different mitigation strategies on the actual climate response that would occur.

 

Roger A. Pielke

 

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