The small number of credible skeptics out there (e.g., Spencer, Lindzen) have spent much of the last decade searching for a negative feedback in our climate system. If a sufficiently big one is found, then it would suggest that warming over the next century may well be small.

Most climate scientists, however, are reasonably certain that a negative feedback big enough to overwhelm the well-known positive feedbacks in the climate system, such as the water vapor feedback [PDF], does not exist. Why?

Negative feedbacks tend to dampen out climate change. If you add greenhouse gases to the atmosphere or the sun brightens, then the hypothetical negative feedback will counteract the warming, leaving the climate nearly unchanged. While it may be comforting to believe that a negative feedback exists, it is extremely difficult to reconcile the existence of a big negative feedback with our past observations of climate variability.

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For example, the ice ages rely on a carbon dioxide feedback to provide their large amplitude. If there were a big negative feedback in the system, then how do you explain the large swings in to and out of ice ages? No way that I know of.

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Similarly, the Paleocene Eocene Thermal Maximum is also thought to be the result of a huge release of greenhouse gases. With a large negative feedback in the system, how do you explain the rapid temperature rise during that event?

Finally, how do you explain the warming of the last few decades if the climate is intrinsically stable?

Climate skeptics, of course, don’t have to look at all the data. In one thread, they can argue “it’s the sun,” while in another thread they can argue “there is a negative feedback.” The problem is that these are mutually contradictory. Any negative feedback working on carbon dioxide would also work on increased solar forcing. One cannot argue both that our present-day warming is due the sun, and that there is a negative feedback that prevents carbon dioxide from warming and climate.

If you look at all the data, the simplest explanation is that there is no negative feedback, and we are experiencing warming due to increases in greenhouse gas concentrations. There is, in fact, no fully consistent, alternative theory.

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