In his presentation that I link to here, Warren Meyer makes the point that most climate change disaster scenarios assume that the earth is dominated by positive feedback, an assumption that is scientifically uncertain at best. In a climate change context, positive feedback is the scientific term used to describe a scenario where things begin to spiral out of control, with each year's increase in global temperature resulting in environmental effects that insure an even larger increase in temperature in following years, and so on.
For instance, some scientist theorize that warmer temperatures will cause more ocean water to evaporate, which in turn results in more water vapor in the atmosphere. Furthermore, because water vapor is among the most potent of greenhouse gases, far more effective than carbon dioxide at trapping heat, more atmospheric water vapor results in higher average global temperatures which in turn insures more evaporation, and so on. This is an example of positive feedback, and the extent to which one scientist's climate model predictions varies from another is determined largely by just how much positive feedback he or she assumes. And I say "assumes" because at present there is very little scientific basis for choosing one level of feedback over another.
Is it reasonable to assume that the earth's climate is dominated by significant levels of positive feedback? There are many reasons to think not, or at least not as much as the more alarmist climate models assume. For example, consider that, as any meteorologist can confirm, increased atmospheric water vapor is likely to result in increased cloud cover. After all, clouds are simply visible water vapor, right? Furthermore, doesn't cloud cover create shade that significantly reduces surface temperatures? Of course it does, as anyone who has sat in the shade on a hot day can attest. This is why cloudy days are typically many degrees cooler than sunny ones. So, if increased temperatures result in levels of atmospheric water vapor sufficient to significantly increase cloud cover and thereby reduce surface temperatures, then we actually have negative feedback loop, not a positive one.
Well, scientifically speaking, would increased atmospheric levels of water vapor produce cloud cover sufficient to lower surface temperatures, interrupting further evaporation? Amazingly, we don't know! And to date, no one seems all that interested in finding out: It is an undisputed fact that the climate models used by the United Nation's International Panel on Climate Change (IPCC) in making its alarming forecast make absolutely no allowance for the effects of increased or decreased cloud cover. In essence they assume that cloud cover is static at all temperature and water vapor levels, and it is only by assuming this that they arrive at some of the more catastrophic forecasts. Is this a reasonable assumption? You be the judge, but it is definitely not a scientifically certain one, as evidenced by the great number of scientists who have pointed out this particular limitation of the UN's models.
Another example of positive feedback loop often offered up by alarmists is the effect of higher temperatures or "black carbon" on polar ice. Because polar ice is reflective, much of the suns energy is reflected back out to space, helping to moderate temperatures. But as the poles melt we get less reflection which causes higher temperatures which in turn causes the poles to melt even more, and so on. But again, is this a reasonable assumption?
Perhaps not. As the Guardian reports:
Collapsing antarctic ice sheets, which have become potent symbols of global warming, may actually turn out to help in the battle against climate change and soaring carbon emissions.
Professor Rob Raiswell, a geologist at the University of Leeds, says that as the sheets break off the ice covering the continent, floating icebergs are produced that gouge minerals from the bedrock as they make their way to the sea. Raiswell believes that the accumulated frozen mud could breathe life into the icy waters around Antarctica, triggering a large, natural removal of carbon dioxide from the atmosphere.
And as rising temperatures cause the ice sheets to break up faster, creating more icebergs, the amount of carbon dioxide removed will also rise.
Few scientists doubt that CO2 contributes to global warming. But the amount of this contribution is very much in dispute and depends largely on how much positive feedback is assumed in a given model. History and common sense suggest that the earth's climate does an excellent job of regulating itself, and that more extreme predictions based on large amounts of positive feedback are unlikely.