Worst Case Scenarios

As the brighter - using the term loosely - members of the denialist crowd like to point out, global warming is not exactly unprecedented. Our current athropogenic warming episode has many natural antecedents. So how bad is it likely to get?

The worst imaginable AGW crisis would be for the warming to trigger a Venusian style runaway greenhouse and exterminate all life on Earth. That will eventually happen as the Sun continues to warm up, but it seems extremely improbable in the short run. Some of the natural global warmings of the past have triggered, or at any rate, coincided with, major extinctions, however, including one which wiped out essentially all large animals. That one, the Paleocene-Eocene Thermal Maximum, or PETM, is the scariest precedent.

In the PETM, some event seems to have led to a fairly rapid build up of carbon dioxide and a global warming by about 6 C (11 F) - perhaps a bit more than the most likely maximum increase from our current CO2 releases. The Earth remained hot for about 20,000 years. From the Wikipedia entry:

Other "hyperthermal" events can be recognised during this period of warming, including the Elmo event (ETM2). During these events – of which the PETM was by far the most severe – around 1,500 to 2,000 gigatons of carbon were released into the ocean/atmosphere system over the course of 1,000 years. This rate of carbon addition almost equals the rate at which carbon is being released into the atmosphere today through anthropogenic activity. [8]
(My emphasis)

So how does such a warming trigger a mass extinction? A (uniform) 3 C or even a 6 C warming might render most of Africa and portions of Asia and the Americas unihabitable, but England might even be more pleasant, right? Maybe not. The problem is that these kinds of warmings aren't uniform and are likely to strongly disrupt the global wind currents. Fertile lands may become arid. The excess CO2 in the atmosphere will change ocean chemistry. The PETM saw life disappear from large chunks of ocean, and major extinctions on land.

I personally doubt that humans are in much danger of extinction as a species due to AGW - we are too versatile. Randomly killing 99 out of every 100 tigers might extinguish the species - the survivors might not be able to find each other. Even 9999 out of every 10,000 casualties for humans would be unlikely to have the same effect.

If warming can be kept to 2-3 C (the current goal is 2), damages may be much less. The atoll nations, I fear, are doomed. There seems little prospect that they can be saved. Bangla Desh and The Netherlands are at extreme risk as well. In such "moderate" scenarios, one of the major risks will be resource and land wars as the displaced struggle to sieze habitable land from the less damaged.

There is one other respect that the denialist crew may have a point. They believe that actions taken to stave off global warming are likely to produce a global economic crisis. Maybe, maybe not, but the alternative might be far worse.

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