Friday, June 02, 2006

Global warming: What do we expect?

It is widely agreed that global CO2 concentrations are increasing. There are complicated models that enable climate scientists to try and predict the effect that these rising concentrations will have on the global climate. However, to get an idea for what one might naively expect, I have found it useful to imagine a world without any greenhouse gases.

Global CO2 concentrations have historically been approximately 280 ppm (parts per million). This concentration of CO2 leads to the temperatures that we have grown accustomed to here on Earth. A simple calculation taking into account the energy flux hitting the Earth from the sun allows one to conclude that in the absence of greenhouse gases the mean surface temperature of the Earth would be around -20C. The presence of greenhouse gases is therefore essential for life on Earth; if there were no greenhouse effect then the Earth would be a giant ball of ice.

Humans are now emitting large amounts of CO2 into the atmosphere, causing the concentration to rise to ~380 ppm. This is a significant increase above historic levels. If 280 ppm were enough CO2 to increase the temperature of the Earth by 40 degrees Celcius, then how much of an increase do we expect from 380 ppm? I would have naively expected temperatures to rise much more than they actually have, which means that the global climate system has a remarkable amount of negative feedback. The question is, how long will this negative feedback continue to operate? Estimates of future CO2 concentrations vary depending upon when the world begins to take the threat seriously, but even relatively optimistic scenarios estimate that CO2 concentrations will not plateau until approximately 750 ppm. This would correspond to an almost tripling of CO2 concentrations. I have no idea what the effect of this would be on global climate. The scary thing is that nobody else does either.


At 6:48 AM, Blogger Arjun said...

Hmm... well, I think this is a bit misleading on a couple of levels. First off, CO2 is not the primary greenhouse gas (water vapor is), so the argument that our current level of CO2 is responsible for the entire 32 degree C increase (not 40, current mean is more like 12C) is incorrect. I forget the exact numbers, but I believe IPCC says that the percentage of the temperature increase due to CO2 is something like 10-30% in their models, although I would take even that with a grain of salt.

To gauge the effects of CO2, one might consider the geological record. Geologically (last 600 million years or so), the earth's temperature has varied from around 12C (now) to around 22C. During that time, the CO2 levels have varied from very low 380ppm that we have today up to an incredible 7000ppm (no typo) during the Cambrian era. The primary explanation for the variability in temperatures on that time scale is not so much CO2 as whether or not there is a large landmass at the poles, resulting in what looks like some sort of temperature "multistability" during the geological time record. Even much of the recent temperature increase (last 100 years) can be explained by increased solar activity--the part of that work that is controversial is the last 30 years or so:

This is not to say, of course, that CO2 has no effect on the temperature of our current world at the time scale relevant to us as a species, and it is almost certain that our current CO2 output is having some effect, climate related or otherwise. My main point is that the climate is anything but simple, and the case for CO2 increasing temperature is not quite as clear as the simple greenhouse gas scenario implies.


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