In terms of the global warming debate, I have generally concentrated on the economics and accepted the consensus of anthropomorphic global warming as a given. The earth is warming, the level of co2 is going up (because of humans), and these things are linked. True, true and true. But recently I’ve been researching the science and the conseqences of these facts aren’t as clear as the global warming fear-mongers would have you believe.
At the heart of the global warming scare is the often repeated message that a doubling of co2 will lead to a temperature increase of 3 (+/- 1.5) degrees. This is possible. It is also possible that a doubling of co2 (from pre-industrial levels) will lead to a temperature increase of 0.4 degrees. Given that temperatures increased 0.6 degrees in the 20th century that implies no further warming. Let me explain.
Doubling co2 (from pre-industrial levels) would give us atmospheric co2 of 560 (2 * 280) ppmv. Using the formula (dF = alpha ln (C/Co) where doubling means C/Co = 2 and alpha is 5.35.) we get a dF (change in “forcing”) of 3.7 WM^-2. No controversy there.
Change in forcing is translated into changed temperatures by multiplying it by lambda. But what is lambda?
As mainstream climate scientist James Hansen explains, compared with today the ice age had about 6.6 (+/- 1.5) WM^-2 less forcing and 5 degrees lower temperature. This implies a lambda of about 0.8 (+/- 0.2). Using this lambda, doubling pre-industrial co2 will produce a temperature increase of about 3 degrees and a range of 2-4 degrees. Another mainstream climate scientist, James Annan roughly agrees with this range and rejects alarmist scenarios of > 5 degree warming.
The skeptical argument comes into play when you consider alternative estimates of lambda. The National Acadamies Climate Change Science Report explains that lambda would be about 0.3 without feedback effects and while noting that 0.8 is the mainstream estimate, also says that “the magnitude and even the sign of the feedback can differ according to the composition, thickness, and altitude of the clouds, and some studies have suggested a lesser climate sensitivity”. Shaopeng Huang estimates 0.4 to 0.7. Sherwood Idso conducted 8 natural experiments and found a lambda of about 0.1.
Another suggested approach is to consider the total forcing from greenhouse gases and the total warming from greenhouse gases. Kiehl and Trenberth indicate a total greenhouse forcing of 324 WM^-2 which gives us 32.5 degrees warming. This implies a lambda of 0.1.
Another approach is to consider the changes in forcing and temperatures since the industrial revolution. Roger Pielke Snr suggests about 27% of recent warming (0.6 degrees) can be explained by atmospheric co2 which implies a lambda of 0.11, but this may be an underestimate because there is a time lag between changed forcing and changed temperatures. *
Taking the above estimates a skeptic might prefer a lambda range of about 0.1 to 0.5, which implies a temperature increase of 0.4 to 1.8 from a doubling of co2 with a best guess of around 1 degree. C8to links to an article by skeptical climate scientist Richard Lindzen which indicates that we might be on track for about a 1 degree increase. This doesn’t seem too scary.
There is lots more that we still don’t fully understand about climate change. The impact of clouds, a potential for a weaker solar impact, consequences and feedback from vegetation growth, or exactly what has happened in the past. Sherwood Idso, President of co2 Science, believes that there are several cooling forces that might offset any co2 based warming.
Further, even if there is some moderate global warming, it is not clear that there will be any significant cost. There is little evidence for catastrophic consequences. Environmental economist Robert Mendelsohn explains that moderate warming will benefit countries in the polar regions and the mid-latitudes while only harming countries in the tropical and sub-tropical regions. He suggests that there will only be a net cost if temperatures increase more than 2.5 degrees when mid-latitude countries start to suffer costs.
In most IIPC scenarios, co2 will not double until late in the 21st century. The actual outcome will depend significantly on the actual level of development in the world and the rate of technological progress. I am a technology optimist and ultimately I think the global warming debate will come be seen in the same way as the food-shortage debate sparked by 19th century economist Thomas Malthus: better energy technology will make the debate irrelevant.
UPDATE 28/11/06: Following a discussion with Tim Lambert (see comments) it became clear to me that I was wrong to use the Pielke co2/total 27% estimate in the way that I did and therefore the 0.11 lambda estimate is not valid. This doesn’t impact on the conclusions of the article.