A Difficult Few Years Ahead for AGW?

I was not going to do much climate change science on this blog, but it has not been lost on me that my recent posting about a climate skeptic provoked more controversy than any other posting. So, I decided to delve a little more into some of the skeptics’ claims. I am not talking about the more mainstream skeptics who accept the science but believe there are better ways to spend our limited resources than on combating climate change, which is a defensible position. Rather I am talking about those who dispute that Anthropogenic Global Warming (AGW) is a reality.

The typical claim here is that global warming stopped in 1998, and more recently that January 2008 was actually colder than January 2007. I decided to try to get to the truth behind the truthiness. I think the first thing to point out is that while it is accepted by almost all scientists that greenhouse gases tend to increase the mean temperature of the earth, nobody has suggested that this is the only factor affecting the earth’s temperature. One well-known additional factor is the presence of CFCs and SO2 in the atmosphere, both of which tend to reflect some of the incoming radiation and make the earth cooler, and this tended to counter the effects of greenhouse gases until regulation reduced emissions of these gases in the 1990’s. (Indeed it has been suggested that we may need to create a similar effect deliberately if we fail to act soon enough on greenhouse gases.)

Two other factors are sun spots, and the “Southern Oscillation” between El Nino and La Nina ocean currents and winds. The latter may not seem relevant to the mean temperature of the earth, but it is; as well as I can understand it, La Nina forces more of the heat into the lower depths of the ocean and thus makes the surface temperature (which is what we measure) cooler than it would otherwise be.

The effect of sun spots is also somewhat unintuitive; since they are dark, one might expect then to reduce the irradiance of the sun, but in fact they do the opposite: more sun spots means more radiation from the sun. Sun spot activity is roughly cyclic, increasing for about 4 years and then declining for about 7, though it is not entirely predictable. Since about 2001 we have been in the declining phase of “cycle 23” and we are now just about at the bottom of the cycle. There has been some concern that the start of cycle 24 might be delayed. (But see http://science.nasa.gov/headlines/y2007/14dec_excitement.htm for the possible first evidence of cycle 24.)

Even if cycle 24 starts on schedule, there will be a couple of years of lower temperatures (than might otherwise pertain) because the earth’s temperature lags the sunspot activity by about 2 years. In addition, we are at the beginning of a La Nina period, which also tends to cool the earth’s surface. What we have is an upward trend due to greenhouse gases superimposed on two somewhat regular cycles both of which are approaching their low points, plus of course quite a bit of noise. The net result is that we probably will not see exceptional temperatures in the next year or so; the skeptics will have a field day and the faithful will be tested. (A good summary of where we are can be found at http://data.giss.nasa.gov/gistemp/2007.) We can be fairly sure however that by 2012 at the latest we will see new records broken. We must all hope that we don’t have to wait that long for more aggressive action.