NASA’s James Hansen has weighed in (PDF) to …
… expose the recent nonsense that has appeared in the blogosphere, to the effect that recent cooling has wiped out global warming of the past century, and the Earth may be headed into an ice age. On the contrary, these misleaders have foolishly (or devilishly) fixated on a natural fluctuation that will soon disappear.
As Hansen explains:
Weather fluctuations or ‘noise’ have a noticeable effect even on monthly-mean global-mean temperature, especially in Northern Hemisphere winter. Weather has little effect on global-mean temperature averaged over several months or more. The primary cause of variations on time scales from a few months to a few years is ocean dynamics, especially the Southern Oscillation (El Nino — La Nina cycle), although an occasional large volcano can have a cooling effect that lasts a few years. The 10-11 year cycle of solar irradiance has a just barely detectable effect on global temperature, no more than about 0.1°C, much less noticeable than El Nino/La Nina fluctuations.
So what happened this winter?
The past year (2007) witnessed a transition from a weak El Nino to a strong La Nina (the latter is perhaps beginning to moderate already, as the ocean waters near Peru are beginning to warm). January 2007 was the warmest January in the period of instrumental data in the GISS analysis, while, as shown in Figure 1, October 2007 was # 5 warmest, November 2007 was #8 warmest, December 2007 was #8 warmest, and January 2008 was #40 warmest. Undoubtedly, the cooling trend through the year was due to the strengthening La Nina, and the unusual coolness in January was aided by a winter weather fluctuation.
Small long-term temperature changes have vastly more consequence than large short-term temperature changes:
The large short-term temperature fluctuations have no bearing on the global warming matter or the impacts of global warming … A global warming much smaller than weather fluctuations has the potential for dramatic effects, e.g., by setting in motion future large sea level change, species extinction, and various other impacts.
His final point goes to the heart of a common misconception among many people:
Cold weather does raise an interesting point, though. People who do not like cold weather, and might have welcomed the idea that Minnesota may become more like Missouri or Massachusetts like Virginia, must give up that notion, unless they wish ill for a large fraction of the planet’s inhabitants, both human and other creatures. We are going to have to figure out a way to keep climate zones pretty much where they are now (winters will continue to happen, as always). It is possible that we can still do that — just barely. But I digress — that will be in our next paper, almost finished.
I’ll post that as soon as he does.