Well, it's official; the average global temperature for 2008 was 14.44° C (57.99° F.) That's 0.13° C (0.23° F) cooler than 2007's 14.57° (58.23° F.) In fact, according to NASA's Goddard Institute for Space Studies (GISS), 2008 was the coolest year since 2000. However, it was also the 9th warmest year since at least 1880, and warmer than any year on record prior to the mid-1990s. The average temperature for the current decade is running 0.2° C warmer than the decade of the 1990s, and 0.3° C warmer than the 1980s. Being as objective as possible, I have a hard time interpreting last year's slight dip as providing much support for the notions of "global cooling" that I began noticing on the Internet a year ago. That's a pity, considering last week's sobering report from the National Oceanic and Atmospheric Administration (NOAA) on the likely duration of the effects of global warming on the climate, even after we eventually get greenhouse gas emissions under control.
As I noted in my posting on the subject last February, the global cooling hypothesis rests not just on the observed slight decline of average temperatures since the peak year of 2005--including a sharp monthly drop last January, compared to January 2007--but also on concerns about tardy sunspots and the recent decline in solar output. NASA's graph of "solar irradiance", the energy in sunlight reaching the earth's orbit, confirms that the sun's activity is at a periodic low point within the approximately 11-year sunspot cycle. It also indicates that this cyclical low is somewhat lower than recent lows, and that it seems a bit overdue for a cyclical uptick. (To put this in perspective, we are currently receiving about 0.02% less solar energy than average.) However, unless the scientists at a recent conference on solar activity were wrong in concluding that were are not headed into a sustained solar minimum of the kind associated with the Little Ice Age, then the implications of this graph and of GISS's commentary on solar irradiance look ominous in the other direction: We could be in for some sharply warmer temperatures in the next decade, a few years after sunspot cycle #24 reaches its peak, when the "non-negligible effect on global temperature" of variability in solar irradiance would reinforce, rather than partially canceling out the effects of greenhouse warming and the Southern Oscillation (El Nino/La Nina.) As a result, my money is still on warming, not cooling.
That makes the NOAA findings worrisome, even if one doesn't expect to be around to see whether their assessment that the adverse consequences of global warming could persist for as long as 1,000 years after our emissions have completely ceased proves correct. On a more personally-relevant timescale, however, it suggests that we shouldn't expect the climate to return smoothly to the previous "normal" after we stop nudging it, whenever that might be. That kind of systemic irreversibility is distinct from the idea that plant and animal species that become extinct along the way won't be retrievable; it speaks to the basic hospitableness of the earth to the levels of human population we're asking it to carry. (If you think that sounds extreme, you should read James Lovelock's recent thoughts on the subject.)
If the aim of policy makers is to create a sufficiently robust public consensus to support a cap on carbon emissions and a big investment in low-carbon energy over the span of time necessary for them to have the desired results, then their explanation of the problem must encompass the variability inherent in the interaction between greenhouse gas emissions and the complex cycles and systems that governed the climate long before the first factory began burning coal, or the first Model T was built. "Global warming" implies a steadier process than we are likely to experience. Some years will be cooler than others, as we've just seen, but the decade-by-decade trend still points higher. Our growing understanding of the consequences of that ought to make this harder to shrug off, even if we assign it a probability lower than 100%. I don't know if we'll end up burying biomass-derived charcoal as Mr. Lovelock suggests, in order to suck CO2 out of the atmosphere, but the longer we delay action, the more dramatic the options we may be forced to consider.