The case for global warming, which has gained much wider acceptance in the US in the last several years, rests in simplest terms on the observation of temperature trends and their extrapolation into the future. The general hypothesis about the role of greenhouse gas emissions and the other factors that nudge the global climate in one direction or another fits those observations pretty well. But what if the key temperature trend broke, or appeared to? Would that undermine the scientific consensus, and by extension the growing policy consensus to address the problem more directly, such as with a cap on emissions? A blog posting making the rounds on the Internet might put that question to the test, though in my assessment it falls well short of demonstrating that the earth is now cooling, rather than warming.
Start with two facts: First, the global temperature this January, measured by both satellite- and land-and-ocean-based instruments, was significantly colder than last January's, which had been significantly warmer than any January in at least a century. From one January to the next we have experienced a fairly dramatic cooling, on the order of 0.7 degrees C or 1.2 deg. F. As the blog cited above put it, that's almost enough to wipe out the last century's worth of warming. Next, consider that this cooling has occurred at a time when the sun's output is at a cyclical minimum, with the onset and intensity of the next sunspot cycle still in doubt. Humans are hard-wired for pattern-seeking, and it's not hard to connect these dots and start to worry that greenhouse gas emissions might not matter as much as we thought, and that we could be on the threshold of another Maunder Minimum, which coincided with the "Little Ice Age"--remember Frost Fairs on the Thames and Washington dodging ice floes while crossing the Delaware?
Given only the information above, one might logically conclude that 2007 must have been an unusually chilly year, globally. In fact, it turns out to have been the second-warmest year on record, behind 2005 and in a statistical tie with 1998. And when you examine the temperature data from NASA's Goddard Institute for Space Studies, you see that despite a 0.75 deg. C drop from one January to the next, only two months last year were 0.1 degrees or more colder than the average for that month this decade, which so far has averaged 0.3 deg. C warmer than the 1990s. The mean global temperature in 2007 was actually a full degree Fahrenheit warmer than the average for the 1950s through 1970s, the baseline period against which the "temperature anomalies" reported by NASA are calculated. If the long-term trend reflected is this data is going to be broken, it will take a lot more than one cold January--a January that incidentally was still slightly warmer than the same-month average for 1951-1980--to signal that we're in a cooling trend, or no longer warming.
The variability of the sun's output hasn't gone unnoticed. Having invested a lot of time in poring over the scientific literature on climate change, I can assure you that climate scientists are not ignoring the behavior of the largest heat source within four light years. As NASA's Dr. Hansen explains, however, even a sustained period of solar output at the current low levels would not overwhelm the warming effect from greenhouse gases, which is accelerating with increasing emissions. Another Maunder Minimum would be unlikely to produce the same results as the last one, because today's atmospheric CO2 is about 25% higher than the prevailing level, then. The earth might not warm as quickly as it has been, but it probably wouldn't start cooling off.
Climate is what we expect, weather is what we get, and both are subject to complex influences and patterns. I'm not prone to dismissing the arguments of climate skeptics out of hand, but at this point the swing from last year's unusually warm January to this year's cold one--with a warm spring and summer between them--seems better explained by the current La Niña episode than by a quiescent sun and the onset of global cooling. If the next couple dozen months turn out to be mostly cooler than the recent average, and if the sunspot cycle hasn't ramped up by then, I would have to revisit that conclusion.
By the way, Energy Outlook just received some very welcome recognition, being cited by The Times (of London) as one its "Top 50 Eco Blogs."