THE VINE JANUARY 23, 2010
So it looks like 2009 has been declared the second-hottest year on record, according to NASA's Goddard Institute for Space Studies. And the 2000s, on the whole, are officially the warmest decade on record—warmer than the 1990s, which in turn were warmer than the 1980s, which in turn beat out the 1970s. For the past 30 years, the Earth's been heating up by about 0.2°C per decade:
At the GISS site, there's some helpful commentary on why long-term trends matter far more than year-to-year fluctuations. There are quite a few factors that can produce short-term blips in temperature: changes in solar activity, oscillations in sea-surface temps (such as El Niño or La Niña events), or changes in aerosol pollution (particles in the air that can reflect or absorb sunlight). But, over a long enough period of time, that "noise" gets drowned out by the most dominant forcing—namely, an increase in heat-trapping greenhouse gases, which are steadily pushing overall temperatures upward.
Here's one example: The 2000s were the warmest decade on record despite the fact that there's been a "deep solar minimum" for the past few years—that is, a lull in sunspot activity, which reduces the amount of energy that the sun gives off by as much as 0.1 percent. Studies have estimated that these variations in solar activity can cool or warm the Earth by as much as 0.1°C. That can make a big difference for any one year, but over time, that effect gets swamped by all the CO2 and other greenhouse gases we're putting into the air.