GISS was already saying that 2010 would likely be the warmest year on record a while ago due partly to the current El Niño (see, for example, section 6 in the Current GISS Global Surface Temperature Analysis from January) and observation data are now bearing this out. NOAA's NCDC just released their April State of the Climate report, and not only was April 2010 the warmest April on record, but 2010's Jan-Apr four month span was the warmest on record as well. A gridded temperature anomaly map is at right.
There's a couple of things about this that are interesting. The first, which relates to some of my own work looking at how people internalize new info about climate change, is that record events seem to be one of the most straightforward ways of conveying to people that the climate is changing. "Climate" is, after all, the general distribution of various properties of the ocean and atmosphere over time, not any specific value at any specific time. Difficulty with the notion that climate change is a shift in this distribution is what often seems to underlie opposition to the theory of anthropogenic climate change; this is what gives rise to, say, your uncle's comment that a cold winter day proves that Al Gore is a liar.
Records, however, seem to be one of the ways of talking about distributions that people intuitively understand. Everyone understands that Usain Bolt is by objective measures a very fast runner cause he holds the world record for the 100m and 200m dashes. Similarly, people intuitively understand that if this is the "hottest year on record" then the climate is in a hot state. Moreover, if we get several hottest years on record in a row, then the climate is probably getting hotter.
So what? Well, given the above it's probably not as much of a surprise that studies like Yale's F&ES Project on Climate Change have been finding a downward trend in beliefs about climate change. After a slew of record hot years the planet has has a host of second-and-third hottest years since 2005, not exactly the sort of news that makes headlines, even if by historical standards the globe is still hot. Should 2010 shape up, as Hansen has repeatedly predicted, to be the hottest year on record, we'll probably see just as many news stories talking about how "global warming is back" as we have recent stories about the "cooling trend" of the past few years.
This, in turn, makes for an interesting counterfactual political economy question: what if Copenhagen had been scheduled for this December rather than last? Not to put too-high a hope on the established mechanisms, but maybe our outcome would have been a lot more positive if delegates had arrived knowing that they had just lived through the hottest calendar year on record...