According to NASA, the first six months of 2010 were officially the hottest half-year on record—and we're now on track to witness the hottest year on record (although that will largely depend on La Niña conditions later this year). A few of the usual bullet points on how this relates to global warming:
1. One hot year doesn't, on its own, prove that humans are warming the planet any more than one cold year disproves it. That said, there's a clear upward trend here, and reams of evidence that the planet is heating up. It's not just the thermometer record, either—as a recent EPA report noted, there are dozens of indicators, from the changing length of the growing season to shifting species habitats. Note also that Arctic sea ice extent just hit its lowest level ever for June.
2. You can't blame the heat on the sun—as a recent NASA paper noted, "the recent minimum of solar irradiance is having its maximum cooling effect." In other words, it's really hot despite the sun being (relatively) weak right now. The best theory to explain the heat is still, as always, that man-made greenhouse-gas emissions are playing a large part in trapping heat in the atmosphere and warming the planet. (If you want a really technical rundown of the greenhouse effect, see this recent post by Rasmus Benestad.)
3. It looks like George Will may have to retire his (always silly) talking point that the globe's been cooling since 1998. It's getting harder and harder to make that trendline fit.
4. What can we expect going forward? Here's one clue: A recent study by Stanford climate modelers suggested that extreme heat waves—similar to what the Northeast endured last week—will become increasingly frequent in the United States over the next 30 years. That's bad news for agriculture (not to mention people who hate sweating). But what caught my eye about the Stanford study is that even if we do take drastic action to cut emissions, we've already warmed up the planet enough to cause serious increases in heat waves in the near future (see below for what we can expect even if we hit that ambitious 2°C target). It's going to be hard to dodge the heat. The only real question left is how much hotter we're going to make it.