In 2008 and 2009, the world was in the grips of a brutal recession and there wasn't much to be cheerful about. There was, however, one tiny consolation: Greenhouse gas emissions were plummeting, thanks to reduced energy use, and it looked like all those worries about global warming could be put on hold for a bit — or at least temporarily postponed. But the reprieve turned out to be more temporary than expected. The International Energy Agency has just released new data showing that carbon dioxide emissions shot way back up in 2010, thanks to rapid growth in developing countries. Since this is Ezra’s blog, here’s the obligatory graph:

I got that chart from John Cook's indispensable Skeptical Science site, which is a great depot for scientific arguments about climate change (if you think you’ve come up with a novel reason why global warming is all hokum, search Cook's site first). He's lined up the observed growth in CO2 emissions against the various scenarios laid out by the Intergovernmental Panel on Climate Change. From 2003 to 2008, we were exceeding even the IPCC's worst-case scenarios. Then we dropped down to safer territory. Now we're going back up. The takeaway message is that we're on pace for about 4°C of warming by 2100.

Now, whenever you see a headline that says, “Scientists predict X degrees of warming by the year 2100,” that's basically a function of two things. The first variable is climate sensitivity — how much the world will warm in response to changes in radiative forcing (such as stronger solar activity or increased carbon-dioxide in the atmosphere, which traps heat). The second variable is how much extra carbon pollution we'll have wafted up into the atmosphere by the time 2100 rolls around.

Unfortunately, climate sensitivity is a function of basic physics and we don't have a lot of wiggle room there. The rough consensus among climatologists is that every doubling of CO2 will warm the Earth around 3°C. Yes, that figure has been largely derived from computer modeling, but there are good external reasons to believe those models. For one, scientists can check the models against natural experiments — looking, for instance, at how the Earth's temperature responds to things like the massive Pinatubo eruption in 1991. And paleoclimatologists can look at how the Earth's climate has reacted in the past to changes in carbon-dioxide (in the past, those changes were often due to shifts in the Earth's orbit rather than coal-fired power plants, but the effect on Earth's average temperature was still measurable). All of this research has tended to converge around one answer — double CO2 in the air, get about 3°C of warming.

(Fun tangent: I'm sure most people are vaguely familiar with the brouhaha over Michael Mann's famous hockey stick graph showing that the Earth is heating up at a faster rate than at any time in the past 1,000 years, right? The climate-skeptic argument is that Mann — and all the other scientists who have reproduced his work — downplayed the Medieval Warm Period. That's doubtful. But if the skeptics did happen to be right, and it was warmer in the medieval era than Mann thinks, that would actually be more worrisome, not less — it would mean the Earth is actually more sensitive to changes in radiative forcing than climatologists have suspected.)

Anyway, that leaves the second variable. It's a lot harder to predict how quickly greenhouse gas emissions will grow between now and 2100. Energy policies could change. New technologies could develop. Solar panels could become so fantastically cheap that coal plants become obsolete. It's extremely tricky for the IPCC to predict these things, which is why the panel laid out so many different emissions scenarios. But right now, we're on pace to hit the IPCC's A2 scenario, which would probably warm the planet roughly 4°C over pre-industrial levels.

And, what a coincidence, not too long ago there was a special January 2011 issue of the “Philosophical Transactions of the Royal Society,” a scientific journal, laying out just what a 4°C warmer world would look like. Here's a sample paper, about the implications for agriculture: “[T]he kind of changes that would occur in a 4°C+ world would be way beyond anything experienced in recent times. There are many options that could be effective in helping farmers adapt even to medium levels of warming ... but it is not difficult to envisage a situation where the adaptive capacity and resilience of hundreds of millions of people in [sub-Saharan Africa] could simply be overwhelmed by events." Good times.

Brad Plumer is an associate editor at the New Republic.