How we know we’re warming the planet
Plenty of media outlets picked up the news last week that a team of skeptical Berkeley scientists tore through the 20th century temperature data and found… well, they found that the Earth has been warming significantly in the past century, just like NASA and the Climate Research Unit have been telling us all along. My colleague Eugene Robinson called it “the scientific finding that settles the climate-change debate.”
In response, some climate skeptics are now claiming that they never doubted the Earth was warming, and in any case, the temperature record is only a small part of the story. Just because the Earth is heating up doesn’t mean that humans are to blame. And even if we are the cause, there’s no telling how bad it will get. Well, fair enough. So what do we know about these second two parts? You can find the detailed scientific explanation in the IPCC’s 2007 report (or a million other places on the Internet), but here’s my stab at a quick and simple way to think about this.
Broadly speaking, here are two reasons why the Earth could be warming up. Either more heat is reaching the Earth’s surface, or else less heat is escaping out into space. On the first, there’s no evidence of a significant increase in heat reaching the earth. True, solar activity can shift from year to year. But satellite data shows that total solar irradiance has declined slightly in the past 30 years, even as the planet continues to warm. Scratch that theory.
So something’s keeping the heat in. Physicists have long known from lab experiments that greenhouse gases like carbon-dioxide can absorb certain frequencies of infrared radiation and scatter them back toward the Earth. We also know these gases are increasing in the atmosphere, largely due to the burning of fossil fuels (checking this involves some fairly straightforward chemistry). And, indeed, satellite data has shown that less and less infrared in the specific frequencies in question is escaping out into space, while more is bouncing back to the Earth’s surface. There's your culprit.
Now, it’s not entirely that simple (see here for the IPCC’s graphic of other, smaller factors influencing the climate), but that’s the basic story. You can see this report for a long list of “fingerprints” that point to a human influence on the Earth’s climate. Disproving all this would involve coming up with some alternate explanation for why the Earth is heating up — and possibly overturning some basic physics in the process. To date, no one’s done this. One popular alternate theory among skeptics is that cosmic rays can somehow influence cloud formation on Earth, but this isn’t even close to proven.
The next, trickier part is figuring out how hot the Earth will get if we keep pumping greenhouse gases into the air. Fortunately there are quite a few ways to measure climate sensitivity. That includes computer models, yes, but also observations from the past 150 years, measurements of the effects of volcanic eruptions, as well as a look at what happened thousands or even millions of years ago, when atmospheric CO2 levels were much higher due to natural causes. As this handy chart from New Scientist shows, most of these studies have been converging around the same range of answers: Doubling the amount of CO2 in the atmosphere will likely heat the Earth by about 3 degrees C, though it’s possible that the end result could be worse, depending on various feedback mechanisms:
In any case, that’s the rough sketch of the situation we’re facing. Technically, the skeptics are right to say that the Berkeley temperature study didn’t, on its own, “prove” man-made global warming. But they have a long, long way to go before knocking down the rest of the case.