Here's how it works. Before the industrial revolution, the Earth's atmosphere contained about 280 parts per million of carbon dioxide. That was a good amount"good" defined as "what we were used to." Since the molecular structure of carbon dioxide traps heat near the planet's surface that would otherwise radiate back out to space, civilization grew up in a world whose thermostat was set by that number. It equated to a global average temperature of about 57 degrees Fahrenheit (about 14 degrees Celsius), which in turn equated to all the places we built our cities, all the crops we learned to grow and eat, all the water supplies we learned to depend on, even the passage of the seasons that, at higher latitudes, set our psychological calendars.
Once we started burning coal and gas and oil to power our lives, that 280 number started to rise. When we began measuring in the late 1950s, it had already reached the 315 level. Now it's at 380, and increasing by roughly two parts per million annually. That doesn't sound like very much, but it turns out that the extra heat that CO2 traps, a couple of watts per square meter of the Earth's surface, is
enough to warm the planet considerably. We've raised the temperature more than a degree Fahrenheit (0.56 degrees Celsius) already. It's impossible to precisely predict the consequences of any further increase in CO2 in the atmosphere. But the warming we've seen so far has started almost everything frozen on Earth to melting; it has changed seasons and rainfall patterns; it's set the sea to rising.
No matter what we do now, that warming will increase somethere's a lag time before the heat fully plays out in the atmosphere. That is, we can't stop global warming. Our task is less inspiring: to contain the damage, to keep things from getting out of control. And even that is not easy. For one thing, until recently there's been no clear data suggesting the point where catastrophe looms. Now we're getting a better picturethe past couple of years have seen a series of reports indicating that 450 parts per million CO2 is a threshold we'd be wise to respect. Beyond that point, scientists believe future centuries will
likely face the melting of the Greenland and West Antarctic ice sheets and a subsequent rise in sea level of giant proportion. Four hundred fifty parts per million is still a best guess (and it doesn't
include the witches' brew of other, lesser, greenhouse gases like methane and nitrous oxide). But it will serve as a target of sorts for the world to aim at. A target that's moving, fast. If concentrations keep increasing by two parts per million per year, we're only three and a half decades away.