Melting Permafrost Will Boost Temps, But Not Quickly
Climate scientists have long known that human-generated greenhouse emissions are only part of the story with global warming. A rising planetary temperature sets in motion all sorts of secondary effects that can boost the temperature even higher — effects like melting Arctic sea ice, rising levels of heat-trapping water vapor in the atmosphere, and more. When researches try to figure out how high the thermometer will go over the next century or two, it’s important to figure in these so-called feedbacks into their calculations. But they don’t fully understand how feedbacks will play out.
A recent study in Nature Geoscience is helping reduce some of that uncertainty. By 2100, say Andrew MacDougall and his co-authors, the melting of Arctic permafrost will release enough carbon dioxide to boost global temperatures by up to an extra half a degree Fahrenheit, and by 2300, the extra heating could add up to as much as 3°F.
So that’s not good, obviously, especially since the permafrost feedback is just one of many that will add to global warming. But two really intriguing things about that become clear when you look more closely. The first is that the half-degree by 2100 is likely to happen whether or not we slow our emissions from fossil-fuel burning or not.
If that sounds crazy, the second seems even crazier: when you go beyond 2100, the extra warming from permafrost is less if our greenhouse emissions are higher.
Both of these facts sound as absurd to me as they probably do to you — but it makes sense when you understand a couple of basic scientific facts.
Fact No. 1: carbon emissions from melting permafrost take a while to get going. First you have to add CO2 to the atmosphere by burning coal or oil. The CO2 starts trapping heat right away, but the heat still takes time to build.
Then the permafrost has to melt, and as MacDougall said in an interview, “the thawing takes several years, because it takes a lot of energy to melt frozen soil. It takes a long time for the temperature signal to penetrate into the ground.”
It’s only after the soil thaws that the organic matter trapped within it can begin to decay and release its load of CO2 — and the decay proceeds slowly as well. “The temperature is still hovering just above freezing,” MacDougall said. It’s similar to the way a head of lettuce rots a lot more quickly out on the kitchen counter than inside the refrigerator.
By the time you add all these delays, it can take decades for permafrost-related warming to kick in. That’s why emissions don’t affect the permafrost feedback all that much for most of this century. By the 2100’s, however, it’s definitely kicking in.
Now comes the really counter-intuitive part: the more CO2 that’s already in the atmosphere from other sources (that is, from human emissions), the less impact the extra CO2 from permafrost will have, thanks to the physics of the greenhouse effect.
“It’s a logarithmic forcing,” MacDougall said, which in plain English means that whatever temperature increase you get from, say doubling the amount of CO2 in the atmosphere, you’ll get the same increase if you double it again.
So for example, doubling of CO2 from its pre-industrial level of about 270 parts per million to 540 parts (we’re now at about 393) should raise temperatures by somewhere between 3.2°F and 7.2°F. But you’d have to double it again to 1080 to get another 3.2°F - 7.2°F. And you’d have to double it to 2160 to get yet another.
So if we fail to cut back emissions and there’s a lot more CO2 in the air by 2100, the extra emissions from permafrost will be less, percentagewise, than they would if we do curb emissions. And their warming effect will be less.
That’s hardly a reason to abandon efforts to reduce greenhouse gas emissions, since the overall heating would be far more without such reductions, even if the contribution from permafrost would be less. It’s kind of like continuing to smoke because it helps you lose weight, even though the health risks from smoking are vastly worse than the risks from chubbiness.
But it is a reason to keep studying the climate. Scientists don’t have all the answers yet, and sometimes those answers can be surprising.