Not-So-Permafrost Could Release as Much Heat-Trapping Pollution as Deforestation

 Scientists call them positive feedback loops, but this is not the kind of positive feedback you give your children when they do well at school. Vicious cycles would be a better term, and they have the potential to make global warming even worse than currently expected. I’m referring to processes in which global warming causes changes that amplify the original warming, which feeds back on itself and causes even more warming, etc., etc., etc….

Perhaps the best know of these viscous cycles is the “ice-albedo feedback loop.” This is a fancy way of saying that global warming melts snow and ice; dark ground and open ocean absorb more solar energy than white snow and shiny ice; that extra solar energy amplifies the initial warming, completing the feedback loop. This process is relatively straightforward to predict and is already accounted for in climate models. (It also shows that the existence of a vicious cycle does not necessarily mean there will be a runaway greenhouse effect; that depends on how much amplification occurs on each loop and whether the process is self-limiting. In this case, the earth would eventually run out of ice so the process can’t continue indefinitely).

Less well understood, but increasingly worrisome, is the “permafrost carbon feedback loop.” In this process global warming causes the arctic to warm up, which is occurring about twice as fast as the average for the earth as a whole; soil that had been permanently frozen starts to thaw out; once the soil thaws the carbon-containing organic matter in the soil starts to decompose, releasing carbon dioxide and methane; these heat-trapping gases cause more warming, completing the loop. This process is generally not included in climate models.

Scientists have known about it for many years (I published a paper on biogeochemical feedbacks back in 1989), but its magnitude is hard to pin down. As explained in a well-done New York Times article over the weekend, the gases released by melting permafrost don’t emanate uniformly from the tundra. Instead they bubble up in particular places and particular times as the physics and biology interact. These variations make it difficult to know whether scientists have obtained representative samples. This also means that the amount of carbon released can’t be predicted as a simple function of temperature—it depends on how the soil shifts as the permafrost physically collapses. Finally, the fraction of the carbon released as methane versus carbon dioxide depends on whether decomposition occurs in oxygen-depleted micro-sites (resulting in methane) or in aerated locations (producing carbon dioxide). This is critical because each carbon atom released as a methane molecule produces about ten times as much warming as it would if it were released as carbon dioxide.

One thing scientists do know is that a lot of carbon is locked up in the permafrost right now—about 1700 billion metric tons, which is about twice as much carbon as is currently in the atmosphere and about three times as much as is contained in all the world’s forests. This estimate is more than three times as much as previously thought, primarily because it accounts for carbon stored below the top three feet of soil, the layer traditionally assumed to hold the vast majority of organic matter.

So we know that permafrost soils contain enough carbon to substantially amplify global warming caused by burning fossil fuels if it were released. The key questions are how much of that carbon will be released and what fraction of it with be released as methane rather than CO2. While scientists are working to develop better models to predict the magnitude of these releases, a recent paper published in Nature took a different approach. The authors surveyed scientists actively studying the permafrost and publishing in the scientific literature and asked them to render their best judgment about what to expect.

Their assessment is scary. These scientists expect that global warming could degrade more than half of the world’s permafrost by the end of this century. That would result in emissions of 230 to 380 billion metric tons carbon-equivalent in the form of CO2 and methane. This is more than would be emitted from deforestation if it continued at current rates throughout this century and is equivalent to about half the total carbon currently stored in forests. Even more troubling, this is equivalent to about 40 to 60 percent of the emission budget the world would have to live within to have a decent chance of keeping global warming below 2 C. And unlike emissions from power plants and factories there is no way to curtail emissions from permafrost once they get going.

While this assessment is substantially higher than estimates produced by recent model studies it is by no means a worst case scenario. It assumes that only about 3% of the carbon is released as methane; a higher fraction is certainly plausible and could amp up the warming impact substantially. Moreover this is only one of many processes that can amplify global warming. Others include reduced carbon uptake by forests and oceans, increased emissions from temperate and tropical soils, and increased solar energy absorption as forests expand into arctic areas, shading the reflective snow cover. And multiple amplifying factors aren’t just additive—they amplify each other, making the overall impact much worse.

What do we do with this information beyond worrying about what the arctic has in store for us? We can and should refine our projections, but once permafrost melts the emissions will be what they will be and there isn’t anything we can do about it. What we can do is try to avoid crossing that threshold by reducing as rapidly as possible the pollution we can control.

Or we can continue pumping heat trapping pollution into our air and hope we get lucky. Forget about $10,000. Are you willing to bet the planet?

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