What is the point of no return for the climate — the level of CO2 concentrations beyond which catastrophic outcomes are virtually unstoppable?

No one knows for sure, but my vote goes for the point at which we start to lose a substantial fraction of the tundra’s carbon to the atmosphere — substantial being 0.1 percent per year! As we saw in my last post, frozen away in the permafrost is more carbon than the atmosphere currently contains (and much of that is in the form of methane, a far more potent greenhouse gas than carbon dioxide).

What is the point of no return for the tundra? A major 2005 study ($ub. req’d) led by NCAR climate researcher David Lawrence found that virtually the entire top 11 feet of permafrost around the globe could disappear by the end of this century.

Using the first “fully interactive climate system model” applied to study permafrost, the researchers found that if we tried to stabilize CO2 concentrations in the air at 550 ppm, permafrost would plummet from over 4 million square miles today to 1.5 million. If concentrations hit 690 ppm, permafrost would shrink to just 800,000 square miles.

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While these projections were done with one of the world’s most sophisticated climate system models, the calculations do not include the feedback effect of the released carbon from the permafrost. That is to say, the CO2 concentrations in the model rise only as a result of direct emissions from humans, with no extra emissions counted from soils or tundra. Thus, they are conservative numbers — or overestimates — of how much CO2 concentrations have to rise to trigger irreversible melting.

In short, those would-be points of atmospheric stabilization, 550 ppm or 690 ppm, aren’t stable at all — they are past the point of no return. We must stay well below 450 ppm to save the tundra and hence the climate.

Significantly, none of the major climate models — including NCAR’s (!) — included this crucial tundra feedback in their forecast of future concentrations atmospheric impacts for the IPCC. Thus, the 2007 Fourth Assessment Report by the U.N.’s Intergovernmental Panel on Climate Change underestimates greenhouse gas forcings and climate change this century — an especially worrisome situation given that the 2007 IPCC report was already incredibly dire.

Yet the IPCC report says that to stabilize below 450 ppm, the world must average under 5 billion tons of carbon emissions a year for the whole century. Annual carbon emissions are currently over 8 billion tons and rising 3 percent per year. We need to cut that to 4 billion by 2050 and below 1 by 2100.

And remember the tundra has some 1,000 billion metric tons of carbon. In the future, losing a mere 0.2 percent per year of the tundra (in the form of CO2) would add two billion tons a year to our carbon emissions, yet that rate would still leave us with over 80 percent of the tundra by 2100, so it is not an especially fast loss rate compared to what we may see at 550 ppm or higher. And, of course, the greenhouse gas impact would be far greater if much of that carbon were released as methane.

The point is that once even a small fraction of the tundra begins to defrost, it makes efforts to stabilize anywhere near 450 ppm almost impossible. But again, should we get to 550 ppm or above for any length of time, then permafrost emissions (and other amplifying feedbacks) are likely to take us to 700 to 1,000 ppm and beyond, which is the end of life on this planet as homo sapiens have come to know it.

So, the only prudent option is to stay below 450 ppm, which is eminently doable from a technological and economic, though not (yet) political, perspective.

My next post will explore some recent research on destructive feedbacks that are internal to the tundra ecosystem.

This post was created for ClimateProgress.org, a project of the Center for American Progress Action Fund.