I’ve argued that scientists are not overestimating climate change, and in fact are underestimating it because they are omitting crucial amplifying feedbacks from their models. In this post, I’ll show how these omissions suggest the climate has a “point of no return” that severely constrains the safe level of human-generated emissions.

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 somehow 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, which has locked in it more carbon than the atmosphere (and much of that is in the form of methane, a potent greenhouse gas).

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.

How do carbon cycle feedbacks constrain future safe levels of CO2 emissions? There’s really only one major climate model that can answer that crucial question.

The United Kingdom’s Hadley Centre for Climate Prediction and Research has one of the few climate models that incorporates a significant number of carbon cycle feedbacks, particularly in soils and tropical forests. In a 2003 study, ($ub. req’d), they found that a typical fossil-fuel emissions scenario for this century, which would have led to carbon dioxide concentrations in 2100 of about 700 ppm without feedbacks, led instead to concentrations of 980 ppm with feedbacks — a huge increase. Even ignoring feedbacks, keeping concentrations below 700 ppm requires the United States and the world to start slowing carbon dioxide emissions from coal, oil, and natural gas significantly by 2015 and to stop the growth almost entirely after 2025.

In 2006, the Hadley Centre, working with other British researchers, published an important study, “Impact of Climate-Carbon Cycle Feedbacks on Emissions Scenarios to Achieve Stabilisation,” that included both ocean and terrestrial carbon cycle feedbacks (though they do not specifically model carbon emissions from defrosting tundra). The study found that such feedbacks reduce the amount of fossil fuel emissions we can release by 21 percent to 33 percent.

We have no room for error. Buried in the IPCC’s Fourth Assessment Report (Working Group 1) is a stunning paragraph, which shows that the consensus is finally shifting on this issue:

Climate-carbon cycle coupling is expected to add carbon dioxide to the atmosphere as the climate system warms, but the magnitude of this feedback is uncertain. This increases the uncertainty in the trajectory of carbon dioxide emissions required to achieve a particular stabilization level of atmospheric carbon dioxide concentration. Based on current understanding of climate carbon cycle feedbacks, model studies suggest that to stabilize at 450 ppm carbon dioxide, code required the cumulative emissions over the 21st-century be reduced from an average of approximately 670 gigatons carbon to approximately 490 GtC.

Ouch. We need to average 5 billion tons of carbon this century to avoid catastrophic warming. We’re already at 8 billion and rising fast.

There thus appears to be a threshold beyond which it becomes more and more difficult for us to fight the feedbacks of the carbon cycle with strong energy policies that reduce fossil-fuel emissions into the air. While the threshold is not known today precisely, it appears to be somewhere between 450 ppm and 650 ppm (and probably between 450 and 550). If we cross that point of no return, we’ll probably shoot to 1000 ppm, and maybe much more — ruining this planet for centuries, if not millennia.

By 2025, we’ll know much better where the point of no return is. Unfortunately, on our current path, the world’s emissions and concentrations will be so high by 2025 that the “easy” technology-based strategy will not be able to stop us from crossing the very high end of the threshold range

That’s why, in my book, I call the 2025-2050 period Planetary Purgatory. Barring a major reversal in U.S. and world policies in the very next decade, come the 2020s, most everyone will know the grim fate that awaits the next 50 generations. But the only plausible way to avoid it will be a desperate effort to cut global emissions by 75 percent in under three decades — a massive, sustained government intervention into every aspect of our lives on a scale that far surpasses what this country did during World War II.

That would indeed be punishment for our sins of inaction. It would also be a great irony if conservative Deniers — who are blocking serious mitigation today because they don’t like (a certain kind of) government intervention in our lives — ended up forcing the country into far more government intervention in the near future.

Failing that desperate effort, we would end up at mid-century with carbon emissions far above current levels, and concentrations at 500 ppm, rising 3 to 4 ppm a year — or even faster if the vicious cycles of the climate system have kicked in. That would propel us to the point of no return in the third quarter of this century.

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