Hit the brakes hard

There is a climate splash in Nature this week, including a cover showing a tera-tonne weight, presumably meant to be made of carbon (could it be graphite?), dangling by a thread over the planet, and containing two new articles (Allen et al and Meinshausen et al), a “News & Views” piece written by two of us, and a couple commentaries urging us to “prepare to adapt to at least 4° C” and to think about what the worst case scenario (at 1000 ppm CO2) might look like.

At the heart of it are the two papers which calculate the odds of exceeding a predefined threshold of 2°C as a function of CO2 emissions. Both find that the most directly relevant quantity is the total amount of CO2 ultimately released, rather than a target atmospheric CO2 concentration or emission rate. This is an extremely useful result, giving us a clear statement of how our policy goals should be framed. We have a total emission quota; if we keep going now, we will have to cut back more quickly later.

There is uncertainty in the climate sensitivity of the Earth and in the response of the carbon cycle, and the papers are extremely useful in the way that they propagate these uncertainties to the probabilities of different amounts of warming. Just looking at the median model results, many people conclude that a moderately optimistic but not terribly aggressive scenario such as IPCC B1 would avoid 2°C warming relative to pre-industrial. But when you take into account the uncertainty, you find that there is a disturbingly high likelihood (roughly even odds) that it won’t.

Schmidt and Archer N and V figureBoth papers come to the same broad conclusion, summarized in our figure, that unless humankind puts on the brakes very quickly and aggressively (i.e. global reductions of 80% by 2050), we face a high probability of driving climate beyond a 2°C threshold taken by both studies as a “danger limit”. Comparing the two papers is obscured by the different units; mass of carbon versus mass of CO2 (moles, anyone? Is there a chemist in the house?). But chugging through the math, we find the papers to be broadly consistent. Both papers conclude that humankind is already about half-way toward releasing enough carbon to probably reach 2°C, and that most of the fossil fuel carbon (the coal, in particular) will have to remain in the ground.

We feel compelled to note that even a “moderate” warming of 2°C stands a strong chance of provoking drought and storm responses that could challenge civilized society, leading potentially to the conflict and suffering that go with failed states and mass migrations. Global warming of 2°C would leave the Earth warmer than it has been in millions of years, a disruption of climate conditions that have been stable for longer than the history of human agriculture. Given the drought that already afflicts Australia, the crumbling of the sea ice in the Arctic, and the increasing storm damage after only 0.8°C of warming so far, calling 2°C a danger limit seems to us pretty cavalier.

Also, there are dangers to CO2 emission other than the peak, such as the long tail of the CO2 perturbation which will dominate the ultimate sea level response, and the acidification of the ocean. A building may be safe from earthquakes but if it is susceptible to fires it is still considered unsafe.

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