Irreversible Does Not Mean Unstoppable

Susan Solomon, ozone hole luminary and chair of the Nobel Prize winning IPCC, and her colleagues, have just published a paper entitled “Irreversible climate change because of carbon dioxide emissions” in the Proceedings of the National Academy of Sciences. We at realclimate have been getting a lot of calls from journalists about this paper, and some of them seem to have gone all doomsday on us. Dennis Avery and Fred Singer used the word Unstoppable as a battle flag a few years ago, over the argument that the observed warming is natural and therefore there is nothing that humanity can do to alter its course. So in terms of its intended rhetorical association, Unstoppable = Burn Baby Burn. But let’s not confuse Irreversible with Unstoppable. One means no turning back, while the other means no slowing down. They are very different words. Despair not!

Solomon et al point out that continued, unabated CO2 emissions to the atmosphere would have climatic consequences that would persist for a thousand years, which they define operationally as “forever”, as in the sense of “Irreversible”. It is not really news scientifically that atmospheric CO2 concentration stays higher than natural for thousands of years after emission of new CO2 to the carbon cycle from fossil fuels. The atmospheric CO2 concentration has a sharp peak toward the end of the fossil fuel era, then after humankind has gone carbon neutral (imagine!) the CO2 concentration starts to subside, quickly at first but after a few centuries settling in a “long tail” which persists for hundreds of thousands of years.

The long tail was first predicted by a carbon cycle model in 1992 by Walker and Kasting. My very first post on realclimate was called “How long will global warming last?”, all about the long tail. Here’s a review paper from Climatic Change of carbon cycle models in the literature, which all show the long tail. A number of us “long tailers” got together (electronically) to do a Long Tail Model Intercomparison Project, LTMIP, just like the big guys PMIP and OCMIP (preliminary results of LTMIP to be appearing soon in Annual Reviews of Earth and Planetary Sciences). I even wrote you guys a book on the topic.

The actual carbon-containing molecules from the fossil fuel spread out into the other carbon reservoirs in the fast parts of the carbon cycle, dissolving in the oceans and getting snapped up by photosynthetic land plants. The spreading of the carbon is analogous to water poured into one part of a lake, it quickly spreads out into the rest of the lake, rather than remaining in a pile where you poured it, and the lake level rises a bit everywhere. In the carbon cycle, translated out of this tortured analogy, the atmospheric carbon dioxide content rises along with the contents of the other carbon reservoirs.

Ultimately the airborne fraction of a CO2 release is determined largely by the buffer chemistry of the ocean, and you can get a pretty good answer with a simple calculation based on a well-mixed ocean, ignoring all the complicated stuff like temperature differences, circulation, and biology. The ocean decides that the airborne fraction of a CO2 release, after it spreads out into the other fast parts of the carbon cycle, will be in the neighborhood of 10-30%. The only long-term way to accelerate the CO2 drawdown in the long tail would be to actively remove CO2 from the air, which I personally believe will ultimately be necessary. But the buffering effect of the ocean would work against us here, releasing CO2 to compensate for our efforts.

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