Irreversible Does Not Mean Unstoppable

Susan Solomon, ozone hole luminary and Nobel Prize winning chair of 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.

As a result of the long tail, any climate impact from more CO₂ in the air will be essentially irreversible. Then the question is, what are the climate impacts of CO₂? It gets warmer, that’s pretty clear, and sea level rises. Sea level rise is a profound consequence of the long tail of global warming because the response in the past, over geologic time scales, is tens of meters per °C change in global mean temperature, about 100 times stronger than the IPCC forecast for 2100 (about 0.2 meters per °C). The third impact which gains immortality from the long tail is precipitation. Here the conventional story has been that climate models are not very consistent in the regional precipitation changes they predict in response to rising CO₂. Apparently this is changing with the AR4 suite of model runs, as Solomon et al demonstrated in their Figure 3. Also, there is a consistent picture of drought impact with warming in some places, for example the American Southwest, both over the past few decades and in medieval time. The specifics of a global warming drought forecast are beginning to come into focus.

Perhaps the despair we heard in our interviewers’ questions arose from the observation in the paper that the temperature will continue to rise, even if CO₂ emissions are stopped today. But you have to remember that the climate changes so far, both observed and committed to, are minor compared with the business-as-usual forecast for the end of the century. It’s further emissions we need to worry about. Climate change is like a ratchet, which we wind up by releasing CO₂. Once we turn the crank, there's no easy turning back to the natural climate. But we can still decide to stop turning the crank, and the sooner the better.

Walker JCG, Kasting JF. 1992. Effects of fuel and forest conservation on future levels of atmospheric carbon dioxide. Palaeogeogr. Palaeoclimatol. Palaeoecol. (Glob. Planet. Change Sect.) 97:151–89