In the last Loose Feathers, I linked to an article stating that warming and its effects would linger for 1,000 years even if carbon emissions stopped today. The authors of the study reported in the article described current climate changes as irreversible. This week, the always-informative RealClimate explains that irreversibility does not imply that climate change is unstoppable.
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.