Chris Field of the Carnegie Institution and Stanford, the lead author on one chapter of the latest IPCC report, spoke today at the Marschak Colloquium. The key issue is when the negative feedbacks that have mitigated the effects of increasing carbon release slow down, or even reverse, making warming a self-sustaining phenomenon. We’re closer to that point than we’d like to be.
It occurred to me as Field was speaking that there’s common territory between the global-warming denialists and the Gaia-hypothesis folks: they share a common belief that, somehow, mystically, negative feedback will always save the planet from whatever we try to do to it. [Update: A reader tells me that this is wrong: the Gaians think the mechanism of negative feedback in this case will be the extinction or near-extinction of the human race.]
Major points from the talk after the jump.
1. All the models are now coming into fairly close agreement. Claims of massive uncertainty simply aren’t justifiable anymore. Warming over the past half-century has been overwhelmingly anthropogenic (fluctuations in solar radiation, for example, explain about 5% of the change).
2. The ocean has been acting as a carbon sink; so has increased plant growth in areas with adequate soil and water. Thus the growth in atmospheric carbon dioxide concentration has been only about half as big as the growth in emissions. In effect, we’re being subsidized by various negative-feedback processes.
3. But none of those sinks is unlimited, and we’re getting to the point where positive feedback will start to replace negative feedback. For example, a little more warming will melt big chunks of permafrost, leading to a huge release of carbon from Pleistocene-era vegetation that suddenly comes out of the deep freeze and quickly rots. For another, some tropical forests, as global warming heats them and dries them up, become vulnerable to fire and transition from forest to savanna, both releasing lots of carbon at once and decreasing their capacity to act as carbon sinks in the future. (On the other hand, the risk of the Gulf Stream suddenly stopping is small.)
4. Growth in carbon emissions has been faster in the current decade than in the previous decade, mostly because economic growth has been higher.
5. No country has done an especially good job at de-coupling its carbon footprint from its GDP.
6. Carbon footprints track GDP, not population. With GDP per capita soaring, reducing population growth can’t do much to reduce the carbon footprint.
7. Given how much coal we’re going to burn, we better figure out how to sequester carbon. Nuclear would be better. Even fusion is worth a shot. Wind and solar run into storage problems, which may or may not be solvable.
8. None of that will happen without charging for carbon emissions, whether as a tax or as a cap-and-trade.
9. We need to plan to adapt to significant warming; stopping it entirely isn’t feasible.
10. “Warming” is shorthand; some areas will warm much more than others. Changes in rainfall may cause more damage than changes in temperature; dry areas will get drier and wet areas wetter.
11. We may need albedo-increasing options, at least to cut the top off the carbon peak for a couple of decades. Simplest approach seems to be to put huge hoses on smokestacks and support them with balloons, in order to release sulphates into the upper troposphere to produce Mount-Pinatubo-like cooling. Cost is manageable; dwell time is about a year, making the process tunable. The big problem would be acid rain. But it’s certainly time to put some substantial effort into figuring out whether increasing the albedo is workable, and if so how to do it.