There’s a fascinating and very timely paper that’s just been published in the PNAS, Reducing abrupt climate change risk using the Montreal Protocol and other regulatory actions to complement cuts in CO2 emissions. As you can tell from the title, it’s an examination of ways to slow down the approach of climate chaos by means other than reducing CO2 emissions.
The six-page article is freely available from the above link in PDF format, and I highly recommend it.
Let me freestyle a little here and comment on the overall approach and the authors’ conclusions.
- The basic idea of looking for other strategies that we can use to complement our CO2 reduction efforts (and notice that the authors even use that word in the title) strikes me as being not just good or common sense, but absolutely imperative. We’re perilously close to some truly horrific tipping points, as the constant drumbeat of “it’s worse than we thought” news stories tell us, and we might even be past at least one of those milestones without having figured it out yet. Therefore, we should take the broadest possible approach to saving ourselves, which means not limiting our focus to CO2. As the authors point out, black carbon and other pollutants play a sizable role in the anthropogenic increase in radiative forcing at the root of this mess, so going after those sources, as well as employing large-scale reforestation efforts looks and feels like something we should have been doing for at least a decade or three (kind of like meaningful reductions in our CO2 emissions, which still haven’t begun).
- I’m a little concerned that any progress on non-CO2 contributors to climate chaos will only reduce the sense of urgency for implementing CO2 emissions reductions. I know that’s a policy issue and beyond the scope of such an article, but here in the real world it’s definitely a factor in play. Given the amount of money and effort being put into blocking any action whatsoever to forestall climate chaos, I shudder to think how non-CO2 based efforts would be twisted and misrepresented.
- The authors point out that there’s some uncertainty involved in their suggestions. The estimates for the radiative forcing effect of black carbon vary quite a bit, for example, and there’s also the concern that burning less coal and oil will reduce not just BC emissions but also sulfate aerosols from the air. Sulfate aerosols have a cooling effect, estimated at 1.15 watts/square meter.[1] In absolute value this is 69% of the warming effect of atmospheric CO2 (1.66), 72% of net anthropogenic forcing (1.6), and 3.8 times the forcing of BC[2]. (See Chapter 2 of The Physical Science Basis in the current IPCC report, especially Table 2.12.) In other words, it’s a hell of a lot of cooling we’re talking about, and stopping those emissions would result in a very quick drop in atmospheric sulfate aerosol levels, on the order of days to months, with an attendant rise in warming.[3] This is not to say that attacking BC emissions is a bad thing, merely that it has to be done in a way that doesn’t backfire.
- They point to sizable potential savings from biosequestration of carbon, through biochar and better forestry management (reduction of dforestation plus more reforestration). These efforts seem to have a sizable potential gain in terms of pulling carbon from the atmosphere, but they’re highly dependent on domestic and international policy efforts, so I don’t have a clue how to assess the probability of their actually being implemented on anything approaching the scale the authors discuss.
- The authors don’t address methane, the second-highest source of anthropogenic warming. Given the source of our methane emissions–largely food production and waste–it would be insanely difficult to reduce them by the usual 80% figure worldwide.[4]
[1] I’ve added the direct forcing for sulfate aerosol (-0.4) and the middle value of the range given for sulfate-induced cloud albedo effect (quoted as 0 to -1.5) in IPCC Table 2.12, linked above.
[2] I’ve added the direct BC forcing (0.2) and the BC/snow forcing (0.1), also from IPCC Table 2.12, linked above.
[3] As I’ve pointed out before, this is perhaps the single most perverse detail of our environmental mess. It’s bad enough that we burn (and are therefore dependent on burning) such an immense amount of coal around the world, but we’d be in very big trouble if we were somehow able to instantly stop using it. In planning and human terms, the CO2 from a couple of centuries of fossil fuel use will be with us forever, but the partially offsetting sulfates from burning all that coal would disappear very quickly. Who says Mother Nature doesn’t have a twisted sense of humor?
[4] But see the article By Degrees – Curbing Climate Change While Capturing Lost Methane about capturing methane that leaks from natural gas wells and facilities. In the US in 2007, we had 699.9 million metric tons of methane emissions (measured in CO2 equivalent units), with the top five sources being natural gas systems (176.6), landfills (169.0) and enteric fermentation (138.5), coal mining (71.1; yes, yet another reason to hate coal), and animal waste (65.0). These values are from the EIA’s publication EIA – Emissions of Greenhouse Gases in the U.S. 2007-Overview; see page five of the document or the linked web page for a large (unlabeled) table with these numbers. Obviously there is at least sizable category of savings that’s centralized enough and with sufficient economic incentive to be worth pursuing.
