Much of the discussion on the "low-hanging fruit" post revolved around a hypothetical question: would tackling secondary climate forcings (such as soot and methane) pave the way for stronger climate policies down the road, or further defray action on carbon dioxide, which happens to be the more pressing long-term threat? At this juncture, political and economic realities would seem to argue in favor of Andy's incrementalism approach, which he laid out here and here in that thread. Indeed, his argument is similar to the one made in the recent NYT op-ed:
Reducing soot and the other short-lived pollutants would not stop global warming, but it would buy time, perhaps a few decades, for the world to put in place more costly efforts to regulate carbon dioxide.
But when I read that passage, I also wondered about this notion of buying time, whether it was true or not. So right after I published my post, I sent my query to the new Climate Science Rapid Response Team. Two days later, John Abraham, a co-custodian of the organization, responded with a nice note, saying they had obtained a response to my question from Dorothy Koch, a research scientist at Columbia University and NASA's Goddard Institute for Space Studies. Koch's areas of expertise (from her NASA page) include "global modeling of aerosol chemistry" and "aerosol impacts on climate." Here is how she answered my question:
I did not read the op-ed, but the statement "Reducing soot and the other short-lived pollutants would not stop global warming, but it would buy time, perhaps a few decades, for the world to put in place more costly efforts to regulate carbon dioxide." is extremely vague. How much reduction? Of what precisely? Buy time for what? Yes, if we were able to reduce soot and methane and ozone this would reduce warming. Any one of these alone would make a small dent, and when you start to look at practical reductions in emissions for any one of these, it gets tricky. Warming "Soot" is co-emitted with shiny cooling aerosols, so one needs to be careful to target sources with lots of dark carbonaceous material but without the shiny aerosols (diesel is a good target). Finding methane sources that are easy to reduce is also not so easy. So the point is, once we find the low-hanging particular sources of the particular warming short-lived species, the benefit is small. But worth pursuing, particularly due to the co-benefits of pollution reduction for both soot and ozone.
Now, just to be clear: if I were writing an article for a publication (as opposed to a blog post), I would follow up with Dr. Koch and ask additional questions. I'm assuming that the Climate Science Rapid Response Team knows this, as I expect would the experts they match up with journalists. However, that is not necessary for the purposes of this post. (I do appreciate the service this organization provides and also the efforts of Dr. Koch and others who make themselves available for public outreach.) But in a quick search of recent work by Veerabhadran Ramanthan, one of the op-ed authors, I did notice this May 2010 PNAS paper, which outlines a "three avenue" approach:
(i) reduce the rate of thickening of the blanket by stabilizing CO2 concentration below 441 ppm during this century (a massive decarbonization of the energy sector is necessary to accomplish this Herculean task), (ii) ensure that air pollution laws that reduce the masking effect of cooling aerosols be made radiant energy-neutral by reductions in black carbon and ozone, and (iii) thin the blanket by reducing emissions of short-lived GHGs.
I'm guessing that political developments since last May probably led Dr. Ramanthan to conclude, as he argues in the NYT op-ed, that
more modest steps, with quick and measurable effects, are a better way to proceed.
And that would be focusing attention on those short-lived GHGs.
