Opinion Government-funded boffins in the USA say that "low-hanging fruit" plans to tackle climate change must be considered alongside CO2 reductions: in particular they say that emissions of black soot and shortlived greenhouse gases must be tackled in order to offset the warming caused by clean-air regulations.
Veerabhadran Ramanathan and Yangyang Xu, researchers at the Scripps Institution in San Diego, produce their results "using a synthesis of National Science Foundation (NSF)-funded research performed over the last 20 years", according to the NSF.
"They stress that carbon dioxide control alone is not enough," says an NSF statement released this week.
According to Ramanathan and Xu, there are other significant human-driven causes of warming besides CO2. In particular, one of these is clean-air law introduced in many industrial nations to put a stop to acid rain and related problems.
This is done by cutting down on emissions of sulphate aerosols, for instance by cleaning up the exhaust from coal-fired power stations. Unfortunately, sulphates in the atmosphere actually work against global warming, by reflecting heat away into space. Thus the clean-air rules mean that the warming tendency gets stronger.
And it gets worse (or better, depending on viewpoint). There's another kind of aerosol pollution, which warms rather than cooling - black soot, which has lately been rising in profile amid the climate debate to much acrimony. Many greens don't want it discussed, fearing that an emphasis on soot might detract from the push to cut CO2 emissions, but nonetheless scientists whose eco-credentials are impeccable have lately acknowledged that soot is a problem on the same level as carbon dioxide.
James Hansen of NASA, for instance, the world high priest of human-driven warming, lately stated that "black soot is probably responsible for as much as half" of the melting effect exerted by humans on the Himalayan glaciers, the planet's so-called "Third Pole". (In fact it is now admitted that the IPCC's suggestion that the glaciers would all be gone in decades was baseless. Even so, the admission by Hansen that soot is a big deal is very significant.)
The good thing about black soot is that like sulphates it disappears from the atmosphere quickly, washed away by rain, unlike CO2, which mainly hangs about until a carbon sink of some sort absorbs it. Thus cutting soot can have a big effect right away: and better still, cutting soot emissions is easy and cheap compared to cutting CO2. Ramanathan and Xu refer to this measure as "low-hanging fruit", in fact.
Soot comes primarily from inefficient burning of fuels, as in poorly adjusted diesel engines and primitive biomass stoves or cooking fires. Filtering diesel exhausts and handing out cleaner cooking gear to the developing world would cost almost nothing compared to major CO2 cuts, and would have big effects.
Some have suggested that the human race should simply press on and cut soot emissions, as binding agreement on carbon dioxide is proving elusive. This usually angers the harder-green lobby, who tend to see such a plan as merely postponing the problem.
'Without the Montreal Protocol, human warming would already be 50 per cent stronger'
Ramanathan and Xu advocate no such plan, however. Instead they say that easy soot reduction should be used to counterbalance planet-warming sulphates cuts; regulations on the two classes of aerosols should be linked in "warming-neutral" pollution laws. Thus as China, for instance, cleans up its sulphate-spewing coal stations it should also cut soot emissions, producing neither a warming nor cooling effect overall.
They also advocate another plan which they see as "low hanging fruit", that of measures to eliminate short-lived but powerful greenhouse gases such as methane and certain hydrofluorocarbons.
The two boffins note that the hydrofluorocarbon measures in particular should be achievable, as they wouldn't be much more troublesome than the ones agreed in the Montreal Protocol of 1987. Quite apart from preserving the ozone layer and preventing worldwide skin-cancer, Ramanathan and Xu calculate that the Montreal chlorofluorocarbon cuts have also saved somewhere between 0.6 and 1.6 watts/m2 of warming: without Montreal, the warming effect of human-generated greenhouse gases would now be as much as 50 per cent more powerful*.
Soot, methane and hydrocarbon measures alongside sulphate controls would mean, according to the two scientists' calculations, that atmospheric CO2 would only need to be stabilised at 441 parts per million (ppm). It currently stands at 389-odd and counting, with many of the world's billions still fantastically energy-poor and keen to start burning fossils like the rich nations; so stopping at 441 is a big ask ("Herculean", according to Ramanathan and Xu).
However this is a rosier picture than that painted by analysts like Hansen, who has stated - before he acknowledged the significance of soot, though - that 350 ppm will be enough to melt all the planet's ice and trigger runaway, civilisation-wrecking heating.
According to Ramanathan and Xu things could be even better, as their analysis assumes only present-day, well-known technologies in use for tackling soot, methane and hydrofluorocarbons. As none of these things have been a major priority so far, they think that better kit could be developed and bigger impacts obtained - conceivably, if the 441 ppm line can be held, cutting warming by 2050 to 1.5 °C or less, well inside the Copenhagen safe zone.
Of course, the relatively easy soot/methane/hydrofluorocarbon package could be pursued as an alternative to CO2 cuts rather than a complement to them: Ramanathan and Xu don't even mention such a possibility, but its obviousness is likely to make them somewhat unpopular in hard-green circles nonetheless.
The scientists' paper, The Copenhagen Accord for limiting global warming: Criteria, constraints, and available avenues, is published here by the Proceedings of the National Academy of Sciences (subscription required). ®
*"The blanket of man-made GHGs has already added 3 (range: 2.6–3.5) watts/m2," the scientists write.