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Soot, hydrofluorocarbs 'low-hanging fruit' to fight warming

Federal boffins insist heat tree be picked bare, though

The smart choice: opportunity from uncertainty

'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 the more Cassandra-esque 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). ®

Bootnotes

*"The blanket of man-made GHGs has already added 3 (range: 2.6–3.5) watts/m2," the scientists write.

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