EEEK! New paper shows that some pollutants have a major COOLING effect
Particularly pollutants emitted by ships, trains and power stations. Quote: "For rail travel however, the warming due to carbon emissions, ozone, and aerosols is more than offset by cooling from sulfate aerosols on short-time horizons. High SO2 emissions notably from the electricity produced in coal fired power plants lead to a strong cooling from sulfate aerosols"
Of the various forms of transport examined by the researchers, shipping is the other one most markedly affected by short-term climate impacts. Here, however, everything is in reverse because the major short-term effect of shipping is sulfate aerosol pollution. While they remain in the air, these aerosol particles bounce sunlight away from the earth and therefore cause cooling rather than warming. The extent of this effect is amazing: if I'm understanding the numbers correctly, over a five-year time frame the world's ships cause enough cooling to offset the total warming caused by every car, plane and bus combined.
Even over a 20-year time frame, shipping pollution still contributes an overall cooling effect – as do electric trains, due to the aerosol pollution kicked out from coal-fired power stations. This throws up a tricky issue for policy makers and industry. If we clean up some kinds of air pollution for the benefit of environmental and human health, then we stand to significantly accelerate global warming in the near-term.
However the world deals with that particular conundrum, the new paper is a useful reminder that carbon footprints are more multi-dimensional than is usually understood.
This issue isn't limited to transport, of course. Any activity that generates lots of methane, nitrous oxide or other non-CO2 greenhouse gases will have a much faster warming effect than its carbon footprint, as traditionally expressed, might suggest. That would include meat and rice farming, landfill sites and fridge production, for example.