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Divergent global-scale temperature effects from identical aerosols emitted in different regions

Geeta G. Persad and Ken Caldeira · Nature Communications 9, 3289 · 2018

Key finding. A fixed quantity and composition of aerosol emission, matched to China's present-day emissions, cools the planet fourteen times more when emitted from Western Europe than when emitted from India, and the efficiency with which a given radiative forcing translates into global cooling differs fivefold depending on the emitting region.

Panel a is a world map with eight emissions regions shaded in different colours — Brazil, China, East Africa, Western Europe, India, Indonesia, South Africa and the United States — with a note that each receives the same emission change of 22.4 teragrams of sulfate precursor, 1.61 of black carbon and 4.03 of organic carbon. Panel b plots global-mean against regional-mean surface air temperature change, both negative, with dashed lines marking regional-to-global ratios of 1, 2, 4, 6, 10 and 20. Western Europe sits highest at about minus 0.30 kelvin global-mean; India lowest at about minus 0.02.
Identical emissions, different planets. Every region in panel a receives the same quantity and composition of aerosol, yet in panel b the resulting global cooling spans a factor of roughly fourteen — Western Europe at the top, India at the bottom — so where an aerosol is emitted matters as much as how much of it there is. Figure 1 from Persad and Caldeira (2018), Nature Communications 9, 3289. Reproduced under CC BY 4.0. Extracted from the published PDF and resized for web display.

What question did this research address?

Greenhouse gases are well mixed, so a tonne emitted in one country has the same climate effect as a tonne emitted in another. Aerosols are not — they are short-lived and stay near their source, so their climate effect depends on the properties of the region they are emitted into.

Despite this, scientific and policy discussions routinely evaluate aerosol climate impacts on the basis of emitted quantity alone, treating emission location as incidental.

This work asked how much difference location actually makes, by emitting one fixed quantity and composition of aerosol from each of eight major emitting regions in turn and comparing the global climate response.

What did we find?

The experiment held the aerosol quantity and composition fixed — emulating China's present-day emissions — and varied only the region of emission, across eight key geopolitical regions, in a climate model.

The global-mean cooling produced was fourteen times larger for the highest-impact region, Western Europe, than for the lowest, India. The same emissions therefore buy very different amounts of global cooling depending on where they occur.

Radiative forcing, the standard proxy for climate response, does not rescue the comparison: the forcing-to-cooling efficacy varied fivefold across emitting regions. A given amount of forcing from one region does not produce the same global cooling as the same forcing from another.

It follows that the geographic redistribution of aerosol emissions, which is already under way as some regions clean up and others industrialise, will itself change the global magnitude and spatial pattern of climate change, independently of any change in global emission totals.

Why does it matter?

Climate accounting frameworks that treat aerosol emissions as fungible across countries are mis-specified. If the same tonne does fourteen times more cooling from one place than another, national aerosol emissions cannot be added into a single global number and interpreted.

Aerosol emissions are shifting from Europe and North America towards South and East Asia. This result implies that shift alone changes the global temperature trajectory, which matters for interpreting the observed warming record as well as for projecting it.

Air quality policy and climate policy interact through this channel. The climate consequence of cleaning up a region's air depends on which region it is.

Citation

Geeta G. Persad and Ken Caldeira (2018). Divergent global-scale temperature effects from identical aerosols emitted in different regions. Nature Communications 9, 3289.