The aerosol story just keeps getting more interesting. In addition to ongoing research about the direct impact of various aerosols on climate and temperature (see here and here, for example), there’s also the indirect impact on photosynthesis and carbon uptake. A study in this week’s Nature explores the latter phenomenon with regard to sulphur dioxide and comes up with some startling results.
The basic gist is that aerosols scatter light, and indirect light is better at penetrating forest canopies and stimulating growth in the understory (to illustrate, imagine the dark shadows cast on the forest floor by trees blocking direct sunlight). The increase in this understory growth is significantly larger than the decrease in canopy growth, because even on a shady day the tallest trees are going to do fairly well.
The implications are substantial. If you assume that pollution regulations around the world will eventually reduce sulphur dioxide emissions to zero in order to clean up the air and save lives, then solar radiation will become more direct and photosynthesis will go down. Which means plants will pull less carbon dioxide out of the atmosphere, which means that addressing global warming will be even more difficult than expected.
We covered this, along with a study about China’s terrestrial carbon sink, in a news story earlier this week. But before you start thinking we’ve got it all figured out, you might want to take a look at another story by Quirin Shiermeier, which suggests that the air might be dirtier than we thought. That might be good news for the plants, at least.