Falling levels of aerosol pollution could make it much harder to curb the total amount of CO2 in the atmosphere. That is the conclusion of climate researchers in the UK and Switzerland, who have found that pollution in the form of aerosol particles gives a dramatic boost to plants’ photosynthesis.

Large amounts of aerosols have been linked previously to a period of “global dimming” from the 1950s to the 1980s, when the amount of visible sunlight reaching the Earth’s surface was fractionally reduced.

However, the new study shows that during the same period the aerosols were diffusing a lot of sunlight into shaded areas of vegetation. The result, which goes contrary to present climate models, is that more plants were able to absorb CO2 through photosynthesis and hence increase the size of the land carbon sink by almost a quarter.

This means that we are going to have to cut the [CO2] emissions even more than we thought Lina Mercado, Centre for Ecology and Hydrology

The study implies that if the levels of aerosol pollutants continue to fall, so too will some of the absorption of CO2. “This means that we are going to have to cut the [CO2] emissions even more than we thought,” says lead author Lina Mercado of the UK’s Centre for Ecology and Hydrology.

Less dimming, less photosynthesis

The global dimming of mid to late last century became recognized only a few years ago. It occurred mostly in urban and industrialized areas, which suggests its main cause was the release of sulphate aerosol particles, particularly from the unrestricted burning of sulphurous coal in the 1960s and 1970s. This is because most aerosols scatter light back into space, either from themselves or by preserving clouds, and thus reduce the total sunlight and heat on the Earth’s surface.

But aerosols also cause hazy atmospheres by diffusing light from the Sun, and this is the effect that Mercado’s group has now studied. They have modified part of a climate model used by the UK MET Office’s Hadley Centre so that it takes into account both direct and diffuse radiation on photosynthesis in shaded and sunlit areas. Then they tested the model using observations taken in broad-leaf, needle and temperate forests.

The improved model showed that, between 1960 and 1999, photosynthesis from diffuse light increased the global land carbon sink by 23.7%. The net effect, allowing for a decreased total radiation over the period, was a carbon sink increased by about 10% .

Lianhong Gu, an environmental scientist at Oak Ridge National Laboratory in the US, says he does not view the group’s findings as conclusive because computer models of clouds and aerosols on global scales tend to be subject to large uncertainties.

It highlights that climate mitigation scenarios must be evaluated from an Earth-system point of view Lianhong Gu, Oak Ridge National Laboratory

Still, he does think it is important research. “It points to the complexity of the driving forces for the terrestrial carbon cycle. It indicates that the two most uncertain factors in projecting climate change — clouds and aerosols — are more uncertain than we thought. [And] it highlights that climate mitigation scenarios must be evaluated from an Earth-system point of view,” he adds.

‘Steeper cuts required’

The study is likely to raise questions about how best to tackle climate change. Many governments have found it relatively easy to cut back on aerosol emissions — as opposed to greenhouse gas emissions — for the sake of the environment, yet present climate models only consider the negative effect aerosols have had on the land carbon sink.

Although the Mercado’s group is unable to say how much the lack of aerosols will affect total CO2 levels in the future, they do note that “steeper cuts will be required” in order to stabilize concentrations at 450 parts per million — the threshold after which many climatologists think there could be severe consequences.

Return to geoengineering?

The study might also bring certain “geoengineering” proposals back into serious scientific debate. One of most enduring proposals is that small quantities of aerosols could be purposefully injected into the atmosphere, in the same way the eruption of Mount Pinatubo in 1991 threw up vast clouds of sulphates and reduced global temperatures by about half a degree. But there are possible side-effects, such as the fact that aerosols provide surfaces on which stratospheric ozone can be destroyed.

Peter Cox, a climate modeller at the University of Exeter and co-author of the study, does not advise injecting aerosols yet. However, he says there is no reason now to think plant growth and crop yields would go down, which has been an argument against the proposal in the past. “If you were to put aerosols into the atmosphere, at least this particular concern — that is, crop yields would go down — is not something you should worry about. In fact, it would be a positive rather than a negative effect.” The results are published in the journal Nature.