Plants absorb more carbon under polluted skies, study shows
New research from Switzerland and the UK reveals that, somewhat paradoxically, plants absorb more carbon dioxide (CO2) when the atmosphere is polluted than they do under cleaner skies. Writing in the journal Nature, the scientists warn that as air pollution levels continue to decline, even steeper greenhouse gas emissions cuts will be needed to stabilise the climate. Plants rely on the sun to absorb CO2 from the atmosphere. Although it seems counter-intuitive, plants actually absorb CO2 more efficiently under hazy sunlight than they do under bright, direct sunlight. When exposed to direct sunlight, the leaves at the top of the plant canopy get more sunlight than they can use, while leaves in the shade do not get enough. However, when clouds and minute particles of pollution scatter the light, leaves lower down on the canopy get comparatively more light than in the previous case. As a result, plants absorb CO2 more effectively in diffuse light than in direct light. From 1950 to 1980, the world underwent a period of 'global dimming', as changing cloud patterns and high levels of pollution meant that less sunlight reached the Earth's surface. However, since the 1980s, the skies over many industrialised parts of the world have brightened considerably, thanks to a fall in emissions of pollutants such as sulphur dioxide. In this study, the scientists investigated the impact of these changes on the amount of carbon absorbed and stored by plants. 'Surprisingly, the effects of atmospheric pollution seem to have enhanced global plant productivity by as much as a quarter from 1960 to 1999,' explained lead author Dr Lina Mercado of the Centre for Ecology and Hydrology in the UK. 'This resulted in a net 10% increase in the amount of carbon stored by the land once other effects were taken into account.' This raises the question of what will happen in the future if, as expected, air pollution continues to decline. The scientists conclude that 'steeper cuts in fossil fuel emissions will be required to stabilise the climate if anthropogenic aerosols decline as expected'. 'As we continue to clean up the air in the lower atmosphere, which we must do for the sake of human health, the challenge of avoiding dangerous climate change through reductions in CO2 emissions will be even harder,' commented Professor Peter Cox of the University of Exeter in the UK. 'Different climate-changing pollutants have very different direct effects on plants, and these need to be taken into account if we are to make good decisions about how to deal with climate change.'
Countries
Switzerland, United Kingdom