Plants saved the Earth from a frozen past
2 July 2009
Land plants may have saved the Earth from a deep frozen fate by stopping levels of atmospheric carbon dioxide (CO2) dropping to zero over the last 24 million years, according to new research.
Pine forest.
Around 50 million years ago, atmospheric CO2 levels were as high as 2000 ppm. The planet was so hot that crocodiles roamed the Arctic. Since then, CO2 levels have steadily dropped.
But scientists noticed that CO2 levels have rarely dipped below around 200 ppm over the last 24 million years.
Over this huge period of time - millions of years - the Andes, the Tibetan plateau and the Himalayas were all born.
'If it wasn't for the plants, the planet may have been much colder over the last 24 million years that it was.'
Professor David Beerling, University of Sheffield
'When CO2 levels are high, a warmer and wetter climate means that these mountain ranges are all eroded more than when it's cool,' explains Professor David Beerling of the University of Sheffield.
When mountain ranges made of silicate rocks like basalt or granite erode, they release calcium and magnesium ions, which finds its way to the oceans via rivers.
Calcium is used by marine creatures to make their shells and it's one of the main components of limestones, a marine rock. Crucially, to do this, shells and limestone also need to use carbonate, which is more abundant in the oceans when atmospheric CO2 levels are high. The gas dissolves in water to make a weak acid - carbonic acid - which acidifies the oceans.
'We were intrigued that all of the CO2 doesn't get sucked out of the atmosphere, during major tectonic events,' says Beerling.
Rock erosion
In the latest research, published in this week's Nature, Beerling and his American colleagues suggest that land plants stop CO2 levels dropping to zero.
Using computer models and the results of previous experiments in which researchers limited CO2 availability to land plants, they show that as CO2 levels drop, the activity of these plants slows down.
Plants are very good at eroding rock when they put down roots and so help to release calcium that is flushed into the oceans. As plant activity slows down when they're starved of CO2, less calcium to finds its way to the oceans, because plants don't erode the rock as much. Not a lot calcium around means fewer marine organisms can grow and less limestone is formed. In consequence, not as much CO2 gets used up from the atmosphere.
'Essentially the plants stop CO2 levels dropping to zero,' says Beerling. 'Current carbon cycle models miss this. If you put plants back into our model, they stabilise CO2 at different values depending on the minimum CO2 level required to sustain an ecosystem.'
The team also suggests that when CO2 levels are higher, there's more erosion. This means that more calcium gets released and so more CO2 gets pulled down out of the atmosphere by marine creatures.
Other research may support this idea. Dr Guy Midgeley of the South African National Biodiversity Institute has shown that trees have trouble making roots when they're starved of CO2.
'If it wasn't for the plants, the planet may have been much colder over the last 24 million years that it was,' adds Beerling.
The role of terrestrial plants in limiting atmospheric CO2 decline over the past 24 million years
Mark Pagani et al.
Nature, 2009
doi:10.1038/nature08133
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