Over geological time periods, volcanoes have released large quantities of CO2 into the atmosphere. Levels of the gas have been kept in check by the weathering of silica-based rocks such as granite, which causes the CO2 to be locked up in carbonates that are then washed into the ocean. The rate at which this sedimentation occurs depends on the upthrust of mountains and subsequent weathering. The creation of the Andes and Himalaya over the past 25 million years should have led to increased sedimentation and hence a huge drop in atmospheric CO2, which would, in turn, have led to a significant drop in temperature – enough to cause the entire planet to freeze over. The fact that it didn’t, with CO2 levels remaining relatively stable,  has long been a puzzle for climate scientists. 

Using simulations of the global carbon cycle and plant growth experiments, researchers at Yale University, the Carnegie Institution of Washington and the University of Sheffield showed that as atmospheric CO2 concentrations dropped towards near-starvation levels for land plants, the capacity of vegetation to weather silicate rocks greatly diminished, slowing the takeup
of atmospheric CO2.

‘The rates of weathering are largely controlled by plants. Their roots secrete acids that dissolve minerals, they hold soils, and they increase the amount of carbon dissolved in groundwater,’
said one of the researchers, Ken Caldeira of the Carnegie Institution.

October 2009