Scientists identify specific climate shifts responsible for an increase in the land carbon sink

Humans have been emitting carbon dioxide (CO₂) into the atmosphere for several hundred years through the combustion of fossil fuels and the burning of vegetation. But this CO₂, the most important anthropogenic greenhouse gas, doesn’t just remain in the air forever; around half of it is taken up by the world’s oceans and continents.

Understanding the way these sinks of carbon work is vital when we are trying to predict how the climate will change in the future.

Recent studies suggest that since the late 1980s the amount of carbon being taken up by the land carbon sink (i.e., soils, trees and other vegetation) has increased, by about 1 petagram (10⁹ tonnes) of carbon per year.

This means that the amount of carbon dioxide in the atmosphere, which reached 400 ppm in 2015, would have been even higher (by around 12 ppm) if not for this increase in the land sink.

To estimate this, scientists add up all the known global carbon emissions and combine this with measurements of the concentration of carbon dioxide in the atmosphere; the difference is the amount of carbon dioxide taken up by the land and ocean sinks. Scientists can then estimate how much carbon dioxide the ocean should take up, and the land carbon sink is what is left over.

But studies that look at the carbon cycle on a global scale are not able to attribute these changes to a particular region and to a particular factor; a recent study led by LEAF scientist Dr Wolfgang Buermann from the School of Earth and Environment, sheds some light on the possible reasons for this shift.

Using both process-based biosphere models and a model driven by satellite data of observed vegetation activity, the team could see evidence of an abrupt increase in plant carbon uptake in the late 1980s consistent with previous findings. They were also able to attribute this increase to two main regions, northern Eurasia and northern Africa.

It seems that in each region, a climate shift occurred that was beneficial for plant productivity. In northern Eurasia this was a warming during springtime, whilst northern Africa saw an increase in rainfall. This boost to plant productivity meant that the vegetation was drawing more CO₂ out of the atmosphere to be stored as carbon on the ground.

It is not yet known whether there was a connection between these two climate phenomena, but they occurred over such a large region that together, they were able to impact the global carbon cycle.

The most common mechanism used to explain increases in the land carbon sink is the process of CO₂ fertilisation, whereby under elevated CO₂ concentrations plant photosynthesis becomes more efficient. But this latest study demonstrates that changes in climate, such as the late 1980s shift, could be just as important.

These climatic shifts have been attributed to natural climate variability, which means that it would be possible for equally substantial changes to occur in the opposite direction, reducing the ability of the land sink to take up carbon in the future.

Read the full study here.

Post by Cat Scott - University of Leeds.