Role of terrestrial biosphere in curbing climate change

It is widely known that the terrestrial biosphere (the collective term for all the world’s land vegetation, soil, etc) is an important factor in mitigating climate change, as it absorbs around 20% of all fossil fuel carbon dioxide emissions. However, its role as a net carbon sink is affected by land-use changes such as deforestation.

Although we know the total net carbon uptake by the terrestrial biosphere very well, understanding the extent to which different processes contribute to this uptake has long presented a challenge for scientists.

The problem arises because we are not able to make measurements of the exchange of carbon between the biosphere and the atmosphere across large spatial scales — for example countries and continents. Instead, we have to combine our best knowledge of the processes involved with small-scale, table-sized to field-sized measurements of carbon exchange and ecosystem properties using computer models. Various modelling approaches agree that ongoing changes in land-use, particularly tropical deforestation, lead to a net emission of carbon from the biosphere when considered globally.

This land-use emission is more than offset by an uptake in carbon by the rest of the global terrestrial biosphere. We believe that the reasons for this uptake include more rapid plant growth as a result of higher levels of carbon dioxide in the atmosphere, a relaxation of temperature constraints on plants growth in colder regions. However, our understanding of the extent to which these, and other processes contribute to the uptake remains limited. Our strongest constraint comes from comparing the net uptake of carbon by the biosphere, which we know very well, with the loss of carbon from land-use change, which we do not.

So what then, if we have underestimated the emission of carbon resulting from land-use change? This is what we set out to investigate in a recent study published in the journal Nature Geoscience. Using multiple models, we calculated the effects of three potentially important processes which had been widely neglected in the previous assessments: cropland management, harvesting of wood from forests, and the effect of including input data with a very high spatial resolution.

What we found is that all of these processes acted to increase the calculated emissions of carbon from land-use change. These higher emissions therefore imply that uptake of carbon by the rest of the biosphere must also be higher than we previously thought.

These results have two very important implications. The first is that we may be emitting much more carbon than we previously thought as a result of land-use change. So, efforts to reduce rates of land-use change, particularly deforestation in the tropics and farming and forest management practices in other parts of the world, may have a significant payoff in terms of reducing the rate of increase of carbon dioxide levels in the atmosphere.

The second implication is that the terrestrial biosphere might have a greater potential than previously thought to mitigate climate change by sequestering carbon emissions from fossil fuels — a potential that will become increasingly evident as we reduce land-use change emissions.

However, our ability to effectively harness this potential uptake is hampered by our lack of knowledge about the processes contributing to it, particularly in terms of which specific ecosystems are providing most of the sink. This provides a further motivation to reduce rates of land-use change; to avoid inadvertently removing the ecosystems which turn out to be critical for the uptake on which we currently depend. Without this strong biospheric carbon uptake, the rate of carbon increase in the atmosphere, and thus the rate of climate change, would be much more rapid.

So, it appears that the global biosphere may be an even greater asset than previously thought in limiting anthropogenic climate change to two degrees. In order to leverage this, we need more understanding of how global environmental changes are affecting the biosphere.

(The author is with the University of Birmingham, UK)