Role of terrestrial biosphere in counteracting climate change may have been underestimated

From the UNIVERSITY OF BIRMINGHAM

New research suggests that the capacity of the terrestrial biosphere to absorb carbon dioxide (CO2) may have been underestimated in past calculations due to certain land-use changes not being fully taken into account.

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 CO2 emissions.

However, its role as a net carbon sink is affected by land-use changes such as deforestation and expanded agricultural practice.

A new study, conducted by an international collaboration of scientists and published in the journal Nature Geoscience, has analysed the extent to which these changing land-use practices affect carbon emissions – allowing the levels of CO2 uptake by the terrestrial biosphere to be more accurately predicted.

The results not only show that CO2 emissions from changing land-use practices are likely to be significantly higher than previously thought, but also imply that these emissions are compensated for by a higher rate of carbon uptake among terrestrial ecosystems.

Co-author of the study, Dr Tom Pugh from the University of Birmingham, says:

‘Our work shows that the terrestrial biosphere might have greater potential than previously thought to mitigate climate change by sequestering carbon emissions from fossil fuels. However, to fully realise this potential we will have to ensure that the significant emissions resulting from land-use changes are reduced as much as possible.’

Co-author Professor Stephen Sitch from the University of Exeter adds:

‘The results imply that reforestation projects and efforts to avoid further deforestation are of the utmost importance in our pursuit to limit global warming to below 2oC, as stated in the Paris climate agreement.’

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The study: http://www.nature.com/ngeo/journal/vaop/ncurrent/full/ngeo2882.html

Historical carbon dioxide emissions caused by land-use changes are possibly larger than assumed

The terrestrial biosphere absorbs about 20% of fossil-fuel CO₂ emissions. The overall magnitude of this sink is constrained by the difference between emissions, the rate of increase in atmospheric CO₂ concentrations, and the ocean sink. However, the land sink is actually composed of two largely counteracting fluxes that are poorly quantified: fluxes from land-use change and CO₂uptake by terrestrial ecosystems. Dynamic global vegetation model simulations suggest that CO₂emissions from land-use change have been substantially underestimated because processes such as tree harvesting and land clearing from shifting cultivation have not been considered. As the overall terrestrial sink is constrained, a larger net flux as a result of land-use change implies that terrestrial uptake of CO₂ is also larger, and that terrestrial ecosystems might have greater potential to sequester carbon in the future. Consequently, reforestation projects and efforts to avoid further deforestation could represent important mitigation pathways, with co-benefits for biodiversity. It is unclear whether a larger land carbon sink can be reconciled with our current understanding of terrestrial carbon cycling. Our possible underestimation of the historical residual terrestrial carbon sink adds further uncertainty to our capacity to predict the future of terrestrial carbon uptake and losses.