Ecosystems provide a tremendous service to society by removing carbon dioxide from the atmosphere thus driving the so-called land carbon sink. Yet, the exact processes driving the relative strength of land carbon uptake have remained elusive and an area of debate amongst scientists. New research from an international team of scientists published today in the journal Nature Climate Change sheds light on this longstanding area of uncertainty. The team combined long-term atmospheric measurements with global satellite observations of vegetation growth to explore the causal mechanisms driving land carbon uptake. “You can think of the land carbon sink as the balance between the amount of carbon inhaled by plants during photosynthesis and exhaled as plants and animals respire carbon back to the atmosphere,” says Ashley Ballantyne of the University of Montana, lead author of the new research. The team found that slight shifts in this delicate balance lead to large consequences for land carbon uptake from year to year. “Over the period 1998-2012, total land carbon uptake was relatively large,” says Bill Smith of the University of Arizona, a co-author on the study and the lead for the satellite remote sensing component of the interdisciplinary effort. “Surprisingly, we found that this observed increase in the land carbon sink was largely driven by reductions in ecosystem respiration, and not increases in photosynthesis”. In other words, global ecosystems were essentially holding their breath over the period.
The team further explained why global ecosystem respiration may have slowed. “We think that the development of global temperatures played a dominant role in the observed trend,” says Steve Running of the University of Montana, senior scientist and member of the international team. “High temperatures promote, while low temperatures generally relax rates of ecosystem respiration. From 1998-2012 ocean temperatures warmed considerably, but land temperatures experienced what’s been termed a ‘warming hiatus’ during which temperatures remained relatively flat”. The team warns that with the rapid increase in land temperatures over the last 3 years, respiration will likely speed up thus causing carbon to return to the atmosphere to further warm the planet.
Ballantyne, A.P., Smith, W.K., Anderegg, W.R.L., Kauppi, P., Sarmiento, J., Tans, P., Shevliakova, E., Pan, Y., Poulter, B., Anav, A., Friedlingstein, P., Houghton, R., Running, S.W. Accelerating net terrestrial carbon uptake during the warming hiatus due to reduced respiration. Nature Climate Change DOI:10.1038/nclimate3204.