The research paper is available online: http://www.nature.com/nclimate/journal/v7/n2/full/nclimate3204.html
Terrestrial ecosystems provide a tremendous service to humanity and the well-being of the Earth by removing carbon dioxide from the atmosphere, acting as a "carbon sink" where carbon inputs from photosynthesis are greater than carbon losses from plant and animal respiration.
Yet, the exact processes influencing the relative strength of land carbon uptake have remained elusive and an area of debate among scientists.
New research from an international team of scientists published recently in the journal Nature Climate Change sheds light on this longstanding area of uncertainty.
The team, which includes University of Arizona researcher Bill Smith of the College of Agriculture and Life Sciences, combined long-term CO2 measurements with global NASA satellite observations of vegetation photosynthesis to explore the causal mechanisms driving land carbon uptake. Smith is a co-author of the study and the lead for the satellite remote sensing component of the interdisciplinary effort. His work was supported through a grant from NASA and a Luc Hoffmann Institute Fellowship.
"The land carbon sink reflects the balance between the amount of carbon 'breathed in' by plants during photosynthesis and 'breathed out' as plants and animals respire carbon back to the atmosphere," said Smith, assistant professor in the School of Natural Resources and the Environment and affiliate of the Earth Dynamics Observatory.
The team explored the temperature sensitivity of these two important processes by separately analyzing periods of rapid warming and extended periods of slower warming at the global scale over the entire satellite observation record from 1982 to 2012.
"Over the period 1998 to 2012, a period of slower warming that has been termed the 'warming hiatus,' we found increased rates of land carbon storage," Smith said. "Interestingly, our findings suggest that this period of accelerated carbon storage was mainly due to reduced rates of ecosystem respiration."
This means that during the 14-year period, Earth absorbed and stored much more carbon from the atmosphere.
However, as global warming ramps up again, this carbon may be rapidly returned to the atmosphere, increasing the rate of warming. This reversal already is being observed with the recent end of the warming hiatus and rapid increase in land temperatures.
Over the last four years, global temperatures have rapidly increased, with 2015 and 2016 being record high temperature years. With these increased temperatures, the research team warns that reductions of land carbon storage should be expected, possibly leading to big increases in atmospheric CO2, which could in turn further increase global temperatures.
"Our work provides new information on how the Earth system may respond to human-caused climate change, and the current outlook is not too bright," Smith said.
"Continued CO2 emissions could work to undermine the ability of ecosystems to soak up CO2 from the atmosphere. Without this tremendous service, rates of global warming will increase faster than expected, with potentially very large negative consequences for humanity, so action is urgently needed to protect natural ecosystems that are strong CO2 sinks, while simultaneously reducing global CO2 emissions."