28 August 2018

Greening of the Land Surface in the World’s Cold Regions Consistent with Recent Warming

Temperature changes are the dominant control on observed increases in maximum vegetation cover since 1982.

Science

We found that satellite observations indicate a greening of the land surface in cold regions, particularly high latitude ecosystems, and show that the observed greening is consistent with the response of ecosystems to recent warming. The inferred sensitivity of vegetation cover to temperature was not reproduced by land surface models – a finding that calls into question previous model-based results in temperature-limited ecosystems. The large biases in individual CMIP5 Earth System Models (ESMs) imply needed model improvements to ecosystem growth and allocation process parameterizations. 

Impact

High-latitude carbon cycle responses to CO2 and climate are critical for global predictions of the terrestrial C cycle. We show here that most current ESMs fail to accurately represent the sensitivity of vegetation cover to temperature changes in high-latitude systems, implying uncertainty in feedbacks with the atmosphere. Our results provide an observationally-inferred benchmark, which can be used to test and improve large-scale land models. The results suggest that the temperature limitation of high-latitude ecosystems will decline quickly under future warming scenarios, and highlight the importance of understanding non-temperature related limitations to growth in these ecosystems. 

Summary

Global ecosystem function is highly dependent on climate and atmospheric composition, yet ecosystem responses to environmental changes remain uncertain. Cold, high-latitude ecosystems in particular have experienced rapid warming, with poorly understood consequences. Here, we use a satellite observed proxy for vegetation cover – the fraction of absorbed photosynthetically active radiation – to identify a decline in the temperature limitation of vegetation in global ecosystems between 1982 and 2012. We quantify the spatial functional response of maximum annual vegetation cover to temperature and show that the observed temporal decline in temperature limitation is consistent with expectations based on observed recent warming. An ensemble of Earth system models from the Coupled Model Intercomparison Project (CMIP5) mischaracterized the functional response to temperature, leading to a large overestimation of vegetation cover in cold regions. We identify a 16.4% decline in the area of vegetated land that is limited by temperature over the past three decades, and suggest an expected large decline in temperature limitation under future warming scenarios. This rapid observed and expected decline in temperature limitation highlights the need for an improved understanding of other limitations to vegetation growth in cold regions, such as soil characteristics, species migration, recruitment, establishment, competition, and community dynamics.

Contact
William J. Riley
Lawrence Berkeley National Laboratory (LBNL)
Publications
Keenan, T, and W Riley.  2018.  "Greening of the Land Surface in the World’s Cold Regions Consistent with Recent Warming."  Nature Climate Change 8(9): 825-828, doi:10.1038/s41558-018-0258-y.