Recent warming has resulted in smaller gains in net carbon uptake in northern high latitudes

Authors: ZHU, PENG - Purdue University; ZHUANG, QIANLAI - Purdue University; WELP, LISA - Purdue University; CIAIS, PHILIPPE - National Council For Scientific Research-Cnrs; HEIMANN, MARTIN - Max Planck Institute For Biogeochemistry; PENG, BIN - University Of Illinois; LI, WENYU - Tsinghua University; Bernacchi, Carl; ROEDENBECK, CHRISTIAN - Max Planck Institute For Biogeochemistry; KEENAN, TREVOR - Lawrence Berkeley National Laboratory

Submitted to: Journal of Climate
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/7/2019
Publication Date: 8/15/2019
Citation: Zhu, P., Zhuang, Q., Welp, L., Ciais, P., Heimann, M., Peng, B., Li, W., Bernacchi, C.J., Roedenbeck, C., Keenan, T.F. 2019. Recent warming has resulted in smaller gains in net carbon uptake in northern high latitudes. Journal of Climate. 32:5849-5863.

Interpretive Summary: The globe is warming because of human activities, and among the areas warming the fastest are the northern high latitudes. Plants in this region use sunlight to grow, in a process called photosynthesis, which removes carbon dioxide from the atmosphere. But can this region continue to remove a large amount of carbon dioxide from the atmosphere as the globe continues to warm? This research used measurements of carbon dioxide in the atmosphere over 40 years to determine whether plants in this region are continuing to remove carbon dioxide at the same, lower, or higher rates as the globe warms. The results showed that the region is removing less carbon dioxide, which can have even more negative impacts on climate than what is occurring directly from human-caused pollution. This impact is not represented in the models of future climate or future plant growth, suggesting that current forecasts of global health may be worse than currently assumed.

Technical Abstract: Carbon balance of terrestrial ecosystems in the northern high latitudes (NHL) is sensitive to climate change. It remains uncertain whether current regional carbon uptake capacity can be sustained under future warming. Here the atmospheric CO2 drawdown rate (CDR) between 1974 and 2014, defined as the CO2 decrease in ppm over the number of days in spring or summer, is estimated using atmospheric CO2 observations at Barrow, Alaska. We found the sensitivity of CDR to inter-annual seasonal air temperature anomalies has trended toward less carbon uptake over this period. Changes in interannual temperature sensitivity of CDR suggest that relatively warm springs now result in less of a carbon uptake enhancement. Similarly, relatively warm summers now result in greater carbon release. These results generally agree with the sensitivity of net carbon exchange (NCE) estimated by atmospheric CO2 inversion. When NCE was aggregated over North America (NA) and Eurasia (EA), separately, temperature sensitivity of NCE in NA has changed more than in EA. In order to explore potential mechanisms of this signal, we also examine trends in interannual variability of other climate variables (soil temperature and precipitation), satellite-derived GPP, and TRENDY model intercomparison results. Our analysis suggests that the weakened spring sensitivity of CDR may be related with the slowdown in seasonal soil thawing rate, while the summer sensitivity change may be caused by the temporally coincident decrease in temperature sensitivity of photosynthesis. This study suggests that the current NHL carbon sink may become unsustainable as temperatures warm further. We also found that current carbon cycle models do not represent the decrease in temperature sensitivity of net carbon flux. We argue that current carbon-climate models misrepresent important aspect of the carbon-climate feedback and underestimate the warming influence on NHL carbon balance.