Continued increase in atmospheric CO2 seasonal amplitude in the 21st century projected by the CMIP5 Earth system models

Wednesday, 17 December 2014
Fang Zhao, University of Maryland, College Park, MD, United States and Ning Zeng, Univ Maryland, College Park, MD, United States
Superimposed on the continued increase in the atmospheric CO2 concentration is a prominent seasonal cycle. Ground-based and aircraft-based observation records show that the amplitude of this seasonal cycle has increased. Will this trend continue into future? In this paper, we analyzed simulations for historical (1850-2005) and future (RCP8.5, 2006-2100) periods produced by 10 Earth System Models participating the Fifth Phase of the Coupled Model Intercomparison Project (CMIP5). Our results show a model consensus that the increase of CO2 seasonal amplitude continues throughout the 21 century. The seasonal amplitude of the multi-model global mean detrended CO2 increases from 1.6 ppm during 1961-1970 to 2.7 ppm during 2081-2090, and the mean relative amplitude increases by 62±19%. This increase is dominated by a 68±25% increase from Net Biosphere Production (NBP). We then show the increase of NBP amplitude mainly comes from enhanced ecosystem uptake during Northern Hemisphere growing season under future CO2 and temperature conditions. Separate analyses on net primary production and respiration reveal that enhanced ecosystem carbon uptake contributes to about 75% of the amplitude increase. Stimulated by higher CO2 concentration and high-latitude warming, enhanced net primary production likely outcompetes increased respiration at higher temperature. Zonal distribution and the spatial pattern of NBP change suggest that regions north of 45°N dominate the amplitude increase. We also found that changes of NBP and its seasonal amplitude are significantly (R=0.73, p<0.05) correlated—models that simulate a stronger carbon uptake tend to show higher change of NBP seasonal amplitude.