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

Abstract:

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.