Author Posting. © American Meteorological Society, 2013. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Climate 26 (2013): 4447–4475, doi:10.1175/JCLI-D-12-00589.1.
Changes in atmospheric CO2 variability during the twenty-first century may provide insight about ecosystem responses to climate change and have implications for the design of carbon monitoring programs. This paper describes changes in the three-dimensional structure of atmospheric CO2 for several representative concentration pathways (RCPs 4.5 and 8.5) using the Community Earth System Model–Biogeochemistry (CESM1-BGC). CO2 simulated for the historical period was first compared to surface, aircraft, and column observations. In a second step, the evolution of spatial and temporal gradients during the twenty-first century was examined. The mean annual cycle in atmospheric CO2 was underestimated for the historical period throughout the Northern Hemisphere, suggesting that the growing season net flux in the Community Land Model (the land component of CESM) was too weak. Consistent with weak summer drawdown in Northern Hemisphere high latitudes, simulated CO2 showed correspondingly weak north–south and vertical gradients during the summer. In the simulations of the twenty-first century, CESM predicted increases in the mean annual cycle of atmospheric CO2 and larger horizontal gradients. Not only did the mean north–south gradient increase due to fossil fuel emissions, but east–west contrasts in CO2 also strengthened because of changing patterns in fossil fuel emissions and terrestrial carbon exchange. In the RCP8.5 simulation, where CO2 increased to 1150 ppm by 2100, the CESM predicted increases in interannual variability in the Northern Hemisphere midlatitudes of up to 60% relative to present variability for time series filtered with a 2–10-yr bandpass. Such an increase in variability may impact detection of changing surface fluxes from atmospheric observations.
The CESM project is supported
by the National Science Foundation and the Office of
Science (BER) of the U.S. Department of Energy.
Computing resources were provided by the Climate
Simulation Laboratory at NCAR’s Computational and
Information Systems Laboratory (CISL), sponsored by
the National Science Foundation and other agencies.
G.K.A. acknowledges support of a NOAA Climate and
Global Change postdoctoral fellowship. J.T.R., N.M.M.,
S.C.D., K.L., and J.K.M. acknowledge support of Collaborative
Research: Improved Regional and Decadal
Predictions of the Carbon Cycle (NSF AGS-1048827,
AGS-1021776,AGS-1048890). TheHIPPO Programwas
supported byNSF GrantsATM-0628575,ATM-0628519,
and ATM-0628388 to Harvard University, University of
California (San Diego), and by University Corporation
for Atmospheric Research, University of Colorado/
CIRES, by the NCAR and by the NOAAEarth System
Research Laboratory. Sunyoung Park, Greg Santoni,
Eric Kort, and Jasna Pittman collected data during
HIPPO. The ACME project was supported by the Office
of Biological and Environmental Research of the U.S.
Department of Energy under Contract DE-AC02-
05CH11231 as part of the Atmospheric Radiation Measurement
Program (ARM), the ARM Aerial Facility,
and the Terrestrial EcosystemScience Program. TCCON
measurements at Eureka were made by the Canadian
Network for Detection of Atmospheric Composition
Change (CANDAC) with additional support from the
Canadian Space Agency. The Lauder TCCON program
was funded by the New Zealand Foundation for Research
Science and Technology contracts CO1X0204,
CO1X0703, and CO1X0406. Measurements at Darwin
andWollongong were supported by Australian Research
Council Grants DP0879468 and DP110103118 and
were undertaken by David Griffith, Nicholas Deutscher,
and Ronald Macatangay. We thank Pauli Heikkinen,
Petteri Ahonen, and Esko Kyr€o of the Finnish Meteorological
Institute for contributing the Sodankyl€a
TCCON data. Measurements at Park Falls, Lamont, and
Pasadena were supported byNASAGrant NNX11AG01G
and the NASA Orbiting Carbon Observatory Program.
Data at these sites were obtained by Geoff Toon, Jean-
Francois Blavier, Coleen Roehl, and Debra Wunch.
In situ atmospheric observations
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