CONTROL ID: 1487680
TITLE: Evaluating carbon dioxide variability in the Community Earth System Model against atmospheric observations
AUTHORS (FIRST NAME, LAST NAME): Gretchen Keppel-Aleks1, James Tremper Randerson1, Keith T Lindsay2, Britton B Stephens2, Jefferson Keith Moore1, Scott C Doney3, Peter E Thornton4, Natalie M Mahowald5, Forrest M Hoffman1, 4, Colm Sweeney6, Pieter P Tans6, Paul O Wennberg7, Steven C Wofsy8
INSTITUTIONS (ALL): 1. Earth System Science, UC-Irvine, Irvine, CA, United States.
2. NCAR, Boulder, CO, United States.
3. Woods Hole Oceanographic Institution, Woods Hole, MA, United States.
4. Oak Ridge National Laboratory, Oak Ridge, TN, United States.
5. Cornell University, Ithaca, NY, United States.
6. National Oceanic and Atmospheric Administration, Boulder, CO, United States.
7. California Institute of Technology, Pasadena, CA, United States.
8. Harvard University, Cambridge, MA, United States.
ABSTRACT BODY: Changes in atmospheric CO2 variability during the 21st century may provide insight on ecosystem responses to climate change and have implications for the design of carbon monitoring programs. We analyzed results from a fully coupled climate-carbon simulation using the Community Earth System Model (CESM1-BGC). We evaluated CO2 simulated for the historical period against surface, aircraft, and column observations. The mean annual cycle in total column atmospheric CO2 was underestimated throughout the northern hemisphere relative to TCCON observations, suggesting that the growing season net flux in the land component of CESM was too weak by 50%. Sampling CESM along HIPPO transects confirmed low growing season uptake, but also showed that spring drawdown in the Northern Hemisphere began too early. The vertical gradients in CESM generally agreed with HIPPO data and with NOAA aircraft profiles outside the growing season, but were too weak during the summer. The seasonal bias suggests that vertical transport in CAM4 (the atmospheric component of CESM) was too weak year round. Model evaluation and improvement based on atmospheric observations is crucial. The simulation of surface exchange and atmospheric transport of CO2 in coupled models such as CESM may help with the design of optimal detection strategies. For example, in the simulations of the 21st century, CESM predicted increases in the mean annual cycle of atmospheric CO2 and larger horizontal gradients. Both north-south and east–west contrasts in CO2 strengthened due to changing patterns in fossil fuel emissions and terrestrial carbon exchange, and northern hemisphere interannual variability increased as well. Our results suggest that using atmospheric observations to gain insight about changing terrestrial and ocean processes over the next several decades may become more challenging as anthropogenic contributions to variability on multiple temporal and spatial scales continue to grow.
KEYWORDS:  ATMOSPHERIC COMPOSITION AND STRUCTURE / Biosphere/atmosphere interactions,  ATMOSPHERIC COMPOSITION AND STRUCTURE / Troposphere: constituent transport and chemistry,  BIOGEOSCIENCES / Carbon cycling,  BIOGEOSCIENCES / Trace gases.
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Previously Presented Material: Similar material was presented at the NOAA GMD annual meeting in May 2012. A paper based on this material was submitted to J. Climate in July 2012.
CONTACT (NAME ONLY): Gretchen Keppel-Aleks
CONTACT (E-MAIL ONLY): gka at gps dot caltech dot edu
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