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PHYSICAL SCIENCES / ENVIRONMENTAL SCIENCES
The changing carbon cycle at Mauna Loa Observatory
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*Berkeley Atmospheric Sciences Center and Department of Geography, University of California, Berkeley, CA 94720; and ||National Aeronautics and Space Administration/Goddard Space Flight Center, Greenbelt, MD 20771
Contributed by Inez Y. Fung, December 29, 2006 (received for review July 6, 2006)
The amplitude of the CO2 seasonal cycle at the Mauna Loa Observatory (MLO) increased from the early 1970s to the early 1990s but decreased thereafter despite continued warming over northern continents. Because of its location relative to the large-scale atmospheric circulation, the MLO receives mainly Eurasian air masses in the northern hemisphere (NH) winter but relatively more North American air masses in NH summer. Consistent with this seasonal footprint, our findings indicate that the MLO amplitude registers North American net carbon uptake during the warm season and Eurasian net carbon release as well as anomalies in atmospheric circulation during the cold season. From the early 1970s to the early 1990s, our analysis was consistent with that of Keeling et al. [Keeling CD, Chin JFS, Whorf TP (1996) Nature 382:146–149], suggesting that the increase in the MLO CO2 amplitude is dominated by enhanced photosynthetic drawdown in North America and enhanced respiration in Eurasia. In contrast, the recent decline in the CO2 amplitude is attributed to reductions in carbon sequestration over North America associated with severe droughts from 1998 to 2003 and changes in atmospheric circulation leading to decreased influence of Eurasian air masses. With the return of rains to the U.S. in 2004, both the normalized difference vegetation index and the MLO amplitude sharply increased, suggesting a return of the North American carbon sink to more normal levels. These findings indicate that atmospheric CO2 measurements at remote sites can continue to play an important role in documenting changes in land carbon flux, including those related to widespread drought, which may continue to worsen as a result of global warming.
atmospheric circulation | atmospheric CO2 seasonal cycle | terrestrial carbon sinks | continental droughts
Author contributions: W.B. and I.Y.F. designed research; W.B., B.R.L., C.D.K., and A.A. performed research; W.B., J.E.P., and C.J.T. analyzed data; W.B. and I.Y.F. wrote the paper; and J.E.P. and C.J.T. contributed NDVI data.
Present address: Department of Atmospheric and Oceanic Sciences and Institute of Geophysics and Planetary Physics, University of California, Los Angeles, CA 90095.
¶Present address: Department of Environmental Sciences and Energy Research, Weizmann Institute of Science, Rehovot 76100, Israel.
The authors declare no conflict of interest.
This article contains supporting information online at www.pnas.org/cgi/content/full/0611224104/DC1.
To whom correspondence may be sent at the present address: Center for Tropical Research, UCLA Institute of the Environment, P.O. Box 951496, Los Angeles, CA 90025-1496. E-mail: buermann{at}ucla.edu
**To whom correspondence may be addressed. E-mail: ifung{at}uclick.berkeley.edu
© 2007 by The National Academy of Sciences of the USA
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