Carbon balance of the terrestrial biosphere in the twentieth century: Analyses of CO₂, climate and land-use effects with four process-based ecosystem models

Global Biogeochemical Cycles, 15:183-206, 2001.

A. D. McGuire^1,2, S. Sitch³, J. S. Clein⁴, R. Dargaville⁴, G. Esser⁵, J. Foley⁶, M. Heimann⁷, F. Joos⁸, J. Kaplan⁷, D. W. Kicklighter⁹, R. A. Meier⁴, J. M. Melillo⁹, B. Moore III¹⁰, I. C. Prentice⁷, N. Ramankutty⁶, T. Reichenau⁵, A. Schloss¹⁰, H.Tian⁹, L. J. Williams¹¹, U. Wittenberg⁵

¹Authorship after McGuire and Sitch is alphabetical.

²U. S. Geological Survey, Alaska Cooperative Fish and Wildlife Research Unit, University of Alaska, Fairbanks, Alaska.

³Potsdam Institute for Climate Impact Research, Potsdam, Germany.

⁴Institute of Arctic Biology, University of Alaska, Fairbanks, Alaska.

⁵Institut fur Pflanzenokologie, Justus-Liebeg-Universitat, Giessen, Germany.

⁶Climate, People, and Environment Program, Institute for Environmental Studies, University of Wisconsin, Madison, Wisconsin.

⁷Max-Planck-Institut fur Biogeochemie, Jena, Germany.

⁸Physics Institute, University of Bern, Bern, Switzerland.

⁹The Ecosystems Center, Marine Biological Laboratory, Woods Hole, Massachusetts.

¹⁰Complex Systems Research Center, Institute for the Study of Earth, Oceans, and Space, University of New Hampshire, Durham, New Hampshire.

¹¹Electric Power Research Institute, Palo Alto, California.

Short Title: CO₂, Climate, Land-Use, and Global Carbon Storage

Abstract.

The concurrent effects of increasing atmospheric CO₂ concentration, climate variability, and cropland establishment and abandonment on terrestrial carbon storage between 1920 and 1992 were assessed using a standard simulation protocol with four process-based terrestrial biosphere models (HRBM, IBIS, LPJ and TEM). Three simulations were performed with each model in which (1) atmospheric CO₂ concentration alone was varied, (2) atmospheric CO₂ and climate were varied, and (3) atmospheric CO₂, climate and cropland extent were varied. Differences between the first two simulations provided estimates of the marginal effect of climate, and difference between the latter two simulations provided estimates of the marginal effect of cropland establishment and abandonment. Results were summarized over two time periods: from 1920 onwards, and for the 1980s. Over the long-term (1920 to 1992), all of the simulations that considered the concurrent effects of CO₂, climate, and land-use yielded a time history of terrestrial uptake that is consistent (within the uncertainty) with a long-term analysis based on ice core and atmospheric CO₂ data. Up to about 1960, three of four analyses indicated a net release of carbon from terrestrial ecosystems to the atmosphere. In the simulations, cropland establishment was the dominant cause of this release. After about 1960, all analyses indicate a net uptake of carbon by terrestrial ecosystems. In the simulations, cropland establishment continued to release carbon in the tropics but this release was exceeded by uptake, in part because of the physiological effects of rapidly rising atmospheric CO₂. During the 1980s, the simulations indicate that terrestrial ecosystems stored between 0.3 to 1.5 Pg C yr^-1, which is within the uncertainty of analysis based on CO₂ and O₂ budgets. Three of the four models indicated (in accordance with O₂ evidence) that the tropics were approximately neutral while a net sink existed in ecosystems north of the tropics. Although all of the models agree that the long-term effect of climate on carbon storage has been small relative to the effects of increasing atmospheric CO₂ and land-use, the models disagree as to whether climate variability and change in the twentieth century has promoted carbon storage or release. Simulated inter-annual variability from 1958 generally reproduced the ENSO-scale variability in the atmospheric CO₂ increase. However, there were substantial differences in the magnitude of inter-annual variability simulated by the models. The models also varied considerably in their ability to simulate the changing amplitude of the seasonal cycle of atmospheric CO₂. However, taken together, the seasonal cycle results strongly suggest that the observed trend is not only a consequence of CO₂ effects, but also that both climate variability and land-use changes have contributed. Thus, the results of this study presented here are to first order consistent with atmospheric data and provide a first consistent quantitative partitioning of total terrestrial biosphere-atmosphere exchanges. The analysis of differences among models and other analyses has identified some of the next steps required for improving the process-based simulation of historical terrestrial carbon dynamics. To enhance the ability of these models to more completely consider the factors influencing changes in terrestrial carbon storage requires improving the understanding of processes represented in the models and improving the data required for driving the models. The evaluation of the performance of the models in the context of a more complete consideration of the factors influencing historical terrestrial carbon dynamics is important for reducing uncertainties in representing the role of terrestrial ecosystems in future projections of the earth system.