Projecting the future of the U.S. carbon sink

^*Institute for the Study of Earth Oceans and Space, University of New Hampshire, Durham, NH 03824; ^‡Department of Ecology and Evolutionary Biology, Princeton University, Princeton, NJ 08544; and ^¶Woods Hole Research Center, Woods Hole, MA 02543

Edited by Christopher B. Field, Carnegie Institution of Washington, Stanford, CA, and approved October 29, 2001 (received for review May 18, 2001)

Abstract

Atmospheric and ground-based methods agree on the presence of a carbon sink in the coterminous United States (the United States minus Alaska and Hawaii), and the primary causes for the sink recently have been identified. Projecting the future behavior of the sink is necessary for projecting future net emissions. Here we use two models, the Ecosystem Demography model and a second simpler empirically based model (Miami Land Use History), to estimate the spatio-temporal patterns of ecosystem carbon stocks and fluxes resulting from land-use changes and fire suppression from 1700 to 2100. Our results are compared with other historical reconstructions of ecosystem carbon fluxes and to a detailed carbon budget for the 1980s. Our projections indicate that the ecosystem recovery processes that are primarily responsible for the contemporary U.S. carbon sink will slow over the next century, resulting in a significant reduction of the sink. The projected rate of decrease depends strongly on scenarios of future land use and the long-term effectiveness of fire suppression.

Footnotes

↵ † To whom reprint requests should be addressed. E-mail: george.hurtt{at}unh.edu.
↵ § Present address: Department of Organismic and Evolutionary Biology, Harvard University, Cambridge, MA 02138.
↵ ‡‡ The hypothesis that the sink in the coterminous U.S. on decadal time scales is primarily caused by net ecosystem recovery and other factors associated with land use and land-use history is consistent with the following lines of evidence: the inconsistency of substantially enhanced forest vital rates with forest inventory data (2), fertilization experiments that suggest that resource limitation may limit the enhancement of carbon sequestration in forests (18), the documentation of sink terms such as woody encroachment that are relatively unambiguously related to factors such as fire suppression (1, 6), the stability of the forest sink over last 40 years based on forest inventories (9–11), and the stability of the U.S. sink generally over last 15 years based on recent atmospheric inversions (1).
↵ †† To maintain constant net emissions, fossil fuel emissions must be reduced by an amount equal to declines in the carbon sink. Thus it is possible to convert model estimates of the decline in the sink into estimates of how much fossil fuel emissions would have to be reduced to compensate. In the calculation presented here, net emissions from the coterminous U.S. in 1990 were estimated by subtracting a sink of 1/3–2/3 Pg C⋅y⁻¹ (1) from fossil fuel emissions of 1.337 Pg C (19). The decline in the sink was estimated by using ED (this study). For simplicity, the necessary cuts in emissions cited in the text are calculated such that the total net emissions over the interval (1990–2100) equal those from maintaining constant 1990 net emissions over the same interval. Holding net emissions to 1990 levels on a year-by-year basis would require cuts in fossil fuel emissions that vary through time and that range from lower to higher than the average values cited. The low estimate (7%) is derived from the scenario in which fire suppression efforts continue. The high estimate (30%) is derived from the scenario in which fire suppression efforts fail. These requirements are in addition to the reduction in fossil fuel emissions currently needed to achieve 1990 net emissions.
This paper was submitted directly (Track II) to the PNAS office.
↵ ‖ The dynamics of ED and Miami-LU are very similar. However, there are differences at finer scales. Miami-LU recovers faster from prior land use than ED, as evidenced in Fig. 3, probably because it lacks mechanisms responsible for the longertime scales of succession.
↵ ** Unlike previous studies, reasonable estimates of the sink on decadal time scales are obtained here without ecophysiological mechanisms. This does not preclude the importance of ecophysiological mechanisms in influencing the sink on seasonal and inter-annual time scales, or over longer time scales under future environmental change. The estimates in this study are similar to estimates from Houghton et al for agricultural lands and fire suppression (ref. 6, but see revision to cropland estimate in ref. 1.) and estimates of the magnitude and cause of the forest sink based on U.S. Forest Inventory Analysis data (1, 2).
Abbreviations:

ED,

Ecosystem Demography;

Miami-LU,

Miami Land Use History