Projecting the future of the U.S. carbon sink

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.

Figure 1

ED tracks patterns of land use and carbon stocks throughout the simulation. Shown here are estimated patterns of land use and average total carbon stocks (kg C⋅m⁻²) at four times in history: 1700, 1850, 1920, and 1990. In the land-use maps, each 1° × 1° grid cell is colored according to the fraction of the grid cell that is estimated to be in each of four land-use classes: primary vegetation (green), secondary vegetation (red), crop (yellow), and pasture (blue). In particular, each grid cell is shown as a stacked bar chart with colors in a fixed order. Spatial patterns of the relative amounts of land use in each of these four classes can be seen on each map. However, subgrid-scale spatial patterns and the impression of bands spanning a series of adjacent cells are the result of consistently applied coloring rules and do not illustrate spatial patterns of land use within grid cells or banded patterns of land use between grid cells.

Figure 2

Estimated average annual air-to-ground net flux in Pg C⋅y⁻¹ from 1700 to 1990. Positive values indicate a land sink and negative values indicate a source to the atmosphere. Light line, ref. without fire suppression. Dark line, ref. with fire suppression. Dark line with ⧫, ED.

Figure 3

(a) Estimated average annual area burned in km²⋅y⁻¹ from 1700 to 2100. (b) Projected average annual air-to-ground net flux in Pg C⋅y⁻¹ from 1700 to 2100. Positive values indicate a land sink and negative values indicate a source to the atmosphere. Diamonds represent ED; squares represent Miami-LU. Solid symbols represent cases assuming continued fire suppression. Empty symbols represent cases assuming fire suppression ceases.