THE INFLUENCE OF LAND USE CHANGE ON GLOBAL-SCALE
FLUXES OF CARBON FROM TERRESTRIAL ECOSYSTEMS
P. E. LEVY
and M. G. R. CANNELL
Centre for Ecology and Hydrology, Bush Estate, Penicuik, Midlothian EH26 0QB, U.K.
NASA GISS, 2880 Broadway, New York NY 10025, U.S.A.
Department of Mathematics, Heriot-Watt University, Edinburgh EH14 4AS, U.K.
Abstract. A process-based approach to modelling the effects of land use change and climate change
on the carbon balance of terrestrial ecosystems was applied at global scale. Simulations were run
both with and without land use change. In the absence of land use change between 1700 and 1990,
carbon storage in terrestrial ecosystems was predicted to increase by 145 Pg C. When land use change
was represented during this period, terrestrial ecosystems became a net source of 97 Pg C. Land use
change was directly responsible for a ﬂux of 222 Pg C, slightly higher but close to estimates from
other studies. The model was then run between 1990 and 2100 with a climate simulated by a GCM.
Simulations were run with three land use change scenarios: 1. no land use change; 2. land use change
speciﬁed by the SRES B2 scenario, and; 3. land use change scaled with population change in the B2
scenario. In the ﬁrst two simulations with no or limited land use change, the net terrestrial carbon
sink was substantial (358 and 257 Pg C, respectively). However, with the population-based land-use
change scenario, the losses of carbon through land use change were close to the carbon gains through
enhanced net ecosystem productivity, resulting in a net sink near zero. Future changes in land use
are highly uncertain, but will have a large impact on the future terrestrial carbon balance. This study
attempts to provide some bounds on how land use change may affect the carbon sink over the next
Emissions of carbon resulting from changes in land use comprise a substantial term
in the global carbon cycle. The major land use change which affects the terrestrial
carbon balance is the clearance of forest for agricultural use. It is estimated that
one sixth of the naturally occurring forest area has been cleared to date (Waring
and Running, 1998). To a substantial degree, estimates of emissions from land
use change determine the estimated size of the so-called “missing sink” in the
contemporary global carbon balance. This missing sink is the residual term re-
maining after the recognised sinks and sources have been accounted for. Emissions
from land use change are the most uncertain of the recognised sources, and largely
determine the residual. The missing sink is usually attributed to terrestrial vegeta-
tion (Keeling et al., 1996; Ciais et al., 1995), and there is considerable interest in
how this may change over the next century because of changing climate and CO
Climatic Change 67: 185–209, 2004.
2004 Kluwer Academic Publishers. Printed in the Netherlands.