Carbon balance of the terrestrial biosphere in the Twentieth Century: Analyses of CO₂, climate and land use effects with four process‐based ecosystem models

Abstract

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. Over the long-term (1920-1992), the simulations yielded a time history of terrestrial uptake that is consistent (within the uncertainty) with a long-term analysis based on ice core and atmospheric CO 2 data. Up to 1958, three of four analyses indicated a net release of carbon from terrestrial ecosystems to the atmosphere caused by cropland establishment. After 1958, all analyses indicate a net uptake of carbon by terrestrial ecosystems, primarily because of the physiological effects of rapidly rising atmospheric CO 2. During the 1980s the simulations indicate that terrestrial ecosystems stored between 0.3 and 1.5 Pg C yr '•, which is within the uncertainty of analysis based on CO2 and 02 budgets. Three of the four models indicated (in accordance with 02 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 CO 2 and land use, the models disagree as to whether climate variability and change in the twentieth century has promoted carbon storage or release. Simulated interannual variability from 1958 generally reproduced the E1 Ni•o/Southern Oscillation (ENSO)-scale variability in the atmospheric CO 2 increase, but there were substantial differences in the magnitude of interannual variability simulated by the models. The analysis of the ability of the models to simulate the changing amplitude of the seasonal cycle of atmospheric CO2 suggested that the observed trend may be a consequence of CO2 effects, climate variability, land use changes, or a combination of these effects. The next steps for improving the process-based simulation of historical terrestrial carbon include (1) the transfer of insight gained from standlevel process studies to improve the sensitivity of simulated carbon storage responses to changes in CO2 and climate, (2) improvements in the data sets used to drive the models so that they incorporate the timing, extent, and types of major disturbances, (3) the enhancement of the models so that they consider major crop types and management schemes, (4) development of data sets that identify the spatial extent of major crop types and management schemes through time, and (5) the consideration of the effects of anthropogenic nitrogen deposition. 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.•Authorship after McGuire and Sitch is alphabetical. 2Also at U.S. Geo...

Amplifying effects of land‐use change on future atmospheric CO2 levels

“…In addition to CO 2 fertilization, nitrogen deposition on ecosystems over industrialized continents, and variability in climate might contribute to modulate the uptake of carbon by the biosphere [Cannell, 1999]. Recent model runs by McGuire et al [2001] indicate that the effects of climate trends and variability over the past century are unclear, and can result either in an extra source or in an extra sink of atmospheric CO 2 depending on which terrestrial biosphere model is used. Similarly, variability or shifts in the ocean circulation, not accounted for in our model, could be responsible for the mismatch.…”

Systematic assessment of terrestrial biogeochemistry in coupled climate–carbon models

“…These tests of coupled models build upon an extensive intercomparison and evaluation history within the terrestrial biogeochemistry and land modeling communities (Schimel et al, 1997;Cramer et al, 2001;McGuire et al, 2001;Dargaville et al, 2002;Morales et al, 2005). However, a systematic framework evaluating the coupled behavior of the land carbon system as well as the interaction between climate and land biogeochemistry has been lacking, and is needed to reduce and assess uncertainties associated with future climate change projections.…”

Impact of tropospheric ozone on the Euro-Mediterranean vegetation

“…Terrestrial ecosystem models (TEMs) have been extensively used to study the processes leading to either carbon loss or gain by land ecosystems (McGuire et al 2001;Prentice et al 2001). However, a factor that has received relatively little attention for its role in terrestrial carbon dynamics is tropospheric ozone (O 3 ).…”