Earth System Models

Tropical ecosystems respond to climate changes, but the ecosystems themselves also exert influences on climate. For example, a number of studies have suggested that removal of the Amazonian forests may cause a warming of surface temperature and

Figure 14.1. Global changes in broadleaf tree cover illustrating importance of tropical forests in C02 feedback effects, (a) Change in cover simulated by the HadCM3LC coupled climate-carbon cycle model (Cox et al., 2000) from I860 to 2100 without C02 climate feedbacks. (b) Additional changes with inclusion of C02 climate feedbacks (© British Crown Copyright 2003, by kind permission of the Met Office).

reduction in precipitation, due to a reduced level of transpiration from the deforested landscape (see, e.g., Lean and Rowntree, 1997 for summary). Such effects may be crucial in maintaining local climates in a state amenable to the forests themselves (Betts, 1999). Changes in forest cover may also influence the climate through changes in the production of aerosol particles, which affect cloud formation and rainfall production. As well as influencing local climate, tropical ecosystem changes may also exert more far-reaching effects. For example, changes in carbon stocks affecting the rate of C02 rise and changes in the near-surface energy balance and cloud processes may modify atmospheric circulatory (Hadley) cells near the equator. Gedney and

Valdes (2000) have used robust atmospheric models to show that such changes in atmospheric circulation may have influences felt across the globe.

Given the potential for major feedbacks from ecosystems, it is clear that predictions of future climate change should consider ecosystem responses and their effect on climate. This has led to the development of "Earth System models'' which couple models of the atmosphere and oceans (GCMs) to models of the terrestrial and marine biosphere (DGVMs) (Foley et al., 1996; Cox et al., 2000; Ganapolski et al., 2001). Physical and biological models interact via biogeochemical cycles and through the impact of life on the physical properties of the Earth's surface. A number of such models have been developed with a wide range of spatial and temporal resolutions, attempting to trade off model complexity and detail against computational efficiency. The models used to study the interactions between climate change and tropical forests typically feature a DGVM and/or an interactive carbon cycle included within a GCM (Cox et al., 2000; Betts et al., 2004).

The inclusion of DGVMs in GCMs allows climate prediction simulations to include feedbacks from ecosystems responding to climatic changes at global and regional scales (Cox et al., 2000). Coupled GCM-DGVMs are therefore potentially valuable for understanding and predicting synergistic responses of ecosystems to climate change over timescales of centuries and spatial scales of hundreds of kilometers (Betts et al., 2004).

Early applications of Earth System models show that widespread increases or decreases in forest cover projected in response to CO2 rise and climate change may indirectly contribute to regional and global climate changes through alterations to land surface properties. Furthermore, net changes in terrestrial carbon stocks in the tropics and elsewhere may influence the rise in CO2 itself. Ecosystems may therefore exert a number of feedbacks on climate change, both at the regional and global scale.

In simulations using the Hadley Centre coupled climate-ecosystem model— HadCM3LC—the forests of Amazonia showed a very large reduction in tree cover as a result of decreased rainfall (Betts et al., 2004; Cox et al., 2004). Some signs of the beginning of this process were already simulated by 2000, with broadleaf tree cover reducing in the northeast of Amazonia in response to a drier climate than that simulated for 1860 (Figure 14.2). The reduction in rainfall spreads towards the southwest through the 21st century, and the tree cover reduces until it is less than 1% in the northeast quarter of Amazonia by 2100. Almost all of the Amazon Basin loses at least 50% of its tree cover by the end of the simulation, to be replaced mainly by C4 grass but also with large areas of bare soil. The general character of the region fundamentally changes from dense evergreen broadleaf forest to savanna, grassland, or even semi-desert.

The changes in tropical forest ecosystems in these simulations had significant impacts on regional climates through changes in the physical properties of the land surface. Although the drying climate in Amazonia emerged even when vegetation was fixed at the present day state, regional climate changes were significantly affected by vegetation feedbacks. In particular, precipitation reduction over Amazonia was found to be enhanced by 25% by feedbacks from the loss of forest cover. In the western part of the basin, the feedback was greater still because of the greater dependency of rainfall on recycling through evapotranspiration in the continental interior. Here

1850 1900 1950 2000 2050 2100 Year

Figure 14.2. Decline of Amazon forest biomass in six different DGYMs, under a climate projection from the HadCM2 climate model (Cox et al., 2004). These simulations do not account for the direct effect of rising atmospheric C02 in fertilization; other simulations including C02 fertilization using the same models showed smaller losses of biomass or small increases (© British Crown Copyright 2003, by kind permission of the Met Office).

1850 1900 1950 2000 2050 2100 Year

Figure 14.2. Decline of Amazon forest biomass in six different DGYMs, under a climate projection from the HadCM2 climate model (Cox et al., 2004). These simulations do not account for the direct effect of rising atmospheric C02 in fertilization; other simulations including C02 fertilization using the same models showed smaller losses of biomass or small increases (© British Crown Copyright 2003, by kind permission of the Met Office).

precipitation reduction was increased by over 30% as a result of drought-induced dieback of the forests, particularly to the east. Forest loss also increased surface albedo which reduced convection and moisture convergence, providing a further positive feedback on rainfall reduction (Charney, 1975).

The model simulations of Friedlingstein et al. (2001) also found a reduction in precipitation to be simulated in Amazonia, but the model did not include dynamic vegetation so there was no feedback on climate through biogeophysical effects. The model used by Thompson et al. (submitted) included the IBIS2 dynamic vegetation model (Foley et al., 1996; Kucharik et al., 2000), but this model did not produce a drying in Amazonia.

In order to estimate some constraints on sustainable conditions for the Amazonian forests with reference to changes that have occurred in the past, Cowling et al. (2003) used HadCM3LC to simulate the coupled paleoclimate-vegetation state in Amazonia at the last glacial maximum (LGM) 25,000 years ago. At the LGM, forest cover was maintained but was less productive, consistent with proxy data from the paleorecord. This was despite a drier climate and lower C02 concentrations, both of which are less favorable for forest cover. Cowling et al. suggested that the variation in forest structure (leaf area index) at the LGM might have acted to drive speciation and diversity in Amazonian forests, through mechanisms somewhat analogous to those that have been proposed for forest refugia, without loss of continuous forest cover. The critical aspect of the climate at the LGM was cooler temperatures, which helped to reduce both photorespiration and evapotranspiration, leading to decreased loss of carbon and water from the vegetation. Cowling et al. noted that—for the future state—warmer conditions are likely to amplify the effects of any drying of the regional climate that may occur despite the likely effects of elevated C02.

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