Setting the Stage The Earth System in the 21st Century

The vulnerability of many carbon cycle processes and pools depends on the magnitude of future climate change. The magnitude of future climate change, in turn, depends on the vulnerability of the carbon cycle. Therefore there is no unique answer, but it is nevertheless instructive for our ensuing analysis to put some bounds on the expected climate change.

Cubasch et al. (2001) provide a comprehensive review of the projected climate change for the 21st century on the basis of coupled atmosphere-ocean-land climate models and for a large range of radiative forcing projections. For the IS92a emission scenario, they project a mean warming of 1.3°C for the mid-21st century (2021-2050) relative to the 1961-1990 average, with a range from +0.8°C to + 1.7°C. This estimate includes the effect of sulphate aerosols, whereas the effect from greenhouse gases alone would be about 0.3°C higher. For the Special Report on Emissions Scenarios (SRES) A2 and B2 emission scenarios,1 the mean and range are very similar for the same time period. For the end of the 21st century (2071-2100), the mean global surface temperature is projected to increase by 3.0°C, with a range from 1.3°C to 4.5°C, while for the B2 scenario the mean temperature change is +2.2°C and the range is +0.9°C to +3.4°C. These individual scenarios can be compared with the full set of scenarios, for which Cubasch et al. (2001) give a range of + 1.4°C to + 5.8°C. Unfortunately, no comprehensive coupled climate models have simulated climate change using WRE (Wigley, Richels, Edmonds 1996) stabilization trajectories. Instead, to estimate the temperature changes associated with the WRE scenarios, Cubasch et al. (2001) used a simple climate model (Raper and Cubasch 1996) that has been tuned to the results of a number of comprehensive climate models. They find for the WRE 450 scenario (assuming stabilization at an atmospheric CO2 concentration of 450 ppm), a temperature change between + 1.2°C and +2.3°C for the latter part of this century, increasing to +1.6°C to +2.9°C for WRE550, and +1.8°C to +3.2°C for WRE650. The general pattern of warming is relatively uniform across a large range of models and generally consists of maximum warming in the high latitudes of the Northern Hemisphere and a minimum in the Southern Ocean. There is also a general tendency for the land to warm more than the ocean.

In some cases, even more important for the global carbon cycle is the projected change in the hydrological cycle. While uncertainties and intermodel differences are substantially larger for the hydrological cycle than for the projected temperature changes, the following trends appear to be quite robust: a general increase in the intensity of the hydro-logical cycle—that is, an increase in mean water vapor, evaporation, and precipitation; and a tendency for an increase in precipitation in the tropics and in the high latitudes and a reduction in precipitation in the subtropical latitudes (Cubasch et al. 2001).

As the coupling between the carbon cycle and human systems is tight, particularly with regard to the terrestrial carbon cycle, common features and regional differences in the development of population, the economy, and social systems in the 21st century are crucial in determining whether certain pools become at risk (Romero Lankao, Chapter 19, this volume). The SRES story lines of Nakicenovic et al. (2000) provide a basis for evaluating these interactions. In the A1 and B1 scenarios, global population is expected to increase to 9 billion people in 2050 and to decrease thereafter to about 7 billion people in 2100. In the A2 and B2 scenarios, global population is expected to grow continuously in the 21st century. Raupach et al. (Chapter 6, this volume) assess some impacts of these scenarios on land use change. We expect that the A2 scenario will lead to highest pressures on local resources, because of the underlying assumption of regionally different impacts on land, and "low incomes in developing countries lessening their ability to meet food demand by improved management or technology" (Raupach et al., Chapter 6).

In summary, the expected stresses on the global carbon cycle might range from relatively small to very substantial, depending on greenhouse gas emission pathways, the magnitude of climate sensitivity, and the development of human systems with regard to their pressure on terrestrial carbon resources. Given this non-negligible risk, it is of great importance to assess the magnitude of the carbon system feedbacks in greater detail. We do not adopt any specific emission scenario or climate sensitivity but tend to use the upper-bound scenarios, consistent with quantifying the maximum expected vulnerability (note that these are nonintervention scenarios, i.e., scenarios that do not contain purposeful human action to curb emissions or increase sinks).

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