Inertia

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Many aspects of the carbon-climate-human system change slowly, with a strong tendency to remain on established trajectories. As a consequence, serious problems may be

Figure 1.2. Effects of inertia in the coupled carbon-climate-human system. If there are delays associated with (1) assembling the evidence that climate has moved outside an acceptable envelope, (2) negotiating agreements on strategy and participation, and (3) developing new technologies to accomplish the strategies, then there will be additional delays associated with internal dynamics of the land and ocean system. As a consequence, the actual climate change may be far greater than that originally identified as acceptable.

Time

Figure 1.2. Effects of inertia in the coupled carbon-climate-human system. If there are delays associated with (1) assembling the evidence that climate has moved outside an acceptable envelope, (2) negotiating agreements on strategy and participation, and (3) developing new technologies to accomplish the strategies, then there will be additional delays associated with internal dynamics of the land and ocean system. As a consequence, the actual climate change may be far greater than that originally identified as acceptable.

effectively entrained before they are generally recognized (Figure 1.2). Effective management may depend on early and consistent action, including actions with financial costs. The political will to support these costs will require the strongest possible evidence on the nature of the problems and the efficiency of the solutions.

The carbon-climate-human system includes processes that operate on a wide range of timescales, including many that extend over decades to centuries. The slow components have added tremendously to the challenge of quantifying human impacts on ocean carbon (Sabine et al., Chapter 2) and ocean heat content (Levitus et al. 2000). They also prevent the ocean from quickly absorbing large amounts of anthropogenic carbon (Sabine et al., Chapter 2) and underlie the very long lifetime of atmospheric CO2.

Several new results highlight the critical role of inertia for the carbon cycle on land. It is increasingly clear that a substantial fraction of the current terrestrial sink, perhaps the majority, is a consequence of ecosystem recovery following past disturbances. Across much of the temperate Northern Hemisphere, changes in forestry practices, agriculture, and fire management have allowed forests to increase in biomass or area (Nabuurs, Chapter 16). Evidence that much of the recent sink on land is a result of land management has important implications for the future trajectory of the carbon cycle. Beginning with Bacastow and Keeling (1973), most estimates of future carbon sinks have assumed that recent sinks were a consequence of CO2 fertilization of plant growth and that past responses could be projected into the future with a CO2-sensi-tivity coefficient or beta factor (Friedlingstein et al. 1995). To the extent that recent sinks are caused by management rather than CO2 fertilization, past estimates of future sinks from CO2 fertilization are likely to be too optimistic (Gruber et al., Chapter 3). Eventual saturation in sinks from management (Schimel et al. 2001) gives them a very different trajectory from that of sinks from CO2 fertilization, especially those calculated by models without nutrient limitation (Prentice 2001).

In the human system, inertia plays a number of critical roles. The dynamics of development tend to concentrate future growth in carbon emissions in countries with developing economies (Romero Lankao, Chapter 19). This historical inertia, combined with potentially limited resources for carbon-efficient energy systems (Sathaye, Chapter 22), creates pressure for massive future emissions growth. Slowly changing institutions and incentive mechanisms in all countries (Lebel, Chapter 20) tend to entrain emissions trajectories further.

Inertia is profoundly important in the energy system, especially in the slow pace for introducing new technologies. The slow pace reflects not only the long time horizon for research and development, but also the long period required to retire existing capital stocks (Caldeira et al., Chapter 5). The long time horizon for bringing technologies to maturity and retiring capital stocks is only part of the timeline for the non-emitting energy system of the future, which also depends on the development of fundamentally new technologies (Hoffert et al. 2002). The search for fundamentally new energy sources cannot, however, constitute the entire strategy for action, because the entrained damage may be unacceptably large before new technologies are ready (Figure 1.2). A diverse portfolio of energy efficiency, new technologies, and carbon sequestration offers the strongest prospects for stabilizing atmospheric CO2 (Caldeira et al., Chapter 5).

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