Climate Change 2001:
Synthesis Report
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Figure 5-5: The recent net uptake of carbon on the land is partly due to enhanced CO2 uptake through plant growth, with a delay before this carbon is returned to the atmosphere via the decay of plant material and soil organic matter. Several processes contribute to the enhanced plant growth: changes in land use and management, fertilizing effects of elevated CO2 and nitrogen, and some climate changes (such as a longer growing season at high latitudes). A range of models (identified by their acronyms in the figure) project a continued increase in the strength of the net carbon uptake on land for several decades, then a leveling off or decline late in the 21st century for reasons explained in the text. The model results illustrated here arise from the IS92a scenario, but similar conclusions are reached using other scenarios.
5.11 Socialstructures andpersonalvaluesinteract withsociety'sphysicalinfrastructure, institutions, and the technologies embodied within them, and the combined system evolves relatively slowly. This is obvious, for instance, in relation to the impact of urban design and infrastructure on energy consumption for heating, cooling, and transport. Markets sometimes "lock in" to technologies and practices that are sub-optimal because of the investment in supporting infrastructure, which block out alternatives. Diffusion of many innovations comes up against people's traditional preferences and other social and cultural barriers. Unless advantages are very clear, social or behavioral changes on the part of technology users may require decades. Energy use and greenhouse gas mitigation are peripheral interests in most people's everyday lives. Their consumption patterns are driven not only by demographic, economic and technological change, resource availability, infrastructure, and time constraints, but also by motivation, habit, need, compulsion, social structures, and other factors.

WGIII TAR Sections 3.2, 3.8.6, 5.2-3, & 10.3, SRTT SPM, & SRTT Chapter 4 ES
5.12 Social and economic time scales are not fixed: They are sensitive to social and economic forces, and could be changed by policies and the choices made by individuals. Behavioral and technological changes can occur rapidly under severe economic conditions. For example, the oil crises of the 1970s triggered societal interest in energy conservation and alternative sources of energy, and the economy in most Organisation for Economic Cooperation and Development (OECD) countries deviated strongly from the traditional tie between energy consumption and economic development growth rates (see Figure 5-6). Another example is the observed reduction in CO2 emissions caused by the disruption of the economy of the Former Soviet Union (FSU) countries in 1988. The response in both case was very rapid (within a few years). The converse is also apparently true: In situations where pressure to change is small, inertia is large. This has implicitly been assumed to be the case in the SRES scenarios, since they do not consider major stresses, such as economic recession, large-scale conflict, or collapses in food stocks and associated human suffering, which are inherently difficult to forecast.

WGIII TAR Chapter 2,
WGIII TAR Sections 3.2 &, & WGII SAR Section 20.1
5.13 Stabilization of atmospheric CO2 concentration at levels below about 600 ppm is only possible with reductions in carbon intensity and/or energy intensity greater than have been achieved historically. This implies shifts toward alternative development pathways with new social, institutional, and technological configurations that address environmental constraints. Low historical rates of improvement in energy intensity (energy use per unit GDP) reflect the relatively low priority placed on energy efficiency by most producers and users of technology. By contrast, labor productivity increased at higher rates over the period 1980 to 1992. The historically recorded annual rates of mprovement of global energy intensity (1 to 1.5% per year) would have to be increased and maintained over long time frames to achieve stabilization of CO2 concentrations at about 600 ppm or below (see Figure 5-7). Carbon intensity (carbon per unit energy produced) reduction rates would eventually have to change by even more (e.g., up to 1.5% per year (the historical baseline is 0.3 to 0.4% per year)). In reality, both energy intensity and carbon intensity are likely to continue to improve, but greenhouse gas stabilization at levels below 600 ppm requires that at least one of them do so at a rate much higher than historically achieved. The lower the stabilization target and the higher the level of baseline emissions, the larger the CO2 divergence from the baseline that is needed, and the earlier it would need to occur.

WGI TAR Section, WGIII TAR Section 2.5, & SRES Section 3.3.4
5.14 Some climate, ecological, and socio-economic system changes are effectively irreversible over many human lifetimes, and others are intrinsically irreversible.

5.15 There are two types of apparent irreversibility. "Effective irreversibility" derives from processes that have the potential to return to their pre-disturbance state, but take centuries to millennia to do so. An example is the partial melting of the Greenland ice sheet. Another is the projected rise in mean sea level, partly as a result of melting of the cryosphere, but primarily due to thermal expansion of the oceans. The world is already committed to some sea-level rise as a consequence of the surface atmospheric warming that has occurred over the past century. "Intrinsic irreversibility" results from crossing a threshold beyond which the system no longer spontaneously returns to the previous state. An example of an intrinsically irreversible change due to crossing a threshold is the extinction of species, resulting from a combination of climate change and habitat loss. WGI TAR Chapter 11, WGII TAR Chapter 5, & WGII TAR Sections 16.2.1 & 17.2.5
Figure 5-6: The response of the energy system, as indicated by the emission of CO2 (expressed as carbon), to economic changes, indicated by GDP (expressed in Purchasing Power Parity (PPP) terms). The response can be almost without inertia if the shock is large. The "oil crisis" -- during which energy prices rose substantially over a short period of time -- led to an almost immediate and sustained divergence of the formerly closely linked emissions and GDP in most developed countries: Japan and United States are shown as examples. At the breakup of the Former Soviet Union, the two indicators remained closely linked, leading the emission to drop rapidly in tandem with declining GDP.
WGIII TAR Table 3.1 & WGII SAR Figure 20-1

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