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Figure 4.1. Carbon emissions, 1990-2100.

1990 2010 2030 2050 2070 2090

Year

Figure 4.1. Carbon emissions, 1990-2100.

concentrations. The effects of energy intensity improvements and the deployment of non-carbon-emitting energy supply technologies are large. Power generation is 75 percent carbon free by the year 2100, and modern commercial biomass provides more energy than the combined global production of oil and gas in 1990.

Even under the advanced technology assumptions of IS92a, emissions will continue to grow. They will increase at a significantly slower rate than they would have without the technology developments envisioned by IS92a. Nevertheless, under the IS92a scenario, the concentration of carbon dioxide will rise to more than 700 parts per million (ppm) by the end of the 21st century—nearly triple the preindustrial level— and will continue rising.

The lower curve depicts an emissions path consistent with a 550-ppm concentration ceiling. This curve is depicted for illustrative purposes. While the Framework Convention on Climate Change commits the governments of the world to stabilize the concentration of greenhouse gases, it is silent as to which concentration. The concentration at which CO2 is stabilized may turn out to be 350 ppm or 1,000 ppm.

Regardless of the concentration at which CO2 is stabilized, the fact that CO2 concentrations are determined by cumulative, not annual, global emissions implies that eventually global CO2 emissions must peak and then begin a long-term, indefinite decline, as in this example.

We define the difference between carbon emissions that are anticipated to occur in a world that places no value on carbon and emissions required to stabilize at a specific concentration level as the "gap." Closing the gap means effecting a change in the technologies that are anticipated to come into use under the IS92a scenario. Substantial development of energy technologies is necessary to achieve the IS92a goals, and even greater development and deployment is needed to achieve stabilization. In the sections that follow, we explore how the size of the gap is affected by assumptions regarding (1) business as usual emissions, (2) the CO2 concentration target, and (3) key components of the carbon cycle. We also show how the gap may be filled under alternative assumptions regarding technology cost and availability.

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