BOX 2.1

STABILIZATION AND NON-CO2GREENHOUSE GASES

Because carbon emissions reductions of more than 80% are required to stabilize carbon dioxide concentrations, small continuing emissions of carbon dioxide, or emissions of CO2-equivalent through other gases, could have surprisingly important implications for stabilizing climate change. For example, emissions of the hydrofluorocarbons (HFCs) currently used as substitutes for chlorofluorocarbons make a small contribution to today’s climate change. However, because emissions of these gases are expected to grow in the future if they are not mitigated, and because of the stringency of the requirement of near zero emissions of CO2-equivalent, these gases could represent a significant future impediment to stabilization efforts. For example, Figure 2.4 below shows that in the absence of mitigation, the HFCs could represent as much as one-third of the allowable CO2-equivalent emissions in 2050 required for a stabilization target of 450 CO2-equivalent. Thus, the analysis presented here underscores that stabilization of climate change requires consideration of the full range of greenhouse gases and aerosols, and of the full suite of emitting sectors, applications, and nations.

FIGURE 2.4 Global CO2 and HFC emissions expressed as CO2-equivalent emissions per year for the period 2000-2050. The emissions of individual HFCs are multiplied by their respective GWPs (direct, 100-year time horizon) to obtain aggregate emissions across all HFCs expressed as equivalent GtCO2 per year. High and low estimated ranges based on analysis of likely demand for these gases and assuming no mitigation of HFCs are shown. HFC emissions are compared to emissions for the range of SRES CO2 scenarios, and two 450-and 550-ppm CO2 stabilization scenarios. The estimated CO2-equivalent emissions due to HFCs in the absence of mitigation reach about 6 GtCO2-equivalent in 2050, or about a third of the emissions due to CO2 itself at that time in the 450 ppm stabilization scenario. Source: Velders et al. (2009).

FIGURE 2.4 Global CO2 and HFC emissions expressed as CO2-equivalent emissions per year for the period 2000-2050. The emissions of individual HFCs are multiplied by their respective GWPs (direct, 100-year time horizon) to obtain aggregate emissions across all HFCs expressed as equivalent GtCO2 per year. High and low estimated ranges based on analysis of likely demand for these gases and assuming no mitigation of HFCs are shown. HFC emissions are compared to emissions for the range of SRES CO2 scenarios, and two 450-and 550-ppm CO2 stabilization scenarios. The estimated CO2-equivalent emissions due to HFCs in the absence of mitigation reach about 6 GtCO2-equivalent in 2050, or about a third of the emissions due to CO2 itself at that time in the 450 ppm stabilization scenario. Source: Velders et al. (2009).

tion, due to the irreversible character of the induced warming driven by carbon dioxide (see Section 3.4).

2.2
INFORMATION FROM SCENARIOS

Figure 2.5 shows the emissions of manmade greenhouse gases from various sectors of the U.S. economy (U.S. EPA, 2008). For highly industri-



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