The Mitigation Challenge Which Sectors and Gases Are Most Important

In order to identify the most productive mitigation strategies, it is necessary to understand the current and projected sources of CO2 and the other greenhouse gases. The author has derived the information in Fig. 1.20 from IEA [6]. This graphic projects world CO2 emissions by sector. The emission growth rates are consistent with the business as usual base case, discussed previously: 1.6% from 2000 to 2030, and 2.2% from 2030 to 2050. It suggests that power generation and transportation sources are the fastest growing sectors and controlling these sources will be the key to any successful mitigation strategy. There is historical evidence that, as a country develops economically, it uses greater quantities of electrical power and experiences a sharp growth in the number and use of motor vehicles and other transportation sources. As mentioned earlier, China and India, with a cumulative population of over 2.5 billion, are projected to continue their rapid economic

Fig. 1.19 Two mitigation scenarios starting in 2025: original (green) assumed 1.6% emission growth rate from 2000 to 2025, followed by an annual 1% reduction; revised (red) assumed a 3.0% growth rate from 2000 to 2025, followed by an annual 1% reduction

expansion with commensurate pressure on the power generation and transportation sectors. It should also be noted that the energy transformation category in Fig. 1.20 includes petroleum refining, natural gas, and coal conversion to liquids and biomass to alcohols, much of which will feed the transportation sector.

■ Energy Transformation

■ Power Generation

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■ Energy Transformation

■ Power Generation

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Fig. 1.20 Projected global CO2 emission growth for key economic sectors, Gt per year

Fig. 1.20 Projected global CO2 emission growth for key economic sectors, Gt per year

US GHG Emission Chart © WRI

Fig. 1.21 U.S. energy and GHG emission flows by sector, end use, and gas in 2003

US GHG Emission Chart © WRI

Fig. 1.21 U.S. energy and GHG emission flows by sector, end use, and gas in 2003

For the United States, the WRI [11] has generated Fig. 1.21, illustrating the relationship between sectors; end use/activities; and greenhouse gas emissions, including methane and nitrous oxide sources in CO2 equivalents, for the year 2003. This graphic illustrates the relative importance and relationship of the power generation (electricity and associated waste heat in the figure), transportation, and industrial production, and the end use of energy in residential, commercial buildings, and industrial operations.

Gases other than CO2 contribute significantly to warming. Figure 1.21 illustrates this for the United States. Although CO2 is the dominant driver, methane and nitrous oxide are significant, together contributing 13% of the warming driving force. For the global view of the relative significance of the key greenhouse gases, Fig. 1.22 was generated using the MAGICC model. This figure illustrates the relative driving force of the key greenhouse gases for 2020, 2050, and 2100 assuming emissions per the modified IEA base case for CO2 and IPCC [1] Scenario WRE750 for the other greenhouse gases. For this scenario, methane emissions are projected to grow at 0.5% per year until 2050, and remain constant for the next 50 years. For N2O, emissions are assumed to grow at 0.4% per year until 2050 and the slow to a 0.1% growth rate until 2100. Also note for the forestry sector CO2 net emissions are projected to decrease at about 2% per year to zero by 2075. Note that mitigating emissions of methane, a short-lived gas, allows for more near-term warming moderation, in contrast to a long-lived gas such as CO2. Also note Fig. 1.22 projects that fine particles contribute a cooling effect in 2020 that transforms to a warming effect in later years. This is explained since emissions of sulfur dioxide are projected to increase until 2020, whereas the emissions will be reduced later in the century as countries install controls to mitigate that health and ecological impact of SO2 and acidic sulfates. With such emission control, concentrations of sulfate particles, which form from SO2 in the atmosphere and reflect incoming solar radiation, will consequently be reduced and their cooling effect reduced, yielding warming relative to 1990.

It is important to note that black carbon (BC), a component of fine particles, is a significant contributor to global warming even though the overall impact of fine

Gases Contributing to Greenhouse Warming IEA Base Case- No Control jj O

Gases Contributing to Greenhouse Warming IEA Base Case- No Control jj O

2020 2050 2100

Fig. 1.22 Thermal driving forces (watts per square meter) of major GHGs relative to 1990

2020 2050 2100

Fig. 1.22 Thermal driving forces (watts per square meter) of major GHGs relative to 1990

particles, dominated by reflective sulfates, is cooling. BC has a short atmospheric lifetime, is not well mixed in the atmosphere, and is a product of incomplete fuel combustion of fossil fuels and biomass. The sources of BC are widely dispersed and not well characterized, but appear dominated by mobile and stationary diesel engines, and residential fuel combustion in developing countries [12].

As mentioned earlier, this and subsequent chapters focus on energy technologies, and only CO2 will be discussed, since it is the critical greenhouse gas and is growing at a fast rate. However, as noted earlier, an aggressive methane mitigation program could add about 0.3°C warming mitigation, to that achieved via CO2 mitigation by 2100. A major mitigation program for ozone precursors and N2O emissions could yield another 0.3°C warming reduction.

BC emission control could also contribute to warming mitigation, but the magnitude of this potential impact is difficult to quantify given the many uncertainties involved. However, Princeton [12] has recently estimated that a global BC mitigation program could yield a best guess value of 0.29 W/m2 decrease in the global thermal driving force in 2100. This is comparable to the level achievable with an aggressive methane mitigation program. Given the short atmospheric lifetimes of BC, such benefits, if available, have the potential to be realized in the near term.

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