The IPCC defines many climate forcing scenarios for the 21st century based on multifarious "story lines" for population growth, economic development, and energy sources. The scenarios lead to a wide range for added climate forcings in the next 50 years (vertical bars in Figure 8).
The IPCC added climate forcing in the next 50 years is 1-3 W/m2 for CO2 and 2-4 W/m2 with other gases and aerosols included. Even their minimum added forcing, 2 W/m , would cause DAI with the climate system, based on our criterion. Further, IPCC studies suggest that the Kyoto Protocol, designed to reduce greenhouse gas emissions from developed countries, would reduce global warming by only several percent. Gloom and doom seem unavoidable.
However, are the IPCC scenarios necessary or even plausible? There are reasons to believe that the IPCC scenarios are unduly pessimistic. First, they ignore changes in emissions, some already underway, due to concerns about global warming. Second, they assume that true air pollution will continue to get worse, with O3, CH4 and BC all greater in 2050 than in 2000. Third, they give short shrift to technology advances that can reduce emissions in the next 50 years.
An alternative way to define scenarios is to examine current trends of climate forcing agents, to ask why they are changing as observed, and to try to understand whether there are reasonable actions that could encourage further changes in the growth rates. Precise data are available for trends of the long-lived greenhouse gases (GHGs) that are well-mixed in the atmosphere, i.e., CO2, CH4, N2O and CFCs (chlorofluorocarbons).
The growth rate of the GHG climate forcing peaked in the early 1980s at a rate of almost 22 0.5 W/m per decade, but declined by the 1990s to about 0.3 W/m per decade (Figure 9). The primary reason for the decline was reduced emissions of CFCs, whose production was phased out because of the destructive effect of CFCs on stratospheric ozone.
The two most important GHGs, with CFCs on the decline, are CO2 and CH4. The growth rate of CO2, after surging between the end of World War II and the mid-1970s, has since almost flattened out to an average growth rate of 1.7 ppm/year over the past decade (Figure 10a). Although the exponential growth rate of CO2 has slowed, the annual increments of atmospheric CO2 continue to increase, and they are likely to continue to grow until annual CO2 emissions flatten out or begin to decline. The annual CO2 increment has exceeded 2 ppm/year in three of the past six years. The CH4 growth rate has declined dramatically in the past 20 years, by at least two-thirds (Figure 10b).
These growth rates are related to the rate of global fossil fuel use (Figure 11). Fossil fuel emissions increased by more than 4%/year from the end of World War II until 1975, but subsequently by just over 1%/year. The change in fossil fuel growth rate occurred after the oil embargo and price increases of the 1970s, with subsequent emphasis on energy efficiency. CH4 growth has also been affected by other factors including changes in rice farming and increased efforts to capture CH4 at landfills and in mining operations.
If recent growth rates of these GHGs continued, the added climate forcing in the next 50 years would be about 1.5 W/m . To this must be added the (positive or negative) change due to other forcings such as O3 and aerosols. These forcings are not well-monitored globally, but it is known that they are increasing in some countries while decreasing in others. Their net effect should be small, but it could add as much as
0.5 W/m2. Thus, if there is no slowing of emission rates, the human-made climate forcing could increase by 2 W/m in the next 50 years.
This "current trends" growth rate of climate forcings, 2 W/m2 in 50 years, is at the low
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