Can we defuse the global warming time bomb?
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Climate forcing scenarios
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 7).
Figure 7. Climate forcing scenario for 20002050 that yields a forcing of 0.85 W/m2 (colored bars) (Reference 1a).
The IPCC added climate forcing in the next 50 years is 13 W/m2 for CO2 and 24 W/m2 with other gases and aerosols included. Even their minimum added forcing, 2 W/m2, 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 black carbon 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 in 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.
The growth rate of the GHG climate forcing peaked in the early 1980s at a rate of almost 0.5 W/m2 per decade, but declined by the 1990s to about 0.3 W/m2 per decade (Figure 8). The primary reason for the decline was reduced emissions of CFCs, the production of which was phased out because of their destructive effect on stratospheric ozone.
Figure 8. Growth rate of climate forcing by well-mixed greenhouse gases (5 year mean). O3 and stratospheric H2O, which were not well measured, are not included (Reference 1a).
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 about 1.6 ppm per year (Figure 9a). The CH4 growth rate has declined dramatically in the past 20 years, by at least two thirds (Figure 9b).
Figure 9. Growth rates of atmospheric CO2 and CH4 (Reference 1a; data update by Ed Dlugokencky and Tom Conway, NOAA Climate Monitoring and Diagnostics Laboratory, Boulder, CO).
These growth rates are related to the rate of global fossil fuel use (Figure 10). Fossil fuel emissions increased by more than 4% per year from the end of World War II until 1975, but subsequently by only about 1% per 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 were to continue, the added climate forcing in the next 50 years would be about 1.5 W/m2. 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/m2 in the next 50 years.
This "current trends" growth rate of climate forcings, i.e., 2 W/m2 in 50 years, is at the low end of the IPCC range of 24 W/m2. The IPCC scenario of 4 W/m2 requires a 4% per year exponential growth rate of CO2 emissions for 50 years and large growth of air pollution. The 4 W/m2 scenario yields dramatic climate change for the media to fixate upon, but it is implausible.
Although the "current trends" scenario of 2 W/m2 in 50 years is at the low end of the IPCC range, it is larger than the 1 W/m2 level that we suggested as our current best estimate for the level of DAI. This raises the question of whether there is a feasible scenario with still lower climate forcing.
A brighter future
I have discussed elsewhere (Reference 6) a specific "alternative scenario" that keeps added climate forcing in the next 50 years at about 1 W/m2. Expected global warming by 2050 is between 1/2 °C and 3/4 °C, i.e., a warming of about 1 °F (References 1b and 1c).
This alternative scenario has two components: (1) a halt or reversal of growth in air pollutants, specifically soot, O3, and CH4, and (2) maintenance of average fossil fuel CO2 emissions over the next 50 years at about the same level as today. The CO2 and non-CO2 portions of the scenario are equally important. I argue that they are both feasible and make sense for other reasons, in addition to climate.
Is it realistic to stop the growth of air pollution, or even achieve some reduction? A million people die every year from air pollution, with large economic cost. Actions to improve air quality have been initiated already in the United States and Europe, and still stricter standards are likely. In developing countries, such as India and China, air pollution is already about as bad as can be tolerated. Discussions among scientists from developed and developing countries (Reference 3) suggest that cleaner air is practical, and achievement could be speeded if there were concerted efforts to develop and share cleaner technologies.
In addressing air pollution, emphasis should be placed on the constituents that contribute most to global warming. Methane, a precursor of O3, is a substance expected to contribute much to future global warming. If human sources of CH4 are reduced, it may even be possible to get the atmospheric CH4 amount to decline, thus providing a cooling that would partially offset the CO2 increase. Reductions in black carbon aerosols would help counter the warming effect of reductions in sulfate aerosols. O3 precursors besides CH4, especially nitrogen oxides and volatile organic compounds, must be reduced to decrease low-level O3, the prime component of smog, which damages the human respiratory system and agricultural productivity.
Actions needed to reduce CH4, such as methane capture at landfills, waste management facilities, and fossil fuel mining, have economic benefits, through reduced health care expenditures, that partially offset their costs. Prime sources of black carbon are diesel fuels and biofuels. The tiny black carbon aerosols generated by the burning of these fuels are microscopic sponges that soak up toxic organic carbon emitted in the same burning process. When these minuscule soot particles are breathed into the lungs they deeply penetrate human tissue. Some enter the bloodstream and are suspected of being the primary carcinogen in air pollution. Diesel could be burned more cleanly with improved technologies. However, there may be even better solutions, such as hydrogen fuel, which would eliminate ozone precursors as well as soot.
CO2 will be the dominant anthropogenic climate forcing in the future. Is the CO2 portion of the alternative scenario feasible? It would require a near-term leveling off of fossil fuel CO2 emissions and a decline in CO2 emissions before mid-century, heading toward stabilization of atmospheric CO2 by the end of the century. Near-term leveling of emissions might be accomplished via improved energy efficiency and increased use of renewable energies, but a long-term decline in emissions will require development of energy technologies that produce little or no CO2 or that capture and sequester CO2.
The plausibility of flattening near-term CO2 emissions is suggested by the history of emissions (Figure 10). The reduction from 4% annual growth to 1% was accomplished mainly via improved energy efficiency and without a concerted global scale effort. Current technologies provide great potential for improvements in efficiency (Reference 5). The reported growth rate of global fossil fuel CO2 emissions in the 1990s was slightly less than 1% per year, despite robust economic growth in the United States, China, and the world as a whole (see Appendix). Concerted efforts to improve efficiencies and develop renewable energies have the potential to squeeze out an additional 1% in the near-term.
Long-term reduction in CO2 emissions is a greater challenge, as energy use will continue to rise. Progress is needed across the board: continued efficiency improvements, more renewable energy, and new technologies. Next-generation nuclear power, if acceptable to the public, could be an important contributor. There may be new technologies before 2050 that we have not imagined. A fallback, should greater fossil fuel use be necessary, is the capture and sequestration of CO2.
The impact of continual energy efficiency improvements must be recognized. Some analysts project a quadrupling of world energy needs by 2050 to 50 TW (power use today is 10 TW of fossil fuel energy and 2 TW from other sources). These same persons have been projecting such energy growth rates for years without comparing their prior predictions with data.
As an informative example, we compare in Figure 11 projections of United States energy use made in the early 1970s with actual energy use. The data show that energy use increased about 1% per year over the past three decades, far below most projections. Only in the past few years has energy use crept above the level that Amory Lovins, an advocate of energy efficiencies, had projected, and then only because the trend toward improving mileage of passenger vehicles was reversed in the past decade. Note that a moderate 1% per year growth in energy use was achieved in a period when the real cost of (fossil fuel) energy was declining. The flat energy usage from the 1970s to the 1980s was aided by energy price increases in the 1970s.
Figure 11. Projections of U.S. energy use made in the early 1970s compared iwth actual use. The growth of "soft" energy technologies (renewable energies, excluding large hydroelectric dams) advocated by Lovins (Reference 5) has not occurred to a noticeable extent, but his projection of total energy use was quite accurate.
The growth of "soft" energy technologies (renewable energies, excluding large hydroelectric dams) advocated by Lovins has not occurred to an extent sufficient to even show up in Figure 11. On the other hand, Lovins's projection of total energy use was accurate. Many opportunities exist for continued improvements in energy efficiency, e.g., of lighting technology and transportation. Thus it may be practical for total energy use in the U.S. to remain nearly constant for a substantial period. Furthermore, U.S. CO2 emissions will increase less than energy use if renewable energy contributions are increased. Thus it seems feasible for U.S. CO2 emissions to be constant or even decline.
Improvements in energy efficiency and moderation of energy growth rates are not limited to the U.S. Indeed, the U.S. fractions of global energy use and CO2 emissions actually increased slightly in the past decade (Reference 1a). Realistic moderate global energy growth rates, coupled with near-term emphasis on renewable energies and long-term technology development, could keep global CO2 emissions constant in the near-term and allow the possibility of long-term reductions, as may be required to avoid DAI with climate. Quantitative CO2 scenarios of this sort are presented in the Appendix.
Observed global CO2 and CH4 are shown in Figure 12. It is apparent that the real world is beginning to deviate from the prototypical IPCC scenario, IS92a. It remains to be proven whether the smaller observed growth rates are a fluke, soon to return to IPCC rates, or are a meaningful difference. The concatenation of the alternative scenario with observations is not surprising, since that scenario was defined with observations in mind. However, in the 2 years since the alternative scenario was defined, observations have continued on that path. Although I have shown that the IPCC scenarios are unrealistically pessimistic, I am not suggesting that the alternative scenario can be achieved without concerted efforts to reduce anthropogenic climate forcings.
Figure 12. Observed CO2 and CH4 amounts, compared with the typical IPCC scenario and the "alternative scenario." The alternative scenario falls below all IPCC scenarios for both CH4 and CO2 (see Appendix). In situ observations are available from the NOAA Climate Monitoring and Diagnostics Laboratory. CH4 in Antarctica is less than the global mean because the CH4 sources are primarily in the Northern Hemisphere (update of Reference 6a).
The alternative scenario falls below all scenarios in IPCC (2001), as illustrated in the Appendix for CO2. The same is true for the other major climate forcings that cause warming: CH4, tropospheric O3, and black carbon aerosols. It is likely that all these forcings are less than the IPCC pathways, but, unfortunately, except for CH4 and CO2, they are not being measured with an accuracy sufficient to define their rates of change.
The strategy for dealing with climate change must evolve as the level of forcing that produces DAI is better defined and as climate forcings are better measured. Monitoring of the ice sheets, together with realistic ice sheet modeling, will help determine how close the ice sheets are to accelerating retreat. Precise monitoring of ocean heat content change, averaged over several years, will yield the sum of all current forcings. Measurements of individual climate forcing agents will help define the most effective ways to stop global warming.