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Research Features

Global Warming in the 21st Century: An Alternative Scenario

Global warming in recent decades has taken global temperature to its highest level in the past millennium (Mann et al. 1999). There is a growing consensus (IPCC 1996) that the warming is at least in part a consequence of increasing anthropogenic greenhouse gases.

Greenhouse gases (GHGs) cause a global climate forcing, i.e., an imposed perturbation of Earth's energy balance with space (Hansen et al. 1997). Specifically, GHGs reduce heat radiation to space, causing Earth to warm. There are many competing natural and anthropogenic climate forcings, but increasing GHGs are estimated to be the largest forcing and to result in a net positive forcing, especially during the past few decades (IPCC 1996, Hansen et al. 1998).

Climate models driven by "business-as-usual" GHG scenarios for the 21st century yield a global warming of several degrees that would almost surely have detrimental effects on humans and wildlife (IPCC 1996). Such GHG scenarios can leave the impression that curtailment of global warming is almost hopeless. The 1997 Kyoto Protocol, which calls for industrialized nations to reduce their CO2 emissions to 95% of 1990 levels by 2012 (Bolin 1998), is itself considered a difficult target to achieve. Yet the climate simulations lead to the conclusion that the Kyoto reductions will have little effect in the 21st century (Wigley 1998), and "thirty Kyotos" may be needed to reduce warming to an acceptable level (Malakoff 1998).

We suggest equal emphasis on an alternative, more optimistic, scenario that emphasizes reduction of non-CO2 GHGs and black carbon during the next 50 years. This scenario derives from our interpretation that observed global warming has been caused mainly by non-CO2 GHGs. Although this interpretation does not alter the desirability of slowing CO2 emissions, it does suggest that it is more practical to slow global warming than is sometimes assumed.

Climate Forcings

Figure 1 shows estimated climate forcings since 1850, measured in Watts per square meter (W/m2). We separate carbon dioxide (CO2), methane (CH4), chlorofluorocarbons (CFCs) and ozone (O3) in Figure 1, because they are produced by different processes and have different growth rates. We associate with CH4 its indirect effects on tropospheric O3 and stratospheric H2 to make clear the importance of CH4 as a climate forcing.

Fig 1

Fig. 1: Estimated climate forcings between 1850 and 2000.

Climate forcing by CO2 is the largest forcing, but it does not dwarf the others. Forcing by CH4 (0.7 W/m2) is half as large as that of CO2 and the total forcing by non-CO2 GHGs (1.4 W/m2) equals that of CO2. Moreover, in comparing forcings due to different activities, note that the fossil fuels producing most of the CO2 are also the main source of atmospheric aerosols, especially sulfates, black carbon, and organic aerosols.

Aerosols cause a climate forcing directly by reflecting sunlight and indirectly by modifying cloud properties. Forcing by atmospheric aerosols is uncertain, but research of the past decade indicates that it is substantial (IPCC 1996). The aerosol forcing that we estimate (4) has the same magnitude (1.4 W/m2) but opposite sign of the CO2 forcing. Fossil fuel use is the main source of both CO2 and aerosols, with land conversion and biomass burning also contributing to both forcings. Although fossil fuels contribute to growth of some of the other GHGs, it follows that the net global climate forcing due to processes that produced CO2 in the past century probably is much less than 1.4 W/m2.

A corollary following from Figure 1 is that climate forcing by non-CO2 GHGs (1.4 W/m2) is nearly equal to the net value of all known forcings for the period 1850-2000 (1.6 W/m2). Thus, assuming only that our estimates are approximately correct, we assert that the processes producing the non-CO2 GHGs have been the primary drive for climate change in the past century.

Observed heat storage in the ocean provides a fundamental consistency check on the estimated climate forcing. The ocean is the only place that the energy from a planetary radiation imbalance can accumulate, because of the low thermal conductivity of land and the limit on ice melting implicit in observed sea level rise. Global ocean data (Levitus et al. 2000) reveal that ocean heat content increased 2*1023 joules between the mid-1950s and the mid-1990s. The simplest interpretation is that the change of ocean heat content, and the implied planetary energy imbalance, are a reflection of the net global climate forcing. Observed heat storage (Levitus et al. 2000) is in good agreement with results in global climate models that use the forcings of Figure 1, thus providing empirical evidence for the sign and approximate magnitude of the net climate forcing of Figure 1.

Fig 2

Fig. 2: Climate forcings by individual greenhouse gases: (A) CO2, (B) CH4 and N2O, (C) CFC-11 and CFC-12, based on trace gas data available from the National Oceanic and Atmospheric Administration Climate Monitoring and Diagnostics Laboratory.

Greenhouse Gas Growth Rates

Atmospheric amounts of the principal human-influenced GHGs have been monitored in recent years and extracted for earlier times from bubbles of air trapped in polar ice sheets.

The growth rate of forcing by carbon dioxide doubled between the 1950s and the 1970s (Figure 2A), but was flat from the late 1970s until the late 1990s despite a 30% increase in fossil fuel use. This implies a recent increase of terrestrial and/or oceanic sinks for CO2, which may be temporary.

Figure 2B shows that a dramatic growth rate change has occurred for methane. Factors that may have slowed the CH4 growth rate are recognized, as discussed below, but most of them are not accurately quantified.

The growth rate of the two principal chlorofluorocarbons is near zero (Figure 2C) and will be negative in the future as a result of production restrictions imposed by the Montreal Protocol.

The Three Largest Climate Forcings

The largest anthropogenic climate forcings, by CO2, CH4 and aerosols (Figure 1), pose the greatest uncertainties in attempts to project future climate change.

Coal and oil are now about equal sources of carbon dioxide emissions. Coal is the source of potentially large future emissions, as its known resources are an order of magnitude greater than those of either oil or gas. The flat growth rate of CO2 forcing, despite increased emissions, is at least in part a reflection of increased terrestrial sequestration of carbon in the 1990s. The prognosis for future sequestration is uncertain, but it is unlikely that a flat growth rate of CO2 forcing can be maintained without a flattening of the growth rate of fossil fuel emissions, which have grown 1.2%/year since 1975.

The decline of the methane growth rate (Figure 2B) is due to some combination of changes in the sinks for CH4 (primarily atmospheric OH, which is affected by chemical emissions) and the sources of CH4. The primary natural source of CH4 is microbial decay of organic matter under anoxic conditions in wetlands. Anthropogenic sources, which in sum may be twice as great as the natural source, include rice cultivation, domestic ruminants, bacterial decay in landfills and sewage, leakage during the mining of fossil fuels, leakage from natural gas pipelines, and biomass burning. Global warming could cause the natural wetland source to increase, but if warming causes a drying of wetlands, it might reduce the CH4 source.

Climate forcing by anthropogenic aerosols may be the largest source of uncertainty about future climate change. The approximate global balancing of aerosol and CO2 forcings in the past (Figure 1) cannot continue indefinitely. As long-lived CO2 accumulates, continued balancing requires a greater and greater aerosol load. This, we have argued (Hansen and Lacis 1990), would be a Faustian bargain. Detrimental effects of aerosols, including acid rain and health impacts, will eventually limit aerosol amount, and thus expose latent greenhouse warming.

We do not have observations that define even the sign of the current trend of aerosol forcing, because that requires the trends of different aerosol compositions. The direct aerosol forcing depends on aerosol absorption. The indirect aerosol forcing also depends on aerosol absorption, which affects both cloud cover and cloud brightness.

An Alternative Scenario

We propose a climate forcing scenario for the next 50 years that adds little forcing, less than or about 1 W/m2 (see Figure 3). The next 50 years is the most difficult time to affect CO2 emissions due to the inertia of global energy systems. The essence of the strategy is to halt and even reverse the growth of non-CO2 GHGs and to reduce black carbon emissions. This will mitigate an inevitable, even if slowing, growth of CO2. By mid-century improved energy efficiency and advanced technologies, perhaps including hydrogen powered fuel cells, should allow policy options with reduced reliance on fossil fuels and, if necessary, CO2 sequestration.

Fig 3

Fig. 3: A scenario for additional climate forcings between 2000 and 2050. Reduction of black carbon moves the aerosol forcing to lower values.

Carbon dioxide. This scenario calls for the mean CO2 growth rate in the next 50 years to be about the same as in the past two decades. Is this plausible? We note that the CO2 growth rate increased little in the past 20 years while much of the developing world had rapid economic growth. The United States had strong growth with little emphasis on energy efficiency, indeed with increasing use of energy-inefficient sports utility vehicles. This suggests that there are opportunities to achieve reduced emissions consistent with strong economic growth. In the near term (2000-2025) this scenario can be achieved via improved energy efficiency and a continued trend toward decarbonization of energy sources, e.g., increased use of gas instead of coal. On the longer term (2025-2050) attainment of a decreasing CO2 growth rate will require still greater use of energy sources that produce little or no CO2. If renewable energy systems are to play a substantial role by the second quarter of the century, it is important to foster research and development investments now on generic technologies at the interface between energy supply and end use, e.g., gas turbines, fuel cells, and photovoltaics.

Methane. Our scenario aims for a forcing of -0.2 W/m2 for CH4 change in the next 50 years. This requires reducing anthropogenic CH4 sources by about 30%. Reduction of CH4 would have the added benefit of increasing atmospheric OH and reducing tropospheric O3, a pollutant that is harmful to human health and agriculture.

CH4 produced by rice cultivation, perhaps the largest anthropogenic source, can be reduced by cultivar choice, fertilizer choice, and use of intermittent irrigation, which has the added advantage of reducing plant pests and malaria-carrying mosquitoes. Ruminants offer potential for emission reduction via dietary adjustments, as the farmer's objective is to produce meat, milk, or power from the carbon in their feed, not CH4. CH4 losses from leaky natural gas distribution lines could be reduced, especially in the former Soviet Union, which is served by an old system that was built without financial incentives to reduce losses. Similarly, CH4 escaping at landfills, in coal and oil mining, and from anaerobic waste management lagoons, can be reduced or captured, with economic benefits that partially or totally offset the costs.

The pollutant carbon monoxide (CO) contributes to increased CH4 and O3 through its effect on OH. A small downward trend of CO has occurred in recent years, apparently a result of pollution control in Western countries. More widespread use of advanced technologies that reduce CO emissions will help achieve CH4 and O3 reductions.

Chlorofluorocarbons. If CFCs are phased out according to the Montreal Protocol the forcing by controlled gases will be about 0.15 W/m2 less in 2050 than at present. Uncontrolled gases, some of them substitutes for ozone-depleting chemicals, are likely to increase and cause a positive forcing of about that same magnitude, with the largest contributor being HFC-134a. The Protocol, which has been a model of international cooperation, recently approved $150M for China and $82M for India, the two largest remaining producers, for complete phase-out of their CFC production. This should make the net change in climate forcing by these gases over the next 50 years about zero. If the phase-out were extended to include additional gases, such as HFC-134a, and destruction of the accessible bank of CFC-12, a negative forcing change of -0.1 W/m2 seems possible.

Tropospheric ozone. Principal precursor emissions of tropospheric O3 are volatile organic compounds and nitrogen oxides (NOx). Primary sources of the precursors are transportation vehicles, power plants and industrial processes. Business-as-usual scenarios have O3 continuing to increase in the future (IPCC 1996). Because O3 in the free troposphere can have a lifetime of weeks, tropospheric O3 is a global problem, e.g., emissions in Asia are projected to have a significant effect on air quality in the United States. High levels of O3 have adverse health and ecosystem effects. Annual costs of the impacts on human health and crop productivity are each estimated to be of the order of $10B/year in the United States alone.

The human and ecological costs of this pollutant suggest that it should be a target for international cooperation in the next half century. Air pollution in some Asian regions is already extreme, with high ecological and health costs. Unlike the Kyoto negotiations on CO2 emissions, which cast the developed and developing worlds as adversaries, all parties should have congruent objectives regarding O3. Analogous to the approach for CFCs, sharing of technology may have mutual environmental and economic benefits.

Aerosols. It is often assumed (IPCC 1996) that aerosol forcing will become more negative in the future, which would be true if all aerosols increased in present proportions. However, it is just as likely that aerosol forcing will become less negative, e.g., if sulfates decrease relative to black carbon. Black carbon reduces aerosol albedo, causes a semi-direct reduction of cloud cover, and reduces cloud particle albedo. All these effects cause warming. Conceivably a reduction of climate forcing by 0.5 W/m2 or more could be obtained by reducing black carbon emissions from diesel fuel and coal. This might become easier in the future with more energy provided via electricity grids from power plants. But quantitative understanding of the absorbing aerosol role in climate change is required to permit reliable policy recommendations.

Aerosols, unlike GHGs, are not monitored to an accuracy defining their global forcing and its temporal change. They must be monitored globally because of their heterogeneity. Measurements must yield precise aerosol microphysics and composition information in order to define the direct forcing and provide data to analyze indirect effects.


Business-as-usual scenarios, which have an additional human-made forcing of about 3 W/m2 in the next 50 years, provide a useful warning about the potential for human-made climate change. Our analysis of climate forcings suggests, as a strategy to slow global warming, an alternative scenario focused on reducing non-CO2 GHGs and black carbon (soot) aerosols. Investments in technology to improve energy efficiency and develop non-fossil energy sources are also needed to slow the growth of CO2 emissions and expand future policy options. The increase of climate forcing would be less than or about 1 W/m2.

A key feature of this strategy is its focus on air pollution, especially aerosols and tropospheric ozone, which have human health and ecological impacts. If the World Bank were to support investments in modern technology and air quality control in India and China, e.g., the reductions in tropospheric ozone and black carbon would not only improve local health and agricultural productivity, but also benefit global climate and air quality.

Non-CO2 greenhouse gases are probably the main cause of observed global warming, with CH4 causing the largest net climate forcing. There are economic incentives to reduce or capture CH4 emissions, but global implementation of appropriate practices requires international cooperation. Definition of appropriate policies requires better understanding of the CH4 cycle, especially CH4 sources.

Climate forcing by CFCs is still growing today, but, if Montreal Protocol restrictions are adhered to, there should be no net growth of the CFC forcing over the next 50 years. A small decrease of the CFC forcing from today's level is possible.

Tropospheric O3 increases in business-as-usual scenarios, which assume that CH4 increases and that there is no global effort to control O3 precursors. The human health and ecological impacts of O3 are so great that it represents an opportunity for effective international cooperation. At least it should be possible to prevent O3 forcing in 2050 from exceeding that of today.

Carbon dioxide will become the dominant climate forcing if its emissions continue to increase and aerosol effects level off. Business-as-usual scenarios understate the potential for CO2 emission reductions from improved energy efficiency and decarbonization of fuels. Based on this potential and current CO2 growth trends, we argue that limiting the CO2 forcing increase to 1 W/m2 in the next 50 years is plausible.

Indeed, CO2 emissions from fossil fuel use declined slightly in 1998 and again in 1999, while the global economy grew. However, achieving the level of emissions needed to slow climate change significantly is likely to require policies that encourage technological developments to accelerate energy efficiency and decarbonization trends.

Climate forcing due to aerosol changes is a wild card. Current trends are uncertain even in the sign of the effect. Unless climate forcings by all aerosols are precisely monitored, it will be difficult to define optimum policies.

We argue that black carbon aerosols, via several effects, contribute significantly to global warming. This suggests one antidote to global warming, if its impacts begin to increase. As electricity plays an increasing role in future energy systems, it should be relatively easy to strip black carbon emissions at fossil fuel power plants. Stripping and disposing of CO2, though more challenging, provides an effective backup strategy.


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Further Information

A description of how this study has been reported in the news and received by the scientific community can be read in Discussion of "An Alternative Scenario".

This webpage is an abbreviated version of an article by James E. Hansen, Makiko Sato, Reto Ruedy, Andrew Lacis, and Valdar Oinas published in Proc. Natl. Acad. Sci.. Many additional references are available in the original paper for unreferenced assertions in the discussion here.