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.

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 imrpoved 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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