The article presents an alternative scenario for global warming in the 21st century, arguing that recent warming has been driven mainly by non-CO2 greenhouse gases (GHGs) such as CH4, N2O, and CFCs, rather than by CO2 and aerosols. The growth rate of non-CO2 GHGs has declined in the past decade, and if sources of CH4 and O3 precursors are reduced, the climate forcing from these gases could be near zero in the next 50 years. Combined with reduced black carbon emissions and plausible CO2 emission reductions, this could lead to a decline in global warming. The focus on air pollution unites the interests of developed and developing countries. However, assessing climate change requires long-term monitoring of aerosol properties. Global surface temperatures have increased by about 0.5°C since 1975, reaching the highest level in the past millennium. There is a growing consensus that this warming is at least partly due to increasing anthropogenic GHGs. GHGs cause a climate forcing, an imposed perturbation of Earth's energy balance. Increasing GHGs are estimated to be the largest forcing, resulting in a net positive forcing, especially in recent decades. Evidence supporting this includes observed heat storage in the ocean, which is positive and of the magnitude of the energy imbalance estimated from climate forcings. The Intergovernmental Panel on Climate Change (IPCC) has considered various scenarios for future GHGs, but simulations have focused on "business as usual" scenarios with rapidly increasing GHGs. These scenarios predict a steep increase in global temperature throughout the 21st century, with warming of several degrees Celsius by 2100. The Kyoto Protocol, which aims to reduce CO2 emissions, is considered a difficult target to achieve. However, climate simulations suggest that the Kyoto reductions will have little effect in the 21st century, and "30 Kyoto" may be needed to reduce warming to an acceptable level. The article suggests equal emphasis on an alternative, more optimistic scenario. This scenario focuses on reducing non-CO2 GHGs and black carbon during the next 50 years. The authors estimate that non-CO2 GHGs have caused most observed global warming. This does not alter the desirability of limiting CO2 emissions, as the future balance of forcings is likely to shift toward dominance of CO2 over aerosols. However, the authors suggest that it is more practical to slow global warming than is sometimes assumed. The article discusses climate forcings from various sources, including CO2, CH4, N2O, and CFCs. It highlights the importance of reducing non-CO2 GHGs and black carbon to mitigate global warming. The article also discusses the role of aerosols in climate forcing, noting that their effects are uncertain but significant. The article concludes that an alternative scenario focusing on reducing non-CO2 GHGs and black carbon isThe article presents an alternative scenario for global warming in the 21st century, arguing that recent warming has been driven mainly by non-CO2 greenhouse gases (GHGs) such as CH4, N2O, and CFCs, rather than by CO2 and aerosols. The growth rate of non-CO2 GHGs has declined in the past decade, and if sources of CH4 and O3 precursors are reduced, the climate forcing from these gases could be near zero in the next 50 years. Combined with reduced black carbon emissions and plausible CO2 emission reductions, this could lead to a decline in global warming. The focus on air pollution unites the interests of developed and developing countries. However, assessing climate change requires long-term monitoring of aerosol properties. Global surface temperatures have increased by about 0.5°C since 1975, reaching the highest level in the past millennium. There is a growing consensus that this warming is at least partly due to increasing anthropogenic GHGs. GHGs cause a climate forcing, an imposed perturbation of Earth's energy balance. Increasing GHGs are estimated to be the largest forcing, resulting in a net positive forcing, especially in recent decades. Evidence supporting this includes observed heat storage in the ocean, which is positive and of the magnitude of the energy imbalance estimated from climate forcings. The Intergovernmental Panel on Climate Change (IPCC) has considered various scenarios for future GHGs, but simulations have focused on "business as usual" scenarios with rapidly increasing GHGs. These scenarios predict a steep increase in global temperature throughout the 21st century, with warming of several degrees Celsius by 2100. The Kyoto Protocol, which aims to reduce CO2 emissions, is considered a difficult target to achieve. However, climate simulations suggest that the Kyoto reductions will have little effect in the 21st century, and "30 Kyoto" may be needed to reduce warming to an acceptable level. The article suggests equal emphasis on an alternative, more optimistic scenario. This scenario focuses on reducing non-CO2 GHGs and black carbon during the next 50 years. The authors estimate that non-CO2 GHGs have caused most observed global warming. This does not alter the desirability of limiting CO2 emissions, as the future balance of forcings is likely to shift toward dominance of CO2 over aerosols. However, the authors suggest that it is more practical to slow global warming than is sometimes assumed. The article discusses climate forcings from various sources, including CO2, CH4, N2O, and CFCs. It highlights the importance of reducing non-CO2 GHGs and black carbon to mitigate global warming. The article also discusses the role of aerosols in climate forcing, noting that their effects are uncertain but significant. The article concludes that an alternative scenario focusing on reducing non-CO2 GHGs and black carbon is