By capturing synergies, some greenhouse gas mitigation actions may yield extensive ancillary benefits for several other environmental problems, but also trade-offs may occur. Examples include, inter alia, reduction of negative environmental impacts such as air pollution and acid deposition; protecting forests, soils, and watersheds; reducing distortionary subsidies and taxes; and inducing more efficient technological change and diffusion, contributing to wider goals of sustainable development. However, dependent on the way climate change or other environmental problems are addressed, and the degree to which interlinking issues are taken into account, significant trade-offs may occur and unanticipated costs may be incurred. For example, policy options to reduce greenhouse gas emissions from the energy and land-use sectors can have both positive and negative effects on other environmental problems:

• In the energy sector, greenhouse gas emissions as well as local and regional pollutants could be reduced through more efficient and environmentally sound use of energy and increasing the share of lower carbon emitting fossil fuels, advanced fossil-fuel technologies (e.g., highly efficient combined cycle gas turbines, fuel cells, and combined heat and power), and renewable energy technologies (e.g., increased use of environmentally sound biofuels, hydropower, solar, wind- and wave-power). Increased use of biomass as a substitute for fossil fuel could have positive or negative impacts on soils, biodiversity, and water availability depending on the land use it replaces and the management regime.
• In the land-use sector, conservation of biological carbon pools not only prevents carbon from being emitted into the atmosphere, it also can have a favorable effect on soil productivity, prevent biodiversity loss, and reduce air pollution problems from biomass burning. Carbon sequestration by plantation forestry can enhance carbon sinks and protect soils and watersheds, but -- if developed improperly -- may have negative effects on biodiversity and water availability. For example, in some implementations, monoculture plantations could decrease local biodiversity.
  
  

Conversely, addressing environmental problems other than climate change can have ancillary climate benefits, but the linkages between the various problems may also lead to trade-offs. Examples include:

• There are likely to be substantial greenhouse gas benefits from policies aimed at reducing air pollution. For example, increasing pollution is often associated with the rapidly growing transportation sector in all regions, involving emissions of particulate matter and precursors of ozone pollution. Addressing these emissions to reduce the impacts on human health, agriculture, and forestry through increasing energy efficiency or penetration of non-fossil-fuel energy can also reduce greenhouse gas emissions.
• Controlling sulfur emissions has positive impacts on human health and vegetation, but sulfate aerosols partly offset the warming effect of greenhouse gases and therefore control of sulfur emissions can amplify possible climate change. If sulfur emissions are controlled through desulfurization of flue gases at power plants, an energy penalty results, with associated increase of greenhouse gas emissions.
  

Reducing vulnerability to climate change can often reduce vulnerability to other environmental stresses and vice versa. Examples include, inter alia:

• Protecting threatened ecosystems: Removing societal stresses and managing resources in a sustainable manner may help unique and threatened systems also to cope with the additional stress posed by climate change. Accounting for potential climatic changes and integration with socio-economic needs and development plans can make biodiversity
  conservation strategies and climate change adaptation measures more effective.
• Land-use management: Addressing or avoiding land degradation also decreases vulnerability to climate change, especially when response strategies consider the social and economic factors defining the land-use practices together with the additional risks imposed by climate change. In regions where deforestation is progressing and leading
  to carbon loss and increased peak runoff, restoring vegetation by reforestation (and when possible by afforestation) and revegetation can help to combat desertification.
• Freshwater management: Problems with availability, abundance, and pollution of freshwater, which are often caused by demographic and development pressures, can be exacerbated by climate change. Reducing vulnerability to water stress (e.g., by water conservation, water-demand management, and more efficient water use) also reduces
  vulnerability to additional stress by climate change.