Based on model simulations of vegetation distribution, which use GCM based climate scenarios, large shifts of vegetation boundaries into higher latitudes and elevations can be expected. The mix of species within a given vegetation class likely will change. Under equilibrium GCM climate scenarios, large regions show drought induced declines in vegetation, even when the direct effects of CO2 fertilization are included. By comparison, under transient climate scenarios in which trace gases increase slowly over a period of years the full effects of changes in temperature and precipitation lag the effects of a change in atmospheric composition by a number of decades; hence, the positive effects of CO2 precede the full effects of changes in climate.

Climate change is projected to occur at a rapid rate relative to the speed at which forest species grow, reproduce and reestablish themselves (past tree species' migration rates are believed to be on the order of 4-200 km per century). For mid-latitude regions, an average warming of 1-3.5°C over the next 100 years would be equivalent to a poleward shift of the present geographic bands of similar temperatures (or 'isotherms') approximately 150-550 km, or an altitude shift of about 150-550 m. Therefore, the species composition of forests is likely to change; in some regions, entire forest types may disappear, while new assemblages of species and hence new ecosystems may be established. As a consequence of possible changes in temperature and water availability under doubled equivalent-CO2 equilibrium conditions, a substantial fraction (a global average of one-third, varying by region from one-seventh to two-thirds) of the existing forested area of the world likely would undergo major changes in broad vegetation types with the greatest changes occurring in high latitudes and the least in the tropics. In tropical rangelands, major alterations in productivity and species composition would occur due to altered rainfall amount and seasonality and increased evapotranspiration, although a mean temperature increase alone would not lead to such changes.

Inland aquatic ecosystems will be influenced by climate change through altered water temperatures, flow regimes, water levels and thawing of permafrost at high latitudes. In lakes and streams, warming would have the greatest biological effects at high latitudes where biological productivity would increase and lead to expansion of cool-water species' ranges and at the low-latitude boundaries of cold- and cool-water species ranges, where extinctions would be greatest. Increases in flow variability, particularly the frequency and duration of large floods and droughts, would tend to reduce water quality, biological productivity and habitat in streams.

The geographical distribution of wetlands is likely to shift with changes in temperature and precipitation, with uncertain implications for net greenhouse gas emissions from non-tidal wetlands. Some coastal eco-systems (saltwater marshes, mangrove ecosystems, coastal wet-lands, coral reefs, coral atolls and river deltas) are particularly at risk from climate change and other stresses. Changes in these ecosystems would have major negative effects on freshwater supplies, fisheries, biodiversity and tourism. Adaptation options for ecosystems are limited, and their effectiveness is uncertain. Options include establishment of corridors to assist the "migration" of ecosystems, land-use management, plantings and restoration of degraded areas. Because of the projected rapid rate of change relative to the rate at which species can reestablish themselves, the isolation and fragmentation of many ecosystems, the existence of multiple stresses (e.g., land-use change, pollution) and limited adaptation options, ecosystems (especially forested systems, montane systems and coral reefs) are vulnerable to climate change.