Biological diversity – the source of enormous environmental, economic, and cultural value – will be threatened by rapid climate change. The composition and geographic distribution of ecosystems will change as individual species respond to new conditions created by climate change. At the same time, habitats may degrade and fragment in response to other human pressures. Species that cannot adapt quickly enough may become extinct – an irreversible loss.

Species and ecosystems have already started responding to global warming. Scientists have observed climate-induced changes in at least 420 physical processes and biological species or communities. Changes include migratory birds arriving earlier in the spring and leaving later in the autumn, a lengthening by 10.8 days of the European growing season for controlled mix-species gardens from 1959 to 1993, earlier springtime reproduction for many birds and amphibians, and the northward movement of cold-sensitive butterflies, beetles, and dragonflies.

Forests adapt slowly to changing conditions. Observations, experiments, and models demonstrate that a sustained increase of just 1^oC in the global average temperature would affect the functioning and composition of forests. The composition of species in existing forests will change, while new combinations of species, and hence new ecosystems, may be established. Other stresses caused by warming will include more pests, pathogens, and fires. Because higher latitudes are expected to warm more than equatorial ones, boreal forests will be more affected than temperate and tropical forests; Alaska’s boreal forests are already expanding northward at the rate of 100 kilometres per degree Centigrade.

Forests play an important role in the climate system. They are a major reservoir of carbon, containing some 80% of all the carbon stored in land vegetation, and about 40% of the carbon residing in soils. Large quantities of carbon may be emitted into the atmosphere during transitions from one forest type to another if mortality releases carbon faster than regeneration and growth absorbs it. Forests also directly affect climate on the local, regional, and continental scales by influencing ground temperature, evapo-transpiration, surface roughness, albedo (or reflectivity), cloud formation, and precipitation.

Deserts and arid and semi-arid ecosystems may become more extreme. With few exceptions, deserts are projected to become hotter but not significantly wetter. Higher temperatures could threaten organisms that now exist near their heat-tolerance limits.

Rangelands may experience altered growing seasons. Grasslands support approximately 50% of the world's livestock and are also grazed by wildlife. Shifts in temperatures and precipitation may reshape the boundaries between grasslands, shrublands, forests, and other ecosystems. In tropical regions such changes in the evapo-transpiration cycle could strongly affect productivity and the mix of species.

Mountain regions are already under considerable stress from human activities. The projected declines in mountain glaciers, permafrost, and snow cover will further affect soil stability and hydrological systems (most major river systems start in the mountains). As species and ecosystems are forced to migrate uphill, those limited to mountain tops may have nowhere to go and become extinct; observations show that some plant species are moving up in the European Alps by one to four metres per decade and that some mountaintop species have already disappeared. Agriculture, tourism, hydropower, logging, and other economic activities will also be affected. The food and fuel resources of indigenous populations in many developing countries may be disrupted.

The cryosphere will continue to shrink. Representing nearly 80% of all freshwater, the cryosphere encompasses all of the earth's snow, ice, and permafrost. Permafrost is thawing worldwide – even around Siberia’s Lake Baikal, the coldest place in the Northern Hemisphere – destabilizing infrastructure and releasing additional carbon and methane into the atmosphere. Mountains glaciers are declining: almost two thirds of Himalayan glaciers have retreated in the past decade, and Andean glaciers have retreated dramatically or disappeared. This will affect nearby ecosystems and communities as well as seasonal river flows and water supplies, which in turn has implications for hydropower and agriculture. The landscapes of many high mountain ranges and polar regions will change dramatically. Reduced sea-ice could lengthen the navigation season for certain rivers and coastal areas. Arctic sea ice has thinned by 40% in the past three decades, and its extent has shrunk by about 10-15%. Despite these many striking effects, the Greenland and Antarctic ice sheets are not expected to change much over the next 50­100 years.

Non-tidal wetlands will also be reduced. Open-water and waterlogged areas provide refuge and breeding grounds for many species. They also help to improve water quality and control floods and droughts. Studies from several countries suggest that a warmer climate will contribute to the decline of wetlands through higher evaporation. By altering their hydrological regimes, climate change will influence the biological, biogeochemical, and hydrological functions of these ecosystems, as well as their geographical distribution.

Human actions can help natural ecosystems adapt to climate change. Creating natural migration corridors and assisting particular species to migrate could benefit forest ecosystems. Reforestation and the "integrated management" of fires, pests, and diseases can also contribute. Rangelands could be supported through the active selection of plant species, controls on animal stocking, and new grazing strategies. Wetlands can be restored and even created. Desertified lands may adapt better if drought-tolerant species and better soil conservation practices are encouraged.