Biological diversity and ecosystems
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 1oC 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