3.3. Terrestrial and Freshwater Ecosystems
Vegetation modeling studies continue to show the potential for significant
disruption of ecosystems under climate change (high confidence6).
Migration of ecosystems or biomes as discrete units is unlikely to occur; instead
at a given site, species composition and dominance will change. The results
of these changes will lag behind the changes in climate by years to decades
to centuries (high confidence6).
Distributions, population sizes, population density, and behavior of wildlife
have been, and will continue to be, affected directly by changes in global or
regional climate and indirectly through changes in vegetation. Climate change
will lead to poleward movement of the boundaries of freshwater fish distributions
along with loss of habitat for cold- and cool-water fishes and gain in habitat
for warm-water fishes (high confidence6).
Many species and populations are already at high risk, and are expected to be
placed at greater risk by the synergy between climate change rendering portions
of current habitat unsuitable for many species, and land-use change fragmenting
habitats and raising obstacles to species migration. Without appropriate management,
these pressures will cause some species currently classified as "critically
endangered" to become extinct and the majority of those labeled "endangered
or vulnerable" to become rarer, and thereby closer to extinction, in the 21st
century (high confidence6).
Possible adaptation methods to reduce risks to species could include: 1) establishment
of refuges, parks, and reserves with corridors to allow migration of species,
and 2) use of captive breeding and translocation. However, these options may
have limitations due to costs. [4.3]
Terrestrial ecosystems appear to be storing increasing amounts of carbon. At
the time of the SAR, this was largely attributed to increasing plant productivity
because of the interaction between elevated CO2 concentration, increasing
temperatures, and soil moisture changes. Recent results confirm that productivity
gains are occurring but suggest that they are smaller under field conditions
than indicated by plant-pot experiments (medium confidence6).
Hence, the terrestrial uptake may be due more to change in uses and management
of land than to the direct effects of elevated CO2 and climate. The
degree to which terrestrial ecosystems continue to be net sinks for carbon is
uncertain due to the complex interactions between the factors mentioned above
(e.g., arctic terrestrial ecosystems and wetlands may act as both sources and
sinks) (medium confidence6).
Contrary to the SAR, global timber market studies that include adaptations
through land and product management, even without forestry projects that increase
the capture and storage of carbon, suggest that a small amount of climate change
would increase global timber supply and enhance existing market trends towards
rising market share in developing countries (medium confidence6).
Consumers may benefit from lower timber prices while producers may gain or lose
depending on regional changes in timber productivity and potential dieback effects.