Climate Change 2001:
Synthesis Report
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3.19 The productivity of ecological systems is highly sensitive to climate change and projections of change in productivity range from increases to decreases (medium confidence). Increasing CO2 concentrations would increase net primary productivity (CO2 fertilization) and net ecosystem productivity in most vegetation systems, causing carbon to accumulate in vegetation and soils over time. Climate change may either augment or reduce the direct effects of CO2 on productivity, depending on the type of vegetation, the region, and the scenario of climate change.

WGI TAR Section 3.7 & WGII TAR Sections 5.2.2 & 5.6.3
3.20 The terrestrial ecosystems at present are a carbon sink which may diminish with increased warming by the end of the 21st century (see Table 3-2) (medium confidence). The terrestrial ecosystems at present are a sink for carbon. This is partly a result of delays between enhanced plant growth and plant death and decay. Current enhanced plant growth is partly due to fertilization effects of elevated CO2 on plant photosynthesis (either directly via increased carbon assimilation, or indirectly through higher water-use efficiency), nitrogen deposition (especially in the Northern Hemisphere), climate change, and land-use practices over past decades. The uptake will decline as forests reach maturity, fertilization effects saturate and decomposition catches up with growth, and possibly through changes in disturbance regimes (e.g., fire and insect outbreaks) mediated through climate change. Some global models project that the net uptake of carbon by terrestrial ecosystems will increase during the first half of the 21st century but may diminish and even become a source with increased warming towards the end of the 21st century.

WGI TAR Section 3.2.2, WGII TAR Sections 5.2, 5.5-6, & 5.9, & SRLULUCF Section 1.4
Table 3-2: Ecosystem effects of climate change if no climate policy interventions are made.*
  2025 2050 2100
CO2 concentrationa 405-460 ppm 445-640 ppm 540-970 ppm
Global mean temperature change from the year 1990b 0.4-1.1°C 0.8-2.6°C 1.4-5.8°C
Global mean sea-level rise from the year 1990b 3-14 cm 5-32 cm 9-88 cm
Ecosystem Effectsc
Corals [WGII TAR Sections 6.4.5, 12.4.7, & 17.2.4] Increase in frequency of coral bleaching and death of corals (high confidenced). More extensive coral
bleaching and death (high confidenced).
More extensive coral bleaching and death (high confidenced). Reduced species biodiversity and fish yields from reefs (medium confidenced).
Coastal wetlands and shorelines [WGII TAR Sections 6.4.2 & 6.4.4] Loss of some coastal wetlands to sea-level rise (medium confidenced). Increased erosion of shorelines (medium confidenced). More extensive loss of coastal wetlands (medium confidenced). Further erosion of shorelines (medium confidenced). Further loss of coastal wetlands (medium
). Further erosion of shorelines
(medium confidenced).
Terrestrial ecosystems [WGII TAR Sections 5.2.1, 5.4.1, 5.4.3, 5.6.2, 16.1.3, & 19.2] Lengthening of growing season in mid- and high latitudes; shifts in ranges of plant and animal species (high confidenced).e,f Increase in net primary productivity of many mid- and high-latitude forests (medium confidenced). Increase in frequency of ecosystem disturbance by fire and insect pests (high confidenced). Extinction of some endangered species; many others pushed closer to extinction (high confidenced).
Increase in net primary productivity may or may not continue. Increase in frequency of ecosystem disturbance by fire and insect pests (high confidenced).
Loss of unique habitats and their endemic species (e.g., vegetation of Cape region of South Africa and some cloud forests) (medium confidenced). Increase in frequency of ecosystem disturbance by fire and insect pests (high confidenced).
Ice environments [WGI TAR Sections 2.2.5 & 11.5; WGII TAR Sections 4.3.11, 11.2.1, 16.1.3, 16.2.1, 16.2.4, & 16.2.7]

Retreat of glaciers, decreased sea-ice extent, thawing of some permafrost, longer ice-free seasons on rivers and lakes (high confidenced).f Extensive Arctic sea-ice reduction, benefiting shipping but harming wildlife (e.g., seals, polar bears, walrus) (medium confidenced). Ground subsidence leading to infrastructure damage (high confidenced). Substantial loss of ice volume from glaciers, particularly tropical glaciers (high confidenced).
* Refer to footnotes a-d accompanying Table 3-1.
e. Aggregate market effects represent the net effects of estimated economic gains and losses summed across market sectors such as agriculture, commercial forestry, energy, water, and construction. The estimates generally exclude the effects of changes in climate variability and extremes, do not account for the effects of different rates of change, and only partially account for impacts on goods and services that are not traded in markets. These omissions are likely to result in underestimates of economic losses and overestimates of economic gains. Estimates of aggregate impacts are controversial because they treat gains for some as canceling out losses for others and because the weights that are used to aggregate across individuals are necessarily subjective.
f. These effects have already been observed and are expected to continue [TAR WGII Sections 5.2.1, 5.4.3, 16.1.3, & 19.2].

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