Synthesis Report - Question 3

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 concentration^a	405-460 ppm	445-640 ppm	540-970 ppm
Global mean temperature change from the year 1990^b	0.4-1.1°C	0.8-2.6°C	1.4-5.8°C
Global mean sea-level rise from the year 1990^b	3-14 cm	5-32 cm	9-88 cm
Ecosystem Effects^c
Corals [WGII TAR Sections 6.4.5, 12.4.7, & 17.2.4]	Increase in frequency of coral bleaching and death of corals (high confidence^d).	More extensive coral bleaching and death (high confidence^d).	More extensive coral bleaching and death (high confidence^d). Reduced species biodiversity and fish yields from reefs (medium confidence^d).
Coastal wetlands and shorelines [WGII TAR Sections 6.4.2 & 6.4.4]	Loss of some coastal wetlands to sea-level rise (medium confidence^d). Increased erosion of shorelines (medium confidence^d).	More extensive loss of coastal wetlands (medium confidence^d). Further erosion of shorelines (medium confidence^d).	Further loss of coastal wetlands (medium confidence^d). Further erosion of shorelines (medium confidence^d).
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 confidence^d).e,f Increase in net primary productivity of many mid- and high-latitude forests (medium confidence^d). Increase in frequency of ecosystem disturbance by fire and insect pests (high confidence^d).	Extinction of some endangered species; many others pushed closer to extinction (high confidence^d). Increase in net primary productivity may or may not continue. Increase in frequency of ecosystem disturbance by fire and insect pests (high confidence^d).	Loss of unique habitats and their endemic species (e.g., vegetation of Cape region of South Africa and some cloud forests) (medium confidence^d). Increase in frequency of ecosystem disturbance by fire and insect pests (high confidence^d).
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 confidence^d).^f	Extensive Arctic sea-ice reduction, benefiting shipping but harming wildlife (e.g., seals, polar bears, walrus) (medium confidence^d). Ground subsidence leading to infrastructure damage (high confidence^d).	Substantial loss of ice volume from glaciers, particularly tropical glaciers (high confidence^d).
* 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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