EXECUTIVE SUMMARY
Ecosystems are subject to many pressures (e.g., land-use change, resource demands,
population changes); their extent and pattern of distribution is changing, and
landscapes are becoming more fragmented. Climate change constitutes an additional
pressure that could change or endanger ecosystems and the many goods and services
they provide.
There now is a substantial core of observational and experimental studies demonstrating
the link between climate and biological or physical processes in ecosystems
(e.g., shifting range boundaries, flowering time or migration times, ice break-up
on streams and rivers), most evident in high latitudes. Recent modeling studies
continue to show the potential for significant disruption of ecosystems under
climate change. Further development of simple correlative models that were available
at the time of the Second Assessment Report (SAR) point to areas where ecosystem
disruption and the potential for ecosystem migration are high. Observational
data and newer dynamic vegetation models linked to transient climate models
are refining the projections. However, the precise outcomes depend on processes
that are too subtle to be fully captured by current models.
At the time of the SAR, the interaction between elevated carbon dioxide (CO2),
increasing temperatures, and soil moisture changes suggested a possible increase
in plant productivity through increased water-use efficiency (WUE). Recent results
suggest that the gains might be small under field conditions and could be further
reduced by human management activities. Many ecosystems are sensitive to the
frequency of El Niño-Southern Oscillation (ENSO) and other extreme events
that result in changes in productivity and disturbance regimes (e.g., fires,
pest and disease outbreak).
Agriculture
Most global and regional economic studieswith and without climate changeindicate
that the downward trend in real commodity prices in the 20th century is likely
to continue into the 21st century, although confidence in these predictions
decreases farther into the future (see Section 5.3.1).
Impacts
- Experiments have shown that relative enhancement of productivity caused
by elevated CO2 usually is greater when temperature rises but may
be less for crop yields at above-optimal temperatures (established but incomplete).
Although the beneficial effects of elevated CO2 on the yield of
crops are well established for the experimental conditions tested, this knowledge
is incomplete for numerous tropical crop species and for crops grown under
suboptimal conditions (low nutrients, weeds, pests and diseases). In experimental
work, grain and forage quality declines with CO2 enrichment and
higher temperatures (high confidence) (see Sections 5.3.3,
5.4.3, and 5.5.3).
- Experimental evidence suggests that relative enhancement of productivity
caused by elevated CO2 usually is greater under drought conditions
than in wet soil. Nevertheless, a climate change-induced reduction in summer
soil moisture (see Table 3-10)which may
occur even in some cases of increased summer precipitationwould have
detrimental effects on some of the major crops, especially in drought-prone
regions (medium confidence).
- Soil properties and processesincluding organic matter decomposition,
leaching, and soil water regimeswill be influenced by temperature increase
(high confidence). Soil erosion and degradation are likely to aggravate the
detrimental effects of a rise in air temperature on crop yields. Climate change
may increase erosion in some regions, through heavy rainfall and through increased
windspeed (competing explanations) (see Section 5.3.3).
- Model simulations of wheat growth indicate that greater variation in temperature
(change in frequency of extremes) under a changing climate reduces average
grain yield. Moreover, recent research emphasizes the importance of understanding
how variability interacts with changes in climate means in determining yields
(established but incomplete) (see Section 5.3.4).
- Crop modeling studies that compare equilibrium scenarios with transient
scenarios of climate change report significant yield differences. The few
studies that include comparable transient and equilibrium climate change scenarios
generally report greater yield loss with equilibrium climate change than with
the equivalent transient climate change. Even these few studies are plagued
with problems of inconsistency in methodologies, which make comparisons speculative
at this time (see Section 5.3.4).
Adaptation and Vulnerability
- Prospects for adaptation of plant material to increased air temperature
through traditional breeding and genetic modification appear promising (established
but incomplete). More research on possible adaptation of crop species to elevated
CO2 is needed before more certain results can be presented (see
Section 5.3.3).
- Simulations without adaptation suggest more consistent yield losses from
climate change in tropical latitudes than temperate latitudes. Agronomic adaptation
abates extreme yield losses at all latitudes, but yields tend to remain beneath
baseline levels after adaptation more consistently in the tropics than in
temperate latitudes (moderate confidence) (see Section
5.3.4).
- The ability of livestock producers to adapt their herds to the physiological
stress of climate change is not known conclusively, in part because of a general
lack of experimentation and simulations of livestock adaptation to climate
change (see Section 5.3.3).
- Crop and livestock farmers who have sufficient access to capital and technologies
are expected to adapt their farming systems to climate change (medium to low
confidence) (see Section 5.3.4). Substantial shifts
in their mix of crops and livestock production may be necessary, however,
and considerable costs could be involved in this processinter alia,
in learning and gaining experience with different crops or if irrigation becomes
necessary. In some cases, a lack of water resulting from climate change might
mean that increased irrigation demands cannot be met (see Section
4.7.2). Although this conclusion is speculative because of lack of research,
it is intuitive that the costs of adaptation should depend critically on the
rate of climate change.
- mpacts of climate change on agriculture after adaptation are estimated to
result in small percentage changes in global income; these changes tend to
be positive for a moderate global warming, especially when the effects of
CO2 fertilization are taken into account (low confidence) (see
Section 5.3.5).
- The effectiveness of adaptation in ameliorating the economic impacts of
climate change across regions will depend critically on regional resource
endowments. It appears that developed countries will fare better in adapting
to climate change; developing countries and countries in transition, especially
in the tropics and subtropics, will fare worse. This finding has particularly
significant implications for the distribution of impacts within developing
countries, as well as between more- and less-developed countries. These findings
provide evidence to support the hypothesis advanced in the SAR that climate
change is likely to have its greatest adverse impacts on areas where resource
endowments are poorest and the ability of farmers to respond and adapt is
most limited (medium confidence) (see Section 5.3.5).
- Degradation of soil and water resources is one of the major future challenges
for global agriculture (see Section 5.3.2). These processes
are likely to be intensified by adverse changes in temperature and precipitation.
Land use and management have been shown to have a greater impact on soil conditions
than the direct effects of climate change; thus, adaptation has the potential
to significantly mitigate these impacts (see Section
5.3.4). A critical research need is to assess whether resource degradation
will significantly increase the risks faced by vulnerable agricultural and
rural populations (see Section 5.3.6).
- It is concluded with low confidence that a global temperature rise of greater
than 2.5°C will result in rising commodity prices. Similarly, a global
temperature rise of greater than 2.5ºC increases by 80 million the absolute
number of people at risk of hunger. It should be noted, however, that these
hunger estimates are based on the assumption that food prices will rise with
climate change, which is highly uncertain (see Section
5.3.6).
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