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).

Most global and regional economic studies—with and without climate change—indicate 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).

• 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 precipitation—would have detrimental effects on some of the major crops, especially in drought-prone regions (medium confidence).
• Soil properties and processes—including organic matter decomposition, leaching, and soil water regimes—will 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).

• 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 process—inter 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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