In the previous sections, we briefly discussed projected changes in climate as a result of current and potential human activities. There are many uncertainties in our predictions, particularly with regard to the timing, magnitude, and regional patterns of climate change. At this point, potential changes in climate globally are better understood than the changes that could occur locally or regionally. However, the impacts of interest from climate change are primarily local to regional in scale. Nevertheless, scientific studies have shown that human health, ecological systems and socioeconomic sectors (e.g., hydrology and water resources, food and fiber production, and coastal systems, all of which are vital to sustainable development) are sensitive to changes in climate as well as to changes in climate variability. Recently, a great deal of work has been undertaken to assess the potential consequences of climate change (e.g., IPCC, 1998). The IPCC (1998) study has assessed current knowledge on how systems would respond to future projections of climate change. Here we restrict our discussion to only a brief overview.
Ecosystems. Ecosystems both affect and are affected by climate. As carbon dioxide levels increase, the productivity and efficiency of water use by vegetation may also increase. As temperature warms, the composition and geographical distribution of many ecosystems will shift as individual species respond to changes in climate. As vegetation shifts, this will in turn affect climate. Vegetation and other land cover determine the amount of radiation absorbed and emitted by the Earth's surface. As the Earth's radiation balance changes, the temperature of the atmosphere will be affected, resulting in further climate change. Other likely climate change impacts from ecosystems include reductions in biological diversity and in the goods and services that ecosystems provide society.
Water resources. Climate change may lead to an intensification of the global hydrological cycle and can have major impacts on regional water resources. Reduced rainfall and increased evaporation in a warmer world could dramatically reduce runoff in some areas, significantly decreasing the availability of water resources for crop irrigation, hydroelectric power production, and industrial/commercial and transport uses. Other regions may see increased rainfall. In light of the increase in artificial fertilizers, pesticides, feedlots excrement, and hazardous waste dumps, the provision of good quality drinking water is anticipated to be difficult.
Agriculture. Crop yields and productivity are projected to increase in some areas, at least during the next few decades, and decrease in others. The most significant decreases are expected in the tropics and subtropics, which contain the majority of the world's population. The decrease could be so severe as to result in increased risk of hunger and famine in some locations that already contain many of the world's poorest people. These regions are particularly vulnerable, as industrialized countries may be able to counteract climate change impacts by technological developments, genetic diversity, and maintaining food reserves.
Livestock production may also be affected by changes in grain prices due to pasture productivity. Supplies of forest products such as wood during the next century may also become increasingly inadequate to meet projected consumption due to both climatic and non-climatic factors. Boreal forests are likely to undergo irregular and large-scale losses of living trees because of the impact of projected climate change. Marine fisheries production are also expected to be affected by climate change. The principal impacts will be felt at the national and local levels.
Sea level rise. In a warmer climate, sea level will rise due to two primary factors: (i) the thermal expansion of ocean water as it warms, and (ii) the melting of snow and ice from mountain glaciers and polar ice caps. Over the last century, the global-mean sea level has risen about 10 to 25 cm. Over the next century, current models project a further increase of 25 to 100 cm in global-mean sea level for typical scenarios of greenhouse gas emissions and resulting climate effects (IPCC, 1996a; Neumann et al., 2000). Figure 1.7 shows the sea level rise calculated with the ISAM model for the four SRES scenarios assuming a climate sensitivity of 2.5°C for a doubling of the CO2 concentration. A sea level rise in the upper part of the range could have very detrimental effects on low-lying coastal areas. In addition to direct flooding and property damage or loss, other impacts may include coastal erosion, increased frequency of storm surge flooding, salt water infiltration and hence pollution of irrigation and drinking water, destruction of estuarine habitats, damage to coral reefs, etc. Little change is expected to occur in the Antarctic over the next century, but if there were to be any major melting, it would potentially increase sea level by a large amount.
Health and human infrastructure. Climate change can impact human health through changes in weather, sea level and water supplies, and through changes in ecosystems that affect food security or the geography of vector-borne diseases. The section of IPCC (1996b) dealing with human health issues found that most of the possible impacts of global warming would be adverse.
In terms of direct effects on human health, increased frequency of heat waves would increase rates of cardio-respiratory illness and death. High temperatures would also exacerbate the health effects of primary pollutants generated in fossil fuel combustion processes and increase the formation of secondary pollutants such as tropospheric ozone. Changes in the geographical distribution of disease vectors such as mosquitoes (malaria) and snails (schistosomiasis) and changes in life-cycle dynamics of both vectors and infective parasites would increase the potential for transmission of disease. Non-vector-borne diseases such as cholera might increase in the tropics and sub-tropics because of climatic change effects on water supplies, temperature and microorganism proliferation. Concern for climate change effects on human health are legitimate. However, impacts research on this subject is sparse and the conclusions reached by the IPCC are still highly speculative.
Indirect effects that would result from climatic changes that decrease food production would reduce overall global food security and lead to malnutrition and hunger. Shortages of food and fresh water and the disruptive effects of sea level rise may lead to psychological stresses and the disruption of economic and social systems.
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