An accumulating body of evidence indicates that global warming (0.7°C over the twentieth century), especially during the last 50 years (0.43°C), has impacted a number of regions. The rates and patterns of climate change and impacts vary over the period as well. Anthropogenic and non-anthropogenic changes may have influenced greater changes in the climate system later in the century than earlier in the period. Impacts observed so far are primarily ecological in nature. These include changes in physiology, spatial distribution, species abundance and diversity, and timing of reproduction.
Much of the evidence of ecosystem changes to date has come from high-latitude (>40°N, 40°S) and high-altitude (>3,000 m) environments, and from species at their high-latitude range limits. Some evidence has been found in tropical and subtropical regions (both terrestrial and coastal ecosystems) and some in temperate oceans and coastal areas.
Observations in 1995-1996 show declines of glacier extent in western Antarctica and elsewhere, sea ice 40 percent thinner than 20 to 40 years ago in the Arctic, and shrinking of the area of perennial Arctic ice at a rate of 7 percent per decade (Vaughan et al., 1999). Satellite data over Northern Hemisphere extratropical lands show a retreat (about 10 percent reduction) of spring snow cover over the period 1973 to 1992 (Groisman, Karl, and Knight, 1994). Glaciers in Latin America have dramatically receded in the past decades. Many of them have disappeared completely. In 18 Peruvian glacial cordilleras, mass balances since 1968 and satellite images show a reduction of more than 20 percent of the glacial surface.
Changes in volume and areal extent of tropical mountain glaciers are among the best indicators of climate change. Himalayan glaciers that feed the Ganges River appear to be retreating at a fast rate. The estimated annual retreat of the Dokriani glacier (one of the several hundred glaciers that feed the Ganges) in 1998 was 20 meters compared to an annual average of 16.5 meters over 1993 to 1998. From observations dating back to 1842, the rate of recession of the snout (the point at which the glacier ice ends) has been found to have increased more than 2.5-fold per year. Between 1842 and 1935, the 26-kilometer long Gangotri glacier was receding at an average of 7.3 m every year, whereas between 1935 and 1990, the rate of recession had gone up to 18 m a year. Almost 67 percent of glaciers in the Himalayan and Tienshan mountain ranges have retreated since the 1970s (Fushimi, 1999).
Increased temperatures in mountainous regions appear to be causing plant species to move to higher altitudes. Approximate moving rates for common alpine plants are calculated to be between zero and four meters per decade (Grabberr, Gottfried, and Pauli, 1995).
In a short grass steppe in Colorado, with recorded temperature increases from 1964 to 1998, aboveground net primary productivity of the dominant grass is on the decrease (U.S. National Assessment, 2000).
A northward and upward shift has been detected in the range of checkerspot butterfly on the western coast of North America over the past century. In a sample of 35 nonmigratory European butterflies, 63 percent have ranges that have shifted north by 35 to 240 km during this century (Parmesan et al., 1999). The disappearance of 20 out of 50 species of frogs and toads in Costa Rica has been linked to recent warming (Pounds, Fogden, and Campbell, 1999). Earlier egg-laying dates have been found for 31 percent of 225 species of birds in the United Kingdom over the period 1971 to 1995 (Crick et al., 1997). In the Netherlands, the availability of caterpillar food has advanced by nine days over the period 1973 to 1995 (Visser et al., 1998).
In marine and littoral ecosystems, there is evidence of coral bleaching, declines of plankton, fish, and bird populations related to warming ocean temperature, mangrove retreat due to sea-level rise, and penguin species increases due to a decrease in sea ice.
Between 1974 and 1993, species richness of reef fishes has fallen and composition shifted from dominance by northern to southern species in the Southern California Bight (Holbrook, Schmitt, and Stephens, 1997).
Agriculture evidence of observed impacts is found in lengthening growing seasons at high latitudes, changing yield trends, and expansion of pest ranges. Carter (1998) observed that the growing season of the Nordic region (Iceland, Denmark, Norway, Sweden, and Finland) has lengthened over the period 1890 to 1995. Climate trends appear to be responsible for 30 to 50 percent of the observed increase in Australian wheat yields, with increases in minimum temperatures (decreases in frosts) being the dominant influence during 1952 to 1992 (Nicholls, 1997). Recent movement of agricultural pests and pathogens related to local climate trends is linked to global warming.
FUTURE SCENARIOS OF CLIMATE CHANGE Uncertainties of Future Climate
Global climate toward the middle and later years of the twenty-first century is projected using general circulation and coupled atmosphere-ocean models (GCMs). However, many uncertainties currently limit the ability to project future climate change. Three main sources of uncertainty with regard to future climate are
1. future greenhouse gas and aerosol emissions;
2. global climate sensitivity due to differences in ways that physical processes and feedback are simulated (some models simulate greater global warming than others do); and
3. regional climate change that is apparent from differences in regional estimates in climate change from the same global warming.
The wide range of projected climate change suggests that caution is required when dealing with any impact assessment based on GCM results. O'Brien (1998) highlighted the fact that the earlier forecasts of greenhouse impact were exaggerated, and new studies are suggesting a postponement of the greenhouse effect. Consequently, there is more time than previously expected to adapt and to take technological action to alleviate global warming. Therefore, decision makers need to be aware of the uncertainties associated with climate projections while formulating strategies to cope with the risk of climate change.
Despite these uncertainties, GCMs provide a reasonable estimate of the important large-scale features of the climate system, including seasonal variations and ENSO-like features. Many climate changes are consistently projected by different models in response to greenhouse gases and aerosols and are explainable in terms of physical processes. The models also produce with reasonable accuracy other variations due to climate forcing, such as interannual variability due to ENSO and the cause of temperature change because of stratospheric aerosols.
A scenario is a coherent, internally consistent, and plausible description of a possible future state of the world (IPCC, 1994). It is not a forecast; rather, each scenario is one alternative image of how the future can unfold. Scenarios are one of the main tools for the assessment of future develop ments in complex systems that are often inherently unpredictable, insufficiently understood, and possessing many scientific uncertainties. Scenarios are also vital aids in evaluating the options for mitigating future emissions of greenhouse gases and aerosols, which are known to affect global climate. There are three main approaches to climate scenario development:
1. Incremental scenarios: In this approach particular climatic or related elements are changed by realistic but arbitrary amounts. They are commonly applied to study the sensitivity of an exposure unit to a wide range of variations in climate.
2. Analogue scenarios: Analogue scenarios are both temporal and spatial. Temporal analogues use climatic information from the past as an analogue of the future climate. Analogue scenarios are based on past climate, as reconstructed from fossil records as well as observed records of the historical period. Spatial analogue scenarios are the climatic conditions in regions that are analogues to those anticipated in the study region in the future.
3. Climate model output-based scenarios: These are based on the results of general circulation model experiments. GCMs are three-dimensional mathematical models that represent the physical and dynamic processes responsible for climate. This is the most commonly used approach in climate change research.
A Generalized Global Climate Scenario of the Twenty-First Century
Based on multimodel output, the current range of twenty-first-century global surface temperature warming is 1.5 to 4.5°C, with a "best estimate" of 2.5°C (Kenitzer, 2000). The increases in surface temperature and other associated changes are expected to increase climate variability.
Climate models simulate a climate change-induced increase in precipitation in high and midlatitudes and most equatorial regions but a general decrease in the subtropics (IPCC, 1996). Across large parts of the world, changes in precipitation associated with global warming are small compared to those due to natural variability.
Global mean sea level is expected to rise as a result of thermal expansion of the oceans and melting of glaciers and ice sheets. The IPCC (1996) estimates sea-level rise from 1995 to the 2050s in the range of 13 and 68 cm and in the range of 15 to 95 cm to the year 2100, with a "best estimate" of 50 cm.
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