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The results of thirty years of research by the scientific community now convincingly suggest that it is fair to call the detection and attribution of human impact on climate a fact. The scientific consensus is clearly expressed in the reports of the Intergovernmental Panel on climate Change (IPPC). Additionally, all major scientific bodies in the U.S. whose members' expertise bears directly on the matter have issued similar consensus including the National Academy of Science, the American Meteorological Society, the American Geophysical Union, and the American Association for the Advancement of science (Oreskes, 2004) Only a small minority of qualified scientists still contest the view that humanity's actions have played a role in increasing recent temperatures. However, uncertainties do exist regarding how much climate change should be expected in the future, and a hotly contested political and public debate exists over what actions should be taken in light of global warming.
The global average surface temperature has increased over the 20th century by about 0.6 degrees Centigrade. Globally, it is very likely that the 1990s was the warmest decade. A new study by NASA indicates that 2005 was the warmest year in at least a century, surpassing 1998. According to their data, the five warmest years over the last century have occurred since 1997: 2005, then 1998, 2002, 2003 and 2004. New analyses of proxy data for the Northern Hemisphere indicate that the increase in temperature in the 20th century is likely to have been the largest of any century during the past 1,000 years. (IPCC, 2002) Snow cover and ice extent have decreased.
Worldwide measurements from tidal gauges indicate that global mean sea level has risen between 10 and 25cm (18cm average) during the last 100 years (Warrick et al., 1996). This rate is greater than the average of the last few thousand years estimated from geological and archaeological records (Warrick et al., 1996).
Prediction: Higher maximum temperatures; more hot days and heat waves over nearly all land areas. Higher minimum temperatures; fewer cold days, frost days, and cold waves over nearly all land areas.
Expected Effect: Temperature affects physiological function and often marine and estuarine organisms already live close to their thermal limit. Thus shifts in biogeographic ranges and zonation patterns can be expected. Different life stages are differently affected by thermal stress such as such planktonic and young benthic stages are more susceptible than adults. Temperature also affects the timing of life stage transition with possible decoupling of larval environment from the cues used by adult populations (Harley et al, 2006). Higher ocean temperatures will increase water stratification.
Prediction: The IPCC Second Assessment Report estimated a sea-level rise for the next 100 years of about 49cm, with a range of uncertainty of 20-86cm. Thermal expansion of the ocean and melting of small glaciers will be the main cause of the increase in the rate of sea level rise.
Expected Effect: Coastal wetlands and lowlands, beaches and barrier islands, and ocean islands and atolls are especially vulnerable to rising seas. Depending on the rate of sea-level rise, the rate of vertical wetland build-up, and the capacity for wetlands to migrate inland, a 50cm sea-level rise could inundate up to 50% of North American coastal wetlands (Shriner and Street, 1998). Wetlands in most areas have been able to keep pace with historic sea-level rise by accreting sediment and growing vertically and by moving inland with the encroaching sea. But the accelerated rates projected for the next 100 years may be too fast for natural accretion and migration to keep up. Sediment deficits and development barriers are among the most important factors that limit the survival of this important coastal habitat. Sea level rise could result in decreased habitat availability within a particular depth zone. Intertidal habitat maybe decreased especially where steep topography and sea walls prevent the landward migration of mudflats and sandy beaches (Galbraith et al, 2002). Sea level rise may reduce the spatial extent of biogenic habitat by outpacing the accretion rates of corals and marshes.
Prediction: Increase in tropical storms and hurricanes intensity with increased rainfall and peak winds over some areas.
Expected Effect: Increase of extreme winds and storm waves has implications for intertidal ad subtidal habitat vulnerable to hydrodynamic disturbance. Researchers are do not agree on the exact nature of climate induced circulation changes, such as upwelling, shifts in nutrient supplies are likely in the future. Species with planktonic life history stages, common in estuarine and marine systems are sensitive to changes in upwelling and along shore advection patterns (Harley et al, 2006)
Prediction: Ocean acidification.
Almost half the carbon dioxide produced by human activity in the past 2 centuries is now dissolved in the oceans. A pH reduction of approximately 0.1 unit in surface waters has occurred already due to oceanic uptake of anthropogenic CO2. Estimates of future atmospheric and oceanic CO2 concentrations, based on the IPCC emission scenarios, indicate that by middle of this century atmospheric CO2 levels could be reach over 500 ppm, and near the end of the century they could be over 800 ppm. Surface water acidity (pH) drop would be approximately 0.4 pH units, and the carbonate ion concentration would decrease almost 50% by the end of the century. This surface ocean pH drop would be lower than it has been for more than twenty million years.
Expected Effect: Ocean acidification could be expected to have major negative impacts on corals and other marine organisms that build calcium carbonate shells and skeletons. (Feely et al, 2004). In addition, lowered pH has a profound effect on in marine organism's physiological processes such as protein synthesis and ion exchange (Harley et al, 2006).
If the Greenland ice sheet melted completely, it would raise global sea levels by about 7 meters. Greenland's contribution to global sea level rise today is two to three times greater than it was in 1996. It is thought the entire Greenland ice sheet could melt in about 1,000 years, but the latest evidence suggests that could happen much sooner.
The IPCC 2001 considered the Antarctic Ice Sheet overall not to be a major cause of the current rise in sea level and its collapse to be very unlikely during the 21st century. However, since 2001 a number of important new results suggest that the contribution of the Antarctic to global sea level rise needs to be reassessed. Ice sheets covering both the Arctic and Antarctic could melt more quickly than expected this century, according to two studies that blend computer modeling with paleoclimate records. Of particular concern is the West is the stability of the west Antarctic ice sheet (WAIS) Much of WAIS rests on bedrock below sea level (as deep as 2km)— with the possibility that a combination of combination of accelerated flow and hydrostatic lift may cause runaway discharge. Melting of WAIS would raise global sea levels by about 7 meters.
Even of we stabilize carbon dioxide levels, sea level is likely to continue rising for the next several centuries. Two global coupled climate models show that even if the concentrations of greenhouse gases in the atmosphere had been stabilized in the year 2000, we are already committed to further global warming of about another half degree and an additional 320% sea level rise caused by thermal expansion by the end of the 21st century (Meehl et al, 2005, Wigley, 2005).