Introduction to Climate Change

3. Current Impact

The Earth's climate has changed over the past century, because of the increase in atmospheric concentrations of greenhouse gases. Although some uncertainty remains over climate change, particularly regarding extreme weather conditions, the IPCC's Second Assessment could nonetheless conclude in 1995 that the observed warming trend was "unlikely to be entirely natural in origin" and that the balance of evidence suggested a "discernible human influence" on the Earth's climate. According to the IPCC, failure to mitigate greenhouse gases will result in a projected increase of between 1.4 to 5.8 Celsius by the year 2100. Temperatures are also expected to increase, even after concentrations of greenhouse gases are stabilized (although at a very slow rate).

A. Impact on the Oceans and Sea level

Global sea level has risen by between 10 and 20 cm over the past century, and this rise is very likely (i.e. between 90-99% likely) caused by this century's observed global warming. The primary variable in modeling future sea level rise, at least over the long term, remains the stability of the ice sheets. Ice sheet models project that a local warming of larger than 3 C, if sustained for millennia, would lead to virtually a complete melting of the Greenland ice sheet with a resulting sea level rise of about 7 meters! Some, but not all models also predict that melting from the West Antarctic Ice Sheet could contribute up to 3 meters to sea level rise as well over the next 1000 years.

B. Impact on Coastal and Marine Ecosystems

Coastal systems are expected to vary widely in their response to changes in climate and sea level. Climate change and sea level rise or changes in storms or storm surges could result in the erosion of shores and associated habitat, increased salinity of estuaries and freshwater aquifers, altered tidal ranges in rivers and bays, changes in sediment and nutrient transport, a change in the pattern of chemical and microbiological contamination in coastal areas, and increased coastal flooding. Changes in coastal ecosystems, such as saltwater marshes, mangroves, coral reefs, and river deltas, would harm tourism, freshwater supplies, fisheries and biodiversity.

In addition to sea level rise, climate change may also alter ocean circulation and vertical mixing, as well as reductions in sea ice cover. As a result, nutrient availability, biological productivity, the structure and functions of marine ecosystems, and heat and carbon storage capacity may be affected.

C. Weather Intensity

Warmer temperatures are very likely to lead to a more vigorous hydrological cycle; this translates into prospects for more severe droughts, floods and heat waves in some places.

D. Public Health Impacts

The increase in global temperatures may have significant impacts on public health, particularly in developing countries. The World Health Organization identified such health impacts of global warming as increased illnesses and deaths from heat waves and air pollution; increased outbreaks of some insect borne infectious diseases; and increased cases of diarrhea and other water borne diseases that are particularly dangerous in developing countries.

E. Deserts and Desertification

Deserts are likely to become more extreme; with few exceptions, they are projected to become hotter but not significantly wetter. Shifts in temperature and precipitation in temperate rangelands may result in altered growing seasons and boundary shifts between grasslands, forests and shrublands.

F. Water and Ice Resources

Climate change will intensify the global hydrological cycle, which could have major impacts on regional water resources. Changes in the total amount and frequency of precipitation directly affect the magnitude and timing of floods and droughts. Access to adequate water supplies already is a serious problem in many regions, including some low lying coastal areas, deltas and small islands, making countries in these regions particularly vulnerable to any additional reduction in indigenous water supplies. Ultimately, the impacts of climate change will depend in part on the ability of water resource managers to respond not only to climate change but also to population growth and changes in demands, technology, and economic and social conditions.