Much of the energy absorbed at the Earth's surface is radiated upwards towards space as infrared (IR) thermal energy.Several gases that occur naturally in the atmosphere absorb this infrared energy and re-radiate it back to the Earth's surface. This re-absorbance of heat causes the Earth's surface to reach higher temperatures than it normally would if there was an atmosphere without IR-trapping gases. The term 'greenhouse' is used as the gases involved in such a phenomenon act like the glass of a greenhouse to trap heat and maintain higher interior temperatures than would normally occur.
There is growing concern that the observed increase in carbon dioxide and other greenhouse gas levels is likely to lead to a global increase in temperature of between 20C and 40C over the next century. Global circulation models predict that the increae is not likely to be uniform; some areas will have a slightly lower increase and other parts considrably more. Increase of greenhouse gas concentrations, causes a reduction in outgoing infrared radiation, thus the Earth's climate must change somehow to restore the balance between incoming and outgoing radiation. This causes a phenominom known as 'global warming¨. However, a small rise in temperature will induce many other changes, for example, cloud cover and wind patterns. Some of these changes may act to enhance the warming.
If global warming takes place, sea level will rise due to two different processes. Firstly, warmer temperature causes sea level to rise due to the thermal expansion of seawater. Secondly, water from melting glaciers and the ice sheets of Greenland and the Antarctica would also add water to the ocean. It is predicted that the Earth's average sea level will rise by 0.09 to 0.88 m between 1990 and 2100. A measurable rise in sea level will have a severe economic impact on low-lying coastal areas and islands, for example, increasing the beach erosion rates along coastlines etc
Agricultural Impact Experiments have shown that with higher concentrations of CO2, plants can grow bigger and faster. However, the effect of global warming may alter the global precipitation pattern as well as changing the soil moisture contents over various continents. Since it is unclear how global warming will affect climate on a regional or local scale, the probable effects on the biosphere remains uncertain.
d) Effects on Hydrological Cycle Global precipitation is likely to increase. However, it is not known how regional rainfall patterns will change. Some regions may have more rainfall, while others may have less. Furthermore, higher temperatures would probably increase evaporation. These changes would probably create new stresses for many water management systems.
Author: Julia Allbutt (document modification date: 22th May 2003)