The rising temperature of ocean water has a number of important consequences. Specifically, as water heats, it expands. This is referred to as thermal expansion. Many scientists believe, therefore, that as ocean temperature rises, the water will take up more space, and sea levels will rise.
Global warming may contribute to a rise in sea levels in other ways as well. As world temperatures rise, large, land-based ice flows called glaciers may begin to melt. This outcome could be especially significant if the massive ice sheets on Antarctica and Greenland were to begin to melt.
Predicting sea-level rise is very difficult, and scientists disagree about how much exactly sea levels can be expected to rise. Some argue that sea-level rise may not be significant. Others worry that a melting of the Greenland ice sheet could mean a rise of 10 to 20 feet (3-6m) in sea level. The general consensus among scientists, however, is that sea-level rise will be between these two extremes. The Intergovernmental Panel on Climate Change (IPCC) predicts that sea-level rise will be between 7 and 23 inches (18-59cm) over the next century.
A significant rise in sea level would affect human beings in numerous ways. Coastal areas would lose land and be subject to increased flooding from storms. Regions that might be affected include the east coast of the United States, California, Nigeria, the Netherlands, and other low-lying areas worldwide. In general, the areas that would be hardest hit are poor regions such as Bangladesh, which do not have the resources to prepare or react to major changes.
Rising sea levels may have similar effects on islands, which could lose land or, in some cases, be submerged altogether. This possibility is a particular concern on some small Pacific islands, though the exact extent of the danger is difficult to assess.
After rising sea level, perhaps the biggest concern about how global warming may affect world water involves rain. Warm air holds more water vapor than cold air, so scientists expect that rainfall worldwide should increase by 3 percent to 5 percent, though such predictions are speculative at best.
Increased rainfall would have a number of benefits. In the first place, more rain might mean more cloud cover, which could slow global warming. Moreover, rain is good for crops and agriculture. More water in the atmosphere might also mean more extreme weather events like hurricanes, however.
In addition, though the world overall should be wetter, some areas may in fact become drier as a result of global warming. For example, the increase in evaporation rates may dry out soils, turning rain forests to savannahs in Latin America.
Warmer temperatures and higher evaporation rates could also contribute to droughts. Scientists are particularly concerned that global warming may be shrinking rivers and lakes around the world. Global warming may also result in more rain and less snowfall, which could have a dangerous effect on areas such as the American Southwest, which rely on melting snow for water during the dry summer months.
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