Increase in Rainfall and Increase in Sea Levels

To maintain the global water evaporation from oceans, worldwide must be balanced by rainfall into the oceans. Earth's atmosphere contains only 0.001 percent of the Earth's water, yet the currents of air carries water vapor over land resulting in precipitation.

Increasing atmospheric concentrations of greenhouse gases, mainly carbon dioxide, has led to warming at the surface, by nearly 0.6°C (1.0°F) during the twentieth century. It is widely believed that this trend will continue in the twenty-first century, leading to a higher sea-surface temperature, among other factors.

One perceptive consequence of a warmer ocean surface is a large vapor-pressure difference between the sea surface and the adjacent atmosphere. This would enhance the evaporation rate, and hence increase rainfall. For instance, a global warming by 4°C (7.2°F) is expected to increase global precipitation by about 10 percent. Research suggests that the increase is more likely to come as heavy rainfall, rather than as more frequent rainfall.

There are few changes suggested for the evidence in change of hydrologic cycle. First, due to global warming there had been an increase in night-time temperature. Daily minimum temperatures have increased at twice the rate of daytime temperatures since 1950 (roughly 1.0°C versus 0.5°C). This suggests an increase in cloudiness and/or humidity at night, and increased evaporation and cooling during the daytime. (This cooling varies according to body heat evaporating).

Second, satellite data suggest, mean (average) atmospheric water-vapor concentration has increased. This enables storms to generate more rainfall.

Third, amounts of rain have changed in different ways in various regions during the last 80 years, but they generally have increased in the middle and high latitudes, often in excess of 10 percent.

In the United States, annual rainfall has increased by about 10 percent during the twentieth century, on an average. The heaviest rainfall are expected to occur near Polar Regions, for two reasons. One, the warming rate has been and will continue to be the highest there, and warmer air can hold more water vapors. Two, the warming will reduce the extent of sea ice, thereby allowing more evaporation.

There has been decrease in rainfall that has been observed in some regions. In the Northern Hemisphere tropics, especially in Africa, a significant decrease in rainfall has occurred since 1950.  Thereby this region is under intense drought.

Changes in rain events, such as monsoon rains, can be either beneficial or harmful. For instance, decrease in rainfall can cause water shortages, and increase in rainfall can aggravate flooding. The potential impact of global warming by monsoon circulation is unknown. In the tropical Pacific, the sea-surface temperature, evaporation rate and rainfall amounts have all increased.

Finally, water vapor is one of the indicators used to measure the amount of greenhouse gases. Increased cloudiness heats the planet. The clouds are high or dark, as is the cases for most storms the clouds reduce the outgoing long-wave radiation more than the net incoming short-wave radiation.