For more information on changes in global precipitation, visit: EPA Global Warming Site: Trends
However, too much precipitation can also have a negative
impact on human activities, business and industry, agriculture, and the environment. For example, too much rain or snowmelt (water from melted
snow) at one time can lead to flooding. Living organisms, including crops, can drown in floodwaters.
For these reasons, many scientists have been monitoring precipitation trends. That is to say, they have been watching changes in the amount of precipitation falling to Earth. In the last 100 years, precipitation has increased by an average of about 1% over all the land surfaces on Earth. Across the United States, alone, precipitation has increased by an average of about 5% in the last 100 years. Note that this is an average. Some areas of the United States have experienced as much as a 20% increase in precipitation over the last 100 years (see figure above). While other areas have experienced a 20% decrease in precipitation. Map courtesy of the United States Environmental Protection Agency.
There are many reasons for changes in precipitation. The
leading cause is a change in temperature. Many
scientists believe an increase in temperature could lead to a more intense water cycle. The rates of
evaporation from soils and water, as well as transpiration from plants, could increase. The amount of
precipitation could also increase.
Of course predicted changes in the water cycle also differ
according to the climate model used. The "business as usual" climate model (described
in Remote Sensing: Carbon
Dioxide" assumes climate changes based solely on the effects of greenhouse gases.
Predictions of changes in precipitation based on this model show an increase in the amount of precipitation
falling in the next 100 years (see figure below). Another model includes in its assumptions the effects of
sulfates. Sulfates cause a cooling of the atmosphere. Therefore, sulfates can decrease the effects of
greenhouse gases on temperature. Models of future precipitation that include the effects of sulfates also
show an increase in the amount of rain and snow. However, the increase in precipitation predicted in these
models is not as great as the increase predicted in the "business as usual" models.
Whether or not the amount of precipitation changes, many climate models show that the timing of precipitation will change. Scientists predict that most precipitation in the future will fall during a smaller number of storms that are heavier in intensity. This is because the elevated temperatures will provide more energy in the atmosphere for storm production.
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