NCDC Frequently Asked Questions
Is the hydrological cycle (evaporation and precipitation) changing?
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Globally-averaged land-based precipitation shows a statistically insignificant upward trend with most of the increase occurring in the first half of the 20th century. Further, precipitation changes have been spatially variable over the last century. On a regional basis increases in annual precipitation have occurred in the higher latitudes of the Northern Hemisphere and southern South America and northern Australia. Decreases have occurred in the tropical region of Africa, and southern Asia. Due to the difficulty in measuring precipitation, it has been important to constrain these observations by analyzing other related variables. The measured changes in precipitation are consistent with observed changes in stream flow, lake levels, and soil moisture (where data are available and have been analyzed).
Northern Hemisphere snow cover extent has consistently remained below average since 1987, and has decreased by about 10% since 1966. This is mostly due to a decrease in spring and summer snow extent over both the Eurasian and North American continents since the mid-1980s. Winter and autumn snow cover extent have shown no significant trend for the northern hemisphere over the same period.
Clouds are also an important indicator of climate change. Surface-based observations of cloud cover suggest increases in total cloud cover over many continental regions. This increase since 1950 is consistent with regional increases in precipitation for the same period. However, global analyses of cloud cover over land for the 1976-2003 period show little change.