Increasing global surface temperatures are very likely to lead to changes in precipitation and atmospheric moisture, because of changes in atmospheric circulation, a more active hydrological cycle, and increases in the water holding capacity throughout the atmosphere. Atmospheric water vapour is also a climatically critical greenhouse gas, and an important chemical constituent in the troposphere and stratosphere.

Precipitation measurement and analysis are made more difficult by accompanying natural phenomena such as wind and the use of different instruments and techniques (Arkin and Ardanuy, 1989). Because of the substantial under-catch of precipitation gauges during solid precipitation, frequent light rainfall events, or windy conditions, the true precipitation in the Arctic is more than 50% higher than the measured values (Førland and Hanssen-Bauer, 2000). Gauge under-catch is substantially less in warmer, less windy climates with heavier rainfall. New, satellite-derived precipitation estimates offer the prospect of near-global climatologies covering at least one or two decades, but multi-decadal global changes cannot be estimated with high confidence.

For all these reasons it is useful to compare changes in many of the moisture-related variables, such as streamflow and soil moisture, with precipitation to help validate long-term precipitation trends.

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