TREND IN RIVER RUN-OFF Millimeters 1948–2004
55 mm to 310 mm large increase 4 mm to 55 mm moderate increase –4 mm to 4 mm little change –55 mm to –4 mm moderate decrease –310 mm to –55 mm large decrease no data
Possible large-scale changes in annual water run-off 2090–99 relative to 1980–99
Decrease 40% or more 20% – 39%
Increase 2% – 4% little change
40% or more
2% – 4% 5% – 9%
10% – 19%
10% – 19%
5% – 9%
20% – 39%
fewer than 66% of models agree more than 90% of models agree
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POTENTIAL CHANGE IN RUN-OFF
Some impacts of climate change have already been observed. The annual trend in run-off (the balance of rainfall and evapotranspiration) in major river basins over the past 50 years may be an early indicator of future climate impacts. The pattern is mixed across the world, although few areas show large increases. Run-off decreases are strongly related to warmer temperatures and decreases in precipitation; both are linked to episodes of the El Niño/Southern Oscillation. However, it is too early to fully attribute such trends to climate change. Water planning often looks forward beyond 30 years to evaluate a range of potential futures. Increasing aridity in the Mediterranean is likely, in contrast to increased run-off in northern temperate regions. However, in large parts of the world, the balance of change is unclear.
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