Climate Change 2001:
Working Group I: The Scientific Basis
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Figure 2.25(i): Trends for 1900 to 1999 for the four seasons. Precipitation trends are represented by the area of the circle with green representing increases and brown representing decreases. Annual and seasonal trends were calculated using the following method. Precipitation anomalies in physical units were calculated for each station based on 1961 to 1990 normals and averaged into 5° x5° grid cells on a monthly basis. The 1961 to 1990 monthly mean precipitation for each grid cell was added to the monthly anomalies and the resulting grid cell values summed into annual and seasonal totals. This series was converted into percentages of normal precipitation, and trends calculated from the percentages. Average trends within six latitude bands (85°N to 55°N, 55°N to 30°N, 30°N to 10°N, 10°N to 10°S, 10°S to 30°S, 30°S to 55°S) are shown in the legend of each map. The 1961 to 1990 monthly mean precipitation for the latitude band was added to the anomaly time-series and the resulting values totalled across all months within the season or year. The significance of each trend (based on a 5% level) was determined using a t-test and a non-parametric test statistic. Trends found to be significant under both tests are indicated with an asterisk.

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