2.5.2 Changes in Precipitation and Related Variables
2.5.2.1 Land
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° x 5° 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. |
Figure 2.25(ii): As in Figure 2.25(i) except
annual trends for the three periods of changing rates of global temperature
(shown in Figure 2.9) and the full period, 1900
to 1999. During the 100-year periods, calculation of grid cell trends required
at least 66% of the years without missing data and at least three years
of data within each decade except the first and last. During the shorter
periods, calculation of grid cell trends required at least 75% of the years
without missing data. Stations with more than one sixth of their data missing
during the normal period and grid cells with more than one season or year
without any measurable precipitation during the normal period were excluded
from consideration. Due to the nature of trend estimation, it is not possible
to cumulatively sum the trends for each of the three periods to obtain an
overall trend. |
Overall, global land precipitation has increased by about 2% since the beginning
of the 20th century (Jones and Hulme, 1996; Hulme et al., 1998). The increase
is statistically significant but has been neither spatially nor temporally uniform
(Karl and Knight, 1998; Doherty et al., 1999). Dai et al. (1997b) found a global
secular increase in precipitation separate from ENSO and other modes of variability.
Data from over 20,000 stations contributed to the changes since 1900 shown in
Figure 2.25. The effects of changes in windshields
on winter precipitation measurements were taken into account for most mid- and
high latitude observations. Dai et al. (1997b) indicated that instrumental discontinuities
are unlikely to significantly impact other observations.
Mid- and high latitudes
Over the 20th century, annual zonally averaged precipitation increased by between
7 to 12% for the zones 30°N to 85°N and by about 2% between 0°S
to 55°S (Figure 2.25(ii)). The increase in
the Northern Hemisphere is likely to be slightly biased because adjustments
have not been made for the increasing fraction of precipitation falling in liquid
as opposed to frozen form. The exact rate of precipitation increase depends
on the method of calculating the changes, but the bias is expected to be small
because the amount of annual precipitation affected by this trend is generally
only about a few per cent. Nevertheless, this unsteady, but highly statistically
significant trend toward more precipitation in many of these regions is continuing.
For example, in 1998 the Northern Hemisphere high latitudes (55°N and higher)
had their wettest year on record and the mid-latitudes have had precipitation
totals exceeding the 1961 to 1990 mean every year since 1995.
Figure 2.25(i) shows mostly increasing precipitation in the Northern Hemisphere
mid- and high latitudes, especially during the autumn and winter, but these
increases vary both spatially and temporally. For example, precipitation over
the United States has increased by between 5 to 10% since 1900 (Figure
2.25(ii)) but this increase has been interrupted by multi-year anomalies
such as the drought years of the 1930s and early 1950s (Karl and Knight, 1998;
Groisman et al., 1999). The increase is most pronounced during the warm seasons.
Using data selected to be relatively free of anthropogenic influences such as
ground water pumpage or land use changes, several recent analyses (Lettenmaier
et al., 1999; Lins and Slack, 1999; Groisman et al., 2001) have detected increases
in streamflow across much of the contiguous United States, confirming the general
tendency to increasing precipitation. However, Lins and Michaels (1994) found
in some regions that increased streamflow did not relate well to an increase
in rainfall. This has been further evaluated by Groisman et al. (2001) who show
that changes in snow-cover extent also influence the timing and volume of streamflow.
Regionally, Mekis and Hogg (1999) showed that precipitation in Canada has increased
by an average of more than 10% over the 20th century. Zhang et al. (2000) report
an increase in Canadian heavy snowfall amounts north of 55°N and Akinremi
et al. (1999) found rainfall significantly increasing in the Canadian prairies
from 1956 to 1995. Multi-decadal streamflow data in Canada are not extensive,
but there are no apparent inconsistencies between observed changes in streamflow
or precipitation (Zhang et al., 2000).
Over the last 50 years there has been a slight decrease
in annual precipitation over China (Zhai et al., 1999a), which is supported
by a significant (5% confidence level) decrease in the number of rainy days
(3.9%/decade; Figure 2.25 (ii)). In contrast, the area affected by the upper
10% of heaviest precipitation has significantly increased. Zhai et al. (1999b)
show a significant increase in precipitation over the middle and lower reaches
of the Yangtze River and west China during the latter part of the 20th century,
while also detecting a declining trend in precipitation over northern China.
There have been marked increases in precipitation in the latter part of the
20th century over northern Europe, with a general decrease southward to the
Mediterranean (Schönwiese and Rapp, 1997; Figure
2.25(i)). Dry wintertime conditions over southern Europe and the Mediterranean
(Piervitali et al., 1998; Romero et al., 1998) and wetter than normal conditions
over many parts of northern Europe and Scandinavia (Hanssen-Bauer and Førland,
2000) are linked to strong positive values of the North Atlantic Oscillation,
with more anticyclonic conditions over southern Europe and stronger westerlies
over northern Europe (Section 2.6.5).
Based on recent research (Bogdanova and Mescherskaya, 1998; Groisman and Rankova,
2001), the precipitation trend for the last century over the former USSR as
reported by the SAR was slightly overestimated. The new results indicate that
precipitation has increased since 1891 by about 5% west of 90°E for both
warm and cold seasons. Georgievsky et al. (1996) also noted increases in precipitation
over the last several decades over western Russia, accompanied by increases
in streamflow and a rise in the level of the Caspian Sea. In eastern Russia
a negative precipitation trend since 1945 is embedded in the century-long positive
precipitation trend (Figure 2.25(ii); Gruza et
al., 1999). Soil moisture data for large regions of Eurasia (Robock et al.,
2000) show large upward trends. The rate of increase is more than 1 cm/decade
in the available soil moisture in the top 1 m of soil. These large positive
trends occur simultaneously with positive trends in temperature that would normally
reduce soil moisture. Increases in precipitation (and cloud cover, Section
2.5.5) are believed to have more than compensated for the increased losses
due to evapotranspiration.
An analysis of rainfall data since 1910 by Haylock and Nicholls (2000) reveals
a large decrease in total precipitation and related rain days in south-western
Australia. Annual total rainfall has increased over much of Australia with significant
increases of 15 to 20% in large areas. The increase in total rainfall has been
accompanied by a significant 10% rise in the average number of rain days over
Australia (Hennessy et al., 1999). Elsewhere in the Southern Hemisphere, a long-term
increase in precipitation in Argentina has been observed for the period 1900
to 1998 (Figure 2.25(i); Dai et al., 1997b).
Tropics and sub-tropics
The increase in precipitation in the mid- and high latitudes contrasts with
decreases in the northern sub-tropics (with marginal statistical significance)
which were largely responsible for the decade-long reduction in global land
precipitation from the mid-1980s through the mid-1990s. Since the SAR, record
low precipitation has been observed in equatorial regions, while the sub-tropics
have recovered from their anomalously low values of the 1980s.
Regionally positive but non-significant trends have occurred in the rainy season
rainfall in north-east Brazil and northern Amazonia (Marengo et al., 1998).
River data from northern Amazonia indicate wetter periods in the mid-1970s,
and in 1990, as well as drier periods between 1980 to 1990, consistent with
rainfall anomalies. Northern Amazonian rainfall appears to be modulated by multi-decadal
climate variations.
There is little evidence for a long-term trend in Indian monsoonal rainfall
but there are multi-decadal variations (Kumar et al., 1999a,b). From 1906 to
about 1960, monsoonal rainfall increased then decreased through 1974 and has
increased since (see Section 2.6). In central America
for much of the period from the early 1940s to present, western Mexico has experienced
an increasingly erratic monsoonal rainfall (Douglas and Englehart, 1999).
Since 1976, increases in precipitation in the South Pacific have occurred to
the north-east of the South Pacific Convergence Zone (SPCZ) while decreases
have occurred to its south-west (Salinger et al., 1996). Manton et al. (2001)
found significant decreases in rain days since 1961 throughout Southeast Asia
and western and central South Pacific, but increases in the north of French
Polynesia and Fiji.
Streamflow data for major rivers in south-eastern South America for the period
1901 to 1995 show that streamflow has increased since the mid-1960s, and was
accompanied by a significant decrease in the amplitude of the seasonal cycle
of most of those rivers (Garcia and Vargas, 1998; Genta et al., 1998). Figure
2.25(i) shows increases in precipitation since 1900 along the South American
eastern coastal areas, with less extensive increases since 1976.
There has been a pattern of continued aridity since the late 1960s throughout
North Africa south of the Sahara. This pattern is most persistent in the western
region. The driest period was in the 1980s with some recovery occurring during
the 1990s, particularly in the easternmost sectors where rainfall in some years
was near or just above the long-term mean (Nicholson et al., 2000). Southern
Africa was relatively moist in the 1950s and 1970s (Nicholson et al., 2000);
but Hulme (1996) found significant decreases in precipitation being observed
since the late 1970s. Early 2000, however, has seen flood-producing rains in
the eastern part of southern Africa.
