Geoscience Reference
In-Depth Information
ability of many fluvial characteristics and the need to take account of possible alternative
causes of change, such as alterations to land use and other human activities. Nonetheless,
the influence of global warming has already been identified in some recent modifications
to fluvial systems. Work on a number of the world's large drainage basins has established a
significant rising trend in the risk of great floods (those with a return period of 100 years)
in the 20th century. Warmer air temperatures are also having a predictable effect on glaciers
- melting and retreat - in many parts of the world. Glaciers are receding particularly fast
in the Himalaya and parts of Tibet, generating worries about long-term water supplies for
hundreds of millions of people in India, Bangladesh, Nepal, and China who rely on rivers
fed by glacial meltwater.
Ice cover has been in general decline since the mid-19th century on most rivers in North
America and Eurasia as gradual warming has meant freeze-up dates have been occurring
later and break-up dates arriving earlier. In the case of the lower Don River in Russia, the
length of the ice season has been reduced by a whole month in about 100 years. Records
for the Tornionjoki River in Finland stretch back to 1692 and show a long-term trend to-
wards earlier break-up dates throughout the entire period. This tendency is not universal,
however. Rivers in central and eastern Siberia display significant trends in the opposite dir-
ection: towards longer periods with ice cover due to earlier freeze-up dates and later break-
up dates.
Northern hemisphere rivers that flow into the Arctic Ocean have been delivering more wa-
ter in line with longer ice-free periods, combined with an increase in precipitation. More
fresh water in the Arctic could slow down or shut off the so-called 'thermohaline circula-
tion', an oceanic current conveyor belt which transports large amounts of warm water to
the North Atlantic region. This circulation is driven by differences in the density of sea wa-
ter, controlled by temperature and salinity, so more fresh water could counteract the flow.
The thermohaline circulation helps to regulate the climate of northern Europe, maintaining
temperatures that are higher than would be otherwise expected given the latitude.
Conversely, a number of other rivers have seen declines in the amount of water they carry
each year since the mid-20th century. Several of the major rivers with dwindling flows
serve large populations, sparking further concerns about future water supplies. These rivers
include the Yellow River in northern China, the Ganges in India, the Niger in West Africa,
and the Colorado in North America.
Drought is thought to be the greatest agent of change associated with global warming in
the Amazon Basin. Many computer-based models of future climate in the region indicate a
reduction of dry-season rainfall, the effects of which will be exacerbated by rising air tem-
