To investigate the possible role of the enhanced greenhouse effect in the recent river flooding events, the relevant physical processes have been analysed and an extensive literature survey has been carried out. In summary, the following conclusions can be drawn:
Physical arguments clearly indicate that global warming will cause an
increase of evaporation from the ocean. Moreover, a warmer atmosphere can
carry more moisture, which leads to larger amounts of precipitable water.
Global warming will also induce higher temperature differences between the
land and sea surfaces, causing an increased transport of precipitable water to
the continents, and an increase of convectional rainfall.
The general circulation models (GCMs) discussed all indicate an
intensification of the (global) hydrological cycle as a result of global
warming. At a doubling of the atmospheric CO2 concentration (expected at
about 2070) the estimated increase in mean annual global precipitation
varies from 3 to 15%. The general consensus is that we should expect greater
amounts of rainfall (10 to 20%) in much of the mid-latitudes during winter
(i.e. northwest Europe), increases at high latitudes throughout the year, and
in areas affected by monsoon rainfall (India, Northern Australia). Whenever
investigated, there is qualitative agreement between most models on a shift
towards more heavy rainfall events and higher rainfall intensities. Similarly,
there is consensus that in arid and semi-arid areas (such as Southern Europe
and North Africa) the amount of rainfall is expected to decrease or remain
more or less constant.
Present CO2 concentrations in the atmosphere have reached a level of 130%
as compared to pre-industrial levels and thus, we are one third underway to
doubling. When equivalent CO2 concentrations are considered, we are even
halfway to doubling. According to the theory, and given the projections for
increased rainfall under doubling of CO2 concentrations, we should already
be experiencing a certain increase in rainfall, say about 1 to 5% in the global
average, and about 5% in the mid-latitudes (i.e. northwest Europe).
Observations of (regional) trends in rainfall are consistent with the GCM
predictions described above. Some studies show that the evaporation rates
for different parts of the ocean have increased since the seventies. There is
observational evidence of more rainfall in northwest Europe. This is due to
an increase in the frequency of the zonal circulations (leading to more winds
from the sea), causing wetter and milder winters. At one station in the
Netherlands, an increase of +5% has been measured over the period
1961-1990 (compared to 1930-1960). In the United States and in the
monsoon regions increases in the frequency of heavy rainfall events have also
been observed.
The possibility of substantial precipitation changes must be considered as
realistic. In spite of the uncertainties and the lack of quantitative data we
are fairly confident that, as a prediction, it is qualitatively correct. Given the
nature of weather statistics, a small change in the mean weather comes with
changes in the frequency and magnitude of what we call extreme events.
Records on recent flooding and on paleoflooding indicate the high sensitivity
of flood occurrence to changing climate for river basins in the USA and
Europe. Analyses also indicate that there is no simple proportionality
between the scale and frequency of floods and climate variations. However, in
general, increases in precipitation lead to proportionally larger increases in
runoff.
Runoff can be considered as the hydrological response of river basins to
climate conditions. Studies presented here indicate that winter runoff
responses of alpine and nonalpine-related German river streams have
increased since 1965 (+26%), and a upward trend in runoff peaks in the
Meuse (10-12%) after 1911 (compared to the period 1882-1910). Another
study shows increases in natural streamflows (unaffected by human
intervention) all over the USA, illustrating that rainfall on this continent has
indeed increased. This supports the hypothesis that enhanced greenhouse
forcing produces an enhanced hydrologic cycle.
Several hydrologic modelling studies indicate that greater precipitation will
cause higher runoff and an increase in soil moisture, as expected. For the
Rhine river, it is estimated that a 20% increase in precipitation will double
the occurrence of what used to be an one-in-two year peak flow event.
However, it is difficult to distinguish climate-change induced effects in the
river regimes from changes caused by the natural variability of the climate,
and various man-made alterations (urbanisation, land use, drainage,
channelisation, dams etc.). Separating these from other effects can be done
by modelling, but such studies are currently in a very early stage.
The recent floods in northwest Europe (1993, 1995), and in the USA (1993)
were caused primarily by unusually high rainfall amounts in combination
with a saturation of soil due to preceding rainfall (and, to a lesser extent,
human interference in the catchment basin). Individual flooding events can
not be attributed directly to the enhanced greenhouse effect. According to
the predictions presented above an increase of flooding is what we expect as a
result of the enhanced greenhouse effect. The recent floods cannot be taken
as a proof of climate change as the associated rainfall events still fit in the
natural variability of our climate, and floods of comparable magnitude have
been observed earlier this century. What can be said is that an increased fre-
quency of unusual amounts of rainfall and floods as we have witnessed over
the last decade are consistent with the climate model predictions.
As greenhouse gases continue to accumulate in the atmosphere we should
expect an increase in rainfall and a (nonproportional) increase in river
flooding events, in particular in mid-latitude regions in the Northern
Hemisphere (i.e. northwest Europe, etc.). Given the long lead times of CO2
build-up in the atmosphere, it is difficult, if not impossible, to prevent (some
of) the expected increase in river flooding events. What can be done in the
short term is slowing down climate change by reducing greenhouse emissions.
In the longer term, such a (preventive) strategy can be aimed at stabilising
greenhouse gas concentrations, and thus decreasing the risk of a potential
destabilisation of the climate system.
