Heavy Rainfall Has Increased as Temperatures Have Risen
Bringing Threat of More Damage in Future
By K. Hennessy and R. Suppiah, CSIRO Division
of Atmospheric Research, Australia
Extreme rainfall events cause significant damage to agriculture,
ecology and infrastructure, disruption to human activities, injury
and loss of life. Any change in the probability of extreme rainfall
would have important implications for engineering, insurance,
town planning and many other activities. What follows is an assessment
of past and future changes in extreme rainfall.
Observed changes
Changes in extreme rainfall have been reported for various regions.
The US National Climatic Data Centre (NCDC) found an increasing
trend in rainfall exceeding 50.8 mm/day since 1910 over the USA,
but not in the former Soviet Union and China. Daily rainfall data
from 1890 to 1980 in Japan show that more stations recorded their
highest, second highest or third highest rainfall event in more
recent decades. An increase in the rainfall threshold exceeded
by the heaviest 10 percent of events (90th percentile) in Australia
has been found since 1910. Unfortunately, these analyses represent
a small fraction of the land surface and a very small part of
the planet, so a global picture cannot be formed at this stage.
The NCDC is addressing this issue in an extended analysis of extreme
rainfall changes in the USA, Canada, Mexico, the former Soviet
Union, China, Australia, Poland and Norway.
Increases in extreme rainfall in Japan, Australia and the USA
have occurred during a period in which global mean temperatures
have increased by 0.3 - 0.6 degrees C. At this stage, it is not
clear what proportion of the observed warming and any associated
increase in rainfall intensity is due to natural variability or
to anthropogenic influences such as land-use change, biomass burning,
ozone depletion and increased levels of greenhouse gases. Attribution
of cause and effect is unlikely to be a simple task.
Future changes
Concentrations of greenhouse gases, particularly carbon dioxide
(CO2), are increasing in the atmosphere. This is expected to lead
to global warming and climate change, some of which may already
be evident. To determine the potential impact on extreme rainfall,
global climate models (GCMs) are used. These models have a simplified
representation of the atmosphere, oceans, land and icecaps. Variables
like temperature, pressure and precipitation are computed at points
300-500 kilometers apart on a three-dimensional grid covering
the planet. GCMs can simulate the continental scale behavior of
the climate system but small-scale features like thunderstorms
are not well resolved due to limited computer power. Climate change
due to enhancement of the greenhouse effect is simulated by increasing
the concentration of carbon dioxide (and sometimes other trace
gases and aerosol) in GCMs.
Return Periods
Global and regional climate models from Australia, the United
Kingdom, and Germany simulate a general increase in heavy rainfall
(over 10 mm/day) frequency and intensity almost globally for a
doubling of the present concentration of CO2. Heavy rainfall return
periods (the average interval between events of the same magnitude)
were analysed in the UKMO, CSIRO4 and CSIRO9 GCMs. For a given
rainfall intensity, the average return period becomes shorter
by a factor of 2 - 5 (i.e. these events occur 2 - 5 times more
often) over selected countries (Australia, India, Europe and the
USA). Alternatively, for a given rainfall return period, the intensity
of heavy rainfall increases by 10-25 percent for each country.
These results are based on simulations with GCMs having a grid
resolution of 300-500 kilometers. Using such coarse resolution
gives grid-box-average rainfall intensities which are much less
than observed at a single location. More detailed and realistic
estimates are to be expected from simulations with finer spatial
resolution. Due to limited computer power, models with such fine
detail can only be run over small regions. These regional climate
models (RCMs) are driven at their boundaries by input from coarse
resolution GCMs.
Regional Models
A number of RCMs have been used in enhanced greenhouse simulations,
but few have been analysed for changes in heavy rainfall. The
United Kingdom Meteorological Office (UKMO) RCM was run at about
50 kilometer resolution over Europe, driven by the UKMO GCM for
10 years of present and doubled CO2 conditions. For the present
climate, the number of heavy events (over 10 mm/day) in the RCM
is generally at least double that in the GCM. For a doubling of
CO2, mean rainfall increases by 7-16 percent in the GCM and by
10-26 percent in the RCM. The frequency of heavy rainfall increases
in the GCM by at least 50 percent and by 20-40 percent in the
RCM. The reason for the smaller percentage increase in the RCM
is the greater number of heavy events in the simulation of present
climate. Actually, the change in the amount of precipitation associated
with heavy events is larger in the RCM than in the GCM.
The CSIRO Division of Atmospheric Research RCM was run at 60
kilometer resolution over southeast Australia. It was driven by
input from the CSIRO Mark 2 GCM (with slab ocean) for 20 years
of current and doubled CO2 conditions. Despite a 5 percent decrease
in mean annual rainfall over Victoria, extreme rainfall events
(50-80 mm/day) with return periods of 5 - 20 years increase in
intensity by 20 - 40 percent. The change in extreme rainfall intensity
is greater for events with longer return periods, implying even
larger increases in the intensity of more-extreme events with
return periods exceeding 20 years.
Summary
Increases in heavy rainfall intensity have occurred over the
last century in Australia, Japan and the USA, during a period
in which global mean temperatures have increased. Concentrations
of carbon dioxide and other greenhouse gases are also increasing
in the atmosphere. This is expected to lead to global warming
and climate change. Climate models can simulate future climate
change due to increasing greenhouse gas concentrations. A general
increase in heavy rainfall intensity is simulated by coarse resolution
global climate models and by fine resolution regional climate
models, with larger increases for more-extreme events. Hence,
increases in damage related to heavy rainfall are anticipated
due to climate change.
Further reading
Fowler, A.M. and Hennessy, K.J. 1995. Potential impacts of global
warming on the frequency and magnitude of heavy precipitation,
Natural Hazards, 11, 283-303.
Hennessy, K.J., Gregory, J.M. and Mitchell, J.F.B. (1997). Changes
in daily precipitation under enhanced greenhouse conditions. Climate
Dynamics, 13(9), 667-680.
Houghton, J.T., Meira Filho, L.G., Callander, B.A., Harris, N.,
Kattenberg, A. and Maskell, K. (eds) 1996. Climate Change 1995.
The science of climate change. Cambridge Uni. Press, U. K., 572p.
Jones, R.G., Murphy, J.M., Noguer, M. and Keen, A.B. 1997. Simulation
of climate change over Europe using a nested regional-climate
model. II: Comparison of driving and regional model responses
to a doubling of carbon dioxide. Q. J. R. Meteorol. Soc., 123,
265-292.
Karl, T.R., Knight, R.W. and Plummer, N. 1995. Trends in high-frequency
climate variability in the twentieth century, Nature, 377, 217-220.
Suppiah, R. and Hennessy, K. J. 1997. Trends in total rainfall,
heavy rain events and dry days in Australia, 1910-1990, Preprint,
Fifth International Conference on Southern Hemisphere Meteorology
and Oceanography, Pretoria, April 7-11, 1997.
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