Transcript US research dealing with climate change impacts on hydrological

U.S. research dealing with climate change
impacts on hydrological extremes
Dennis P. Lettenmaier
Department of Civil and Environmental Engineering
University of Washington
for presentation at
European Geophysical Society
Nice
April 7, 2003
1) The future as indicated by
climate models
 Increasing T -> increased atmospheric
moisture -> increased P
 Hence increased risk of hydrologic
extremes
source: Ziegler et al, J. Clim, 2003
A generally accepted hypothesis regarding
acceleration of the global water cycle:
• “According to model predictions, the most significant
manifestation of climate change would be an acceleration
of the global water cycle, leading to … a general
exacerbation of extreme hydrologic regimes, floods and
droughts” (NASA Global Water and Energy Cycle
solicitation, 2000).
• “There is evidence that suggests that the global hydrologic
cycle may be intensifying, leading to an increase in the
frequency of extremes” (Hornberger et al, USGCRP water
cycle science plan)
• Climate models generally project an acceleration in the
rate of global water cycling and an increase in global
precipitation … (Morel, GEWEX News, 2001)
2) The situation as indicated by
observations over the last ~ century
• Increased in mean and “extreme” P over
much of continental U.S. except winter
• But no apparent changes in floods (although
many upward trends in low flows over
much of the country)
(from Lins and Slack, 1999)
“Since 1910, precipitation has increased by
about 10% across the contiguous United
States. The increase in precipitation is
reflected primarily in the heavy and extreme
daily precipitation events. For example,
over half (53%) of the total increase of
precipitation is due to positive trends in the
upper 10 percentiles of the precipitation
distribution.”
(Karl and Knight, BAMS, 1998)
Percent contribution of upper 10th percentile daily
precipitation to annual total, averaged over U.S.
from Karl and Knight, 1998
from Karl and Knight, 1998
from Karl and Knight, 1998
From Easterling et al, BAMS, 2000
Groisman et al (2001)
• In three of five regions of the eastern two-thirds of
the contiguous U.S., a significant increase in the
frequency of “very heavy” precipitation events (>
101.6 mm/day) occurred during the 20th century.
• The return period of “very heavy” precipitation
events changed during the past century in the
Midwest from 10 to 7 years, in the South from 4
to 2.7 years, and in the Northeast from 26 to 11
years.
3) Is there any relationship between
trends in heavy precipitation and
(lack of) trends in floods?
Source: Groisman et al, BAMS, 2001
“In the Eastern half of the United States we
found a significant relationship between the
frequency of heavy precipitation and high
streamflow events both annually and during
the months of maximum streamflow. An
increase of spring heavy precipitation events
over the eastern United States indicates with
high probability that during the 20th century
an increase of high streamflow conditions has
also occurred.”
Groisman et al, BAMS, 2001
4) Trends in number of “great floods”
exceeding Tr = 100 yrs in basins of size >
105 km2 (Milly et al, Nature 2002)
• More floods in second half of 20th century
than in first half; statistically significant
even accounting for record differences
• Detectability consistent with GFDL GCM
for very large rivers given observed 20th
century CO2 increase and associated
warming
source: Milly et al, 2001
Conclusions
• Results of U.S. studies seemingly inconsistent, but
based on statistical analysis, number of trends in
annual maximum flood is barely larger than would be
expected by chance (and probably not field significant)
• Natural variability is large enough to obscure fairly
large changes, suggests aggregation and/or compositing
approaches (but these tend to complicate
interpretation)
• Some of the apparent inconsistencies may well have to
do with attempts to perform “simple” time series type
approaches to a complicated nonlinear process (issues
e.g. with spatial scale of precipitation-runoff
interactions and their variability with season,
antecedent conditions, temporal signature of extreme
precipitation, and surface conditions
• Is it really possible to make more progress on the
problem without a dynamic modeling approach?
REFERENCES
Easterling, D.R., J.L. Evans, P. Ya. Groisman, T.R. Karl, K.E. Kunkel, and P.
Ambenje, 2000. Observed variability and trends in extreme climate events: A
brief review, Bull. Amer. Meteorol. Soc. 81, 417-425.
Groisman. P., T.R. Karl, D.R. Easterling, R.W. Knight, P.F. Jamason, K.J.
Hennessy, R. Suppiah, C.M. Page, J. Wibig, K. Fortuniak, V.N. Razuvaev, A.
Douglas, E. Forland, and P.-M. Zhai, 1999. Changes in the probability of
heavy precipitation: Important indicators of climatic change, Climatic Change
42, 243-283.
Groisman, O., R.W. Knight, and T.R. Karl, 2001. Heavy precipitation and high
streamflow in the contiguous United States: Trends in the 20th century, Bull.
Amer. Meteorol. Soc. 82, 219-246.
Karl, T.R., and R.W. Knight, 1998. Secular trends of precipitation amount,
frequency, and intensity in the United States, Bull. Amer. Meteorol. Soc., 79,
231-241.
Karl, T.R., R.W. Knight, and N. Plummer, 1995. Trends in high frequency climate
variability in the twentieth century, Nature 377, 217-220.
Lins, H.F., and J.R. Slack, 1999. Streamflow trends in the United States,
Geophysical Research Letters 26, 227-230.
Milly, P.C.D., R.T. Wetherald, K.A. Dunne, and T.L. Delworth, 2002. Increasing
risk of great floods in a changing climate, Nature 415, 514 – 517.
Ziegler, A.D., J. Sheffield, and others, 2003. Detection of intensification in
global- and continental-scale hydrological cycles: Temporal scale of
evaluation, Journal of Climate 16, 535-547.