4 Papers on Climate Change and Hydrology

1 ——————————————-
“Our results indicate that future reductions in Arctic sea ice cover could
significantly reduce available water in the American west….”
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GEOPHYSICAL RESEARCH LETTERS
VOL. 31, L06209, doi:10.1029/2003GL019133, 2004
Copyright 2004 by the American Geophysical Union.

Disappearing Arctic sea ice reduces available water in the American west

Jacob O. Sewall and Lisa Cirbus Sloan
Earth Sciences Department, University of
California, Santa Cruz, California, USA

ABSTRACT – Recent decreases in Arctic sea ice cover and the
probability of continued decreases have raised the question of how
reduced Arctic sea ice cover will influence extrapolar climate. Using
a fully coupled earth system model, we generate one possible future
Arctic sea ice distribution. We use this ”future” sea ice
distribution and the corresponding sea surface temperatures (SSTs) to
run a fixed SST and ice concentration experiment with the goal of
determining direct climate responses to the reduction in Arctic sea
ice that is projected to occur in the next 50 years. Our results
indicate that future reductions in Arctic sea ice cover could
significantly reduce available water in the American west and
highlight the fact that the most severe impacts of future climate
change will likely be at a regional scale.

2 ———————————
“Although the results reported in this study were derived from an
ensemble of regional climate simulations driven by a global climate
model that displays low climate sensitivity compared with most other
models, climate change was found to significantly affect water
resources in the western U.S. by the mid twenty-first century.”
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Climatic Change 62: 75-113, 2004

MID-CENTURY ENSEMBLE REGIONAL CLIMATE CHANGE
SCENARIOS FOR THE WESTERN UNITED STATES

L. RUBY LEUNG, YUN QIAN, XINDI BIAN , WARREN M. WASHINGTON,
JONGIL HAN  and JOHN O. ROADS

Abstract: To study the impacts of climate change on water resources in
the western U.S., global climate simulations were produced using the
National Center for Atmospheric Research/Department of Energy
(NCAR/DOE) Parallel Climate Model (PCM). The Penn State/NCAR Mesoscale
Model (MM5) was used to downscale the PCM control (20 years) and three
future (2040-2060) climate simulations to yield ensemble regional
climate simulations at 40 km spatial resolution for the western U.S.
This paper describes the regional simulations and focuses on the
hydroclimate conditions in the Columbia River Basin (CRB) and
Sacramento-San Joaquin River (SSJ) Basin. Results based on global and
regional simulations show that by mid-century, the average regional
warming of 1 to 2.5 ? C strongly affects snowpack in the western U.S.
Along coastal mountains, reduction in annual snowpack was about 70% as
indicated by the regional simulations. Besides changes in mean
temperature, precipitation, and snowpack, cold season extreme daily
precipitation increased by 5 to 15 mm/day (15-20%) along the Cascades
and the Sierra. The warming resulted in increased rainfall at the
expense of reduced snowfall, and reduced snow accumulation (or earlier
snowmelt) during the cold season. In the CRB, these changes were
accompanied by more frequent rain-on-snow events. Overall, they
induced higher likelihood of wintertime flooding and reduced runoff
and soil moisture in the summer. Changes in surface water and energy
budgets in the CRB and SSJ basin were affected mainly by changes in
surface temperature, which were statistically significant at the 0.95
confidence level. Changes in precipitation, while spatially
incoherent, were not statistically significant except for the drying
trend during summer. Because snow and runoff are highly sensitive to
spatial distributions of temperature and precipitation, this study
shows that (1) downscaling provides more realistic estimates of
hydrologic impacts in mountainous regions such as the western U.S.,
and (2) despite relatively small changes in temperature and
precipitation, changes in snowpack and runoff can be much larger on
monthly to seasonal time scales because the effects of temperature and
precipitation are integrated over time and space through various
surface hydrological and land-atmosphere feedback processes. Although
the results reported in this study were derived from an ensemble of
regional climate simulations driven by a global climate model that
displays low climate sensitivity compared with most other models,
climate change was found to significantly affect water resources in
the western U.S. by the mid twenty-first century.

3
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“The atmospheric modeling results of 1998-2002 suggest an increased
risk for severe and synchronized drying of the mid-latitudes if Š”
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SCIENCE www.sciencemag.org
VOL 299
31 JANUARY 2003

R E P O R TS

The Perfect Ocean for Drought
Martin Hoerling  and Arun Kumar

Abstract:
The 1998 -2002 droughts spanning the United States, southern Europe,
and South- west Asia were linked through a common oceanic influence.
Cold sea surface temperatures (SSTs)in the eastern tropical Pacific
and warm SSTs in the western tropical Pacific and Indian oceans were
remarkably persistent during this period. Climate models show that the
climate signals forced separately by these regions reacted
synergistically, each contributing to widespread mid-latitude drying :
an ideal scenario for spatially expansive, synchronized drought.

From concluding remarks:
It is an open question whether such tropical oceanic forcings will
become more prevalent during the 21st century. Because of deficiencies
in coupled ocean-atmosphere models, little confidence exists with
regard to projections of the future statistics of ENSO (such as its
duration and amplitude) or of the regional pattern of mean tropical
SST change itself. The atmospheric modeling results of 1998-2002
suggest an increased risk for severe and synchronized drying of the
mid-latitudes if the tropical mean SSTs or their interannual
variability increase the ocean’s west-east contrast over the
equatorial Pacific.

4 ———————————————————————–
“Future projections of drought in the twenty-first century … show
regions of strong wetting and drying with a net overall global drying
trend. For example, the proportion of the land surface in extreme
drought is predicted to increase from 1% for the present day to 30% by
the end of the twenty-first century.”
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JOURNAL OF HYDROMETEOROLOGY
OCTOBER 2006

Modeling the Recent Evolution of Global Drought and Projections for the
Twenty-First Century with the Hadley Centre Climate Model

ELEANOR J. BURKE, SIMON J. BROWN, AND NIKOLAOS CHRISTIDIS
Hadley Centre for Climate Prediction and Research, Met Office, Exeter,
United Kingdom

ABSTRACT

Meteorological drought in the Hadley Centre global climate model is
assessed using the Palmer Drought Severity Index (PDSI), a commonly
used drought index. At interannual time scales, for the majority of
the land surface, the model captures the observed relationship between
the El Niño-Southern Oscillation and regions of relative wetness and
dryness represented by high and low values of the PDSI respectively.
At decadal time scales, on a global basis, the model reproduces the
observed drying trend (decreasing PDSI) since 1952. An optimal
detection analysis shows that there is a significant influence of
anthropogenic emissions of greenhouse gasses and sulphate aerosols in
the production of this drying trend. On a regional basis, the specific
regions of wetting and drying are not always accurately simulated. In
this paper, present-day drought events are defined as continuous time
periods where the PDSI is less than the 20th percentile of the PDSI
distribution between 1952 and 1998 (i.e., on average 20% of the land
surface is in drought at any one time). Overall, the model predicts
slightly less frequent but longer events than are observed. Future
projections of drought in the twenty-first century made using the
Special Report on Emissions Scenarios (SRES) A2 emission scenario show
regions of strong wetting and drying with a net overall global drying
trend. For example, the proportion of the land surface in extreme
drought is predicted to increase from 1% for the present day to 30% by
the end of the twenty-first century.
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