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Krishna Ramanujan					For Release:
Goddard Space Flight Center, Greenbelt, Md.		March 6, 2003
(Phone: 301/286-3026)
Kramanuj{at}pop900.gsfc.nasa.gov

Release: 03-25

CLIMATE CHANGES MAY INCREASE EXTREME RAIN/SNOW EVENTS IN CALIFORNIA

Increasing carbon dioxide levels in the atmosphere may lead to a rise
in the number of annual extreme precipitation events in the Sierra
Nevada Mountains, which in turn could increase the frequency of
flooding in California, a NASA-funded study finds.

One of the missions of NASA's Earth Science Enterprise (ESE), which
funded this research, is to better understand how the Earth system is
changing. Within this framework, NASA is committed to studying
variability in global precipitation, how well we can predict future
changes in the Earth system, and what are the consequences of change
in the Earth system for human civilization.

Based on computer model simulations of the next 40 to 50 years, Jinwon 
Kim, an atmospheric scientist at the University of California, Los 
Angeles (UCLA), found that the Sierra Nevada region may experience 
substantial increases in heavy precipitation (exceeding 2 inches of 
rain/day), and extreme precipitation events (exceeding 4 inches of 
rain/day). Most of these increases occur during the winter, currently 
the wettest season in the region.

"The frequency of extreme precipitation may increase, in general, and
the most notable increase of extreme events may occur in areas
characterized by heavy winter precipitation in today's climate," Kim
said. Kim recently presented his results at the 83rd Annual Meeting of 
the American Meteorological Society in Long Beach, Calif.

Existing projections from Hadley Centre for Climate Prediction and
Research (HCCPR) HadCM2 computer model suggests that increases in
carbon dioxide (CO2) are likely to substantially alter the hydrologic
cycle in the Western U.S. That's because increasing levels of CO2 in
the atmosphere trap heat, and warm the air. Warmer air holds more
water, and when parcels of saturated air rise, they tend to rain water 
back to Earth.

Kim used his regional computer model (MAS) to make two fine-scale
precipitation projections for the decade of 2040 to 2049 based on
different values of CO2 in the atmosphere from the coarse global
projections by HCCPR, United Kingdom.

Some of the background data input into the computer model included
NASA-derived Normalized Difference Vegetation Index (NDVI) data, which 
measures the amount of solar energy reflected and absorbed by
vegetation. This is important data for computing transpiration. NDVI
was created by Compton Tucker of NASA Goddard, using data from the
National Oceanic and Atmospheric Administration's (NOAA) Geostationary 
Environmental Orbiting Satellite (GOES) Advanced Very High Resolution 
Radiometer (AVHRR) instrument.

The first projection assumed that greenhouse gas concentrations will
stay at levels equal to those of the late 1900s. The second projection 
represented the climate of the same period assuming increases in 
greenhouse gas levels by 1 percent per year from the year 1990.

Compared to the first projection, the second projection showed
increases in both the number of wet days and, more importantly, large
increases in heavy precipitation events for the region during the cold 
season from October to March. The model showed increases of heavy 
precipitation events increased by 10 to 15 days per year. It also 
showed that extreme precipitation events increased by 5 to 10 days per 
year.

Comparing the two projections, the average number of wet days per year 
over the southern and northern Sierra Nevada basins (divided along the 
area near Sacramento) increased by 37 percent for the southern basin 
and 32 percent for the northern basin in the second projection. While 
light precipitation events (less than 5mm or .2 inches/day), stayed 
the same or decreased slightly for both basins, the occurrence of 
heavy precipitation events rose from 1 percent of wet days annually to 
3 percent in the second projection. Extreme events rose from .1 
percent of wet days annually to 1 percent.  Similar changes are 
projected for all major California basins. These projections suggest 
that the intensity of the hydrologic cycle will increase as levels CO2 
continue to climb.

The second model-based projection scenario also showed that elevation
levels in the mountains where freezing occurs will rise as
temperatures rise. That means that much of the precipitation that
currently falls in higher altitudes as snow may come down as rain in
future years. Snow stores water during the cold season and releases it 
gradually in spring and summer. Hence, a substantial increase of cold 
season rainfall at the expense of snowfall reduces the buffering
effects of snow and could result in more flooding.

These changes, combined with more heavy rain events and steep mountain 
slopes, could therefore lead to a greater frequency of flooding in the 
future.

"Since the primary concern for reservoir management is to reduce
flooding risks that require maintaining the storage space to capture
excessive runoff, the reservoirs may have to maintain lower water
levels," Kim said. "This directly decreases the water resources."

The climate change signals projected in this study are based on a
single global projection and are expected to include an unknown amount 
of uncertainties. Hence, the projections here must be taken as
qualitative, rather than quantitative. The author is planning
additional studies using global projections from multiple Global
Climate Models.

This research was funded by NASA's ESE and NOAA. NASA's ESE
Applications Division applies the results of the nation's investment
in ESE to issues of national concern, such as water and resource
management, environmental quality, community growth, and disaster
management to support policy makers at the state and local levels.

For more information, please see:
http://www.gsfc.nasa.gov/topstory/2003/0210extremeprecip.html

Normalized Difference Vegetation Index (NDVI)
http://daac.gsfc.nasa.gov/CAMPAIGN_DOCS/LAND_BIO/ndvi..html

The Advanced Very High Resolution Radiometer (AVHRR)
http://www.ngdc.noaa.gov/seg/globsys/avhrr.shtml

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