Only one of four large regions of the United States showed a significant relationship between carbon dioxide (CO2)
in the atmosphere and the size of floods over the last 100 years. This
was in the southwestern region, where floods have become smaller as CO2 has increased.
This does not mean that no strong relationship between flooding and greenhouse gases will emerge in the future.
An increase in flood magnitudes remains one of the most anticipated
impacts of climate change, and land and water resource managers are
asking questions about how to estimate future flood risks and develop
effective flood mitigation strategies for the future.
A new report published by U.S. Geological Survey scientists in the
Hydrologic Sciences Journal looks at this potential linkage using
historical records of floods throughout the nation. Scientists studied
flood conditions at 200 locations across the United States looking back
127 years through 2008.
"Currently we do not see a clear pattern that enables us to
understand how climate change will alter flood conditions in the future,
but the USGS will continue to collect new data over time and conduct
new analyses as conditions change," said USGS scientist and lead author
Robert Hirsch. "Changes in snow packs, frozen ground, soil moisture and
storm tracks are all mechanisms that could be altered by greenhouse gas
concentrations and possibly change flood behavior. As we continue
research, we will consider these and other factors in our analyses."
The decrease of floods in the southwestern region is consistent with
other research findings that this region has been getting drier and
experienced less precipitation as a likely result of climate change.
"The relationship between greenhouse gas concentrations and floods is
complex, demonstrating the need for long-term streamflow data to help
guide future flood hazard mitigation and water resources planning," said
Matthew Larsen, USGS Associate Director for Climate and Land Use
Change. "USGS streamgages provide real-world data to help scientists
understand this relationship. Planning for water supplies and flood
hazards should be informed by a combination of predictive modeling
approaches as well as statistical approaches such as this study."
The USGS operates over 7600 streamgages across the United States,
collecting data on the amount of water flowing in streams and rivers.
This study used data from 200 of these USGS streamgages, each of which
had between 85 and 127 years of data through 2008.
Climate changes that could influence flood magnitudes include shifts
in the intensity and tracks of various types of storms and changes in
the type of precipitation (rain versus snow). The conditions on the
landscape when large storms arrive can also change (for example, smaller
snowpacks, less soil moisture and less frozen soil). All of these can
influence the size of floods. Of course, human activities within the
watershed can also have a major influence in the size of floods. These
include urbanization, building of dams and levees, and shifts in
vegetation types and drainage of soils and wetlands. At the present
time, we see much larger changes in flooding from these causes than we
can see from greenhouse forcing.
In this study, the United States was divided into four large regions,
and research showed some regional differences in the way that flood
magnitudes have varied with CO2 concentrations over the past
century. In the northwestern and southeastern regions of the United
States, there is virtually no evidence of increases or decreases in
flood magnitudes over the study period. The northeastern region (which
stretches from the middle of the Dakotas and Nebraska all the way east
to the New York and New England area) shows a tendency towards increases
in flooding over this period.
The article, "Has the magnitude of floods across the USA changed with global CO2 levels," is available online.
The article contains a map of the results and extensive tables, which
identify the streamgages used in the study and the site-specific
findings for each of them.
Keep visiting thegeoscienceforum.blogspot.com
0 comments:
Post a Comment