Have floods changed with increasing CO2 levels?

US Geological Survey: 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 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 1927 Mississippi flood in Greenville, Mississippi