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Have Floods Changed with Increasing CO2 Levels?

Have Floods Changed with Increasing CO2 Levels?

U.S. Geological Survey

Only one of four large regions of the U.S. showed a significant relationship between carbon dioxide (CO2) in the atmosphere and the size of floods over the past 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 (USGS) 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 U.S. 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 Southwest 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 more than 7600 streamgages across the U.S., 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 U.S. 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 Northwest and Southeast, 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.

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.

To learn more about USGS streamgages, click here.

For more information, click here.

A plume of smoke from wildfires burning in the Angeles National Forest is seen from downtown Los Angeles on Aug. 29, 2009 in Los Angeles, California. Kevork Djansezian / Getty Images

California is bracing for rare January wildfires this week amid damaging Santa Ana winds coupled with unusually hot and dry winter weather.

High winds, gusting up to 80- to 90 miles per hour in some parts of the state, are expected to last through Wednesday evening. Nearly the entire state has been in a drought for months, according to the U.S. Drought Monitor, which, alongside summerlike temperatures, has left vegetation dry and flammable.

Utilities Southern California Edison and PG&E, which serves the central and northern portions of the state, warned it may preemptively shut off power to hundreds of thousands of customers to reduce the risk of electrical fires sparked by trees and branches falling on live power lines. The rare January fire conditions come on the heels of the worst wildfire season ever recorded in California, as climate change exacerbates the factors causing fires to be more frequent and severe.

California is also experiencing the most severe surge of COVID-19 cases since the beginning of the pandemic, with hospitals and ICUs over capacity and a stay-at-home order in place. Wildfire smoke can increase the risk of adverse health effects due to COVID, and evacuations forcing people to crowd into shelters could further spread the virus.

As reported by AccuWeather:

In the atmosphere, air flows from high to low pressure. The setup into Wednesday is like having two giant atmospheric fans working as a team with one pulling and the other pushing the air in the same direction.
Normally, mountains to the north and east of Los Angeles would protect the downtown which sits in a basin. However, with the assistance of the offshore storm, there will be areas of gusty winds even in the L.A. Basin. The winds may get strong enough in parts of the basin to break tree limbs and lead to sporadic power outages and sparks that could ignite fires.
"Typically, Santa Ana winds stay out of downtown Los Angeles and the L.A. Basin, but this time, conditions may set up just right to bring 30- to 40-mph wind gusts even in those typically calm condition areas," said AccuWeather Senior Meteorologist Mike Doll.

For a deeper dive:

AP, LA Times, San Francisco Chronicle, Washington Post, Weather Channel, AccuWeather, New York Times, Slideshow: New York Times; Climate Signals Background: Wildfires, 2020 Western wildfire season

For more climate change and clean energy news, you can follow Climate Nexus on Twitter and Facebook, sign up for daily Hot News, and visit their news site, Nexus Media News.

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