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Climate change causing one-third of flood damage in U.S., study finds

San Francisco Chronicle logo San Francisco Chronicle 1/13/2021 By Kurtis Alexander

Increasingly strong storms are responsible for more than a third of the nation’s flood costs, swelling the tab by billions of dollars a year as climate change continues to fuel more extreme weather, according to new research at Stanford University.

The research, which is among the first to put a price tag on heavier rainfall, found that the changing weather is responsible for $75 billion of the cumulative $199 billion of U.S. flood damage from 1998 to 2017. Many of the losses over that period were in California.

The worsening storms are leaving more homes and communities underwater and exacerbating what’s already the costliest natural disaster in the United States. Scientists and flood experts have long known that heavier rain is increasing flood damage. But parsing out how much has been difficult given the range of factors at play, which include population growth, new housing and rising property values.

“Our study is an important step in untangling this,” said Noah Diffenbaugh, senior author of the study and a climate scientist at Stanford. “Over the last three decades, we’ve taken on tens of billions of dollars in additional costs as a result of changing precipitation. This helps us quantify those costs and what it will cost to adapt to future climate change.”

The problem, according to the research, is not necessarily more rain, but more intense bouts of rain. Some parts of the country, including California, have seen little if any variation in total precipitation in recent decades, though when storms come, they’re bigger.

“The wettest events have increased pretty uniformly across the U.S., whether the mean precipitation has changed,” Diffenbaugh said.

This trend has long been expected. As the climate warms because of more industrial and vehicle emissions, evaporation of the oceans increases and so does the amount of moisture packed into the atmosphere, which wrings out when storms blow through.

The Stanford researchers were able to estimate the cost of the additional rain by analyzing historical state-level flood damage and teasing out what models suggest would be the climate impact. The methodology accounts for regional variation, which differs from prior studies that either focused too broadly on the United States or homed in on a single place.

How much flood damage will increase going forward hinges on future greenhouse gas emissions as well as future development in the nation’s floodplains.

Planning and environmental groups have long pushed for stricter rules on where and how new construction can take place. This month, the Natural Resources Defense Council and the Association of State Floodplain Managers petitioned the Federal Emergency Management Agency to update its flood maps to better address climate risk and limit the potential for damage.

The high toll of flooding was underscored last week when Munich Re, an insurance giant that underwrites other insurance companies, released a report on worldwide 2020 damage. According to the report, the Atlantic hurricane season caused $43 billion in losses last year while thunderstorms in the U.S. caused $40 billion of damage.

Isolating the role of human-caused global warming in such disasters is a relatively new and improving science.

“Accurately and comprehensively tallying the past and future costs of climate change is key to making good policy decisions,” said Marshall Burke, a co-author of the Stanford study and an economist and professor of Earth system science. “This work shows that past climate change has already cost the U.S. economy billions of dollars, just due to flood damages alone.”

The Stanford research was published Monday in the journal Proceedings of the National Academy of Sciences.

Kurtis Alexander is a San Francisco Chronicle staff writer. Email: kalexander@sfchronicle.com Twitter: @kurtisalexander

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