It’s been getting rainier across much of the United States over the past few decades, largely due to the planet’s warming. These are not just more frequent pleasant showers. Instead, increasingly intense storms release a deluge of water that cost the U.S. billions of dollars in flood damage each year. Converting increased rainfall caused by climate change to costs from flood damage is a tricky task. But it’s one that scientists have recently completed. The researchers found that around a third of the $200 billion in flood damages over the past thirty years is due to climate change’s effect on precipitation. The study, led by doctoral student Frances Davenport and Professor Noah Diffenbaugh at Stanford University, was published this January in Proceedings of the National Academy of Sciences.
To tackle the cost of flooding induced by climate change, the scientists first dug up and analyzed records of intense rainstorms and flood damage for each state and each month since 1988. The authors then checked their monthly trends with 24 climate simulations that estimate how rainfall has transformed in the past thirty years. To separate humanity’s impact from natural climate variability, they compared two versions of their simulations. The first version is based on recent CO2 trends, and the second assumes much lower CO2 levels associated with the late 1800s, when fossil fuel usage was still in its infancy. Comparing these rainstorm simulations and applying that comparison to recent rainstorm trends and flood damage is the final step to determining how much climate change has worsened flood damage across the U.S.
Their approach takes a more detailed stab at the question of climate change and flooding than previous studies, which have either only looked at annual trends or only looked at the United States as a whole. Since the U.S. isn’t experiencing uniformly heavier rainfall across the entire country or over the entire year, results from less detailed assessments are much less conclusive. By evaluating these more detailed trends, they determined that the increase in rainstorms since 1988 can at least be partly attributed to Earth’s warming. Taking it one step further, they tie climate change to about $73 billion—or 36% of the total—U.S. flood damage during that period.
Higher air temperatures lead to more intense precipitation in many areas because warmer air can retain more water. When clouds finally do swell to the point of raining down on a warmer planet, there is therefore much more water. Higher temperatures also increase evaporation, often leaving soils drier and harder. This further encourages flooding because the hard ground is less willing to absorb water. (Paradoxically, this also leads to increased drought in many areas of the U.S.). Global temperatures between 1988 and 2017 have risen around half a degree Celsius or almost one degree Fahrenheit. As the planet continues warming, floods and the immense damage they cause can only be expected to increase.
Frances Devnport is a PhD student at Stanford University, working under Climate Science Professor Noah Diffenbaugh.
Managing Correspondent: Jordan Wilkerson
Original Science Article: Contribution of historical precipitation change to US flood damages
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