This is a re-post from Yale Climate Connections by Jeff Masters When a weak 45-mph tropical storm named Harvey moved through the Lesser Antilles Islands in August 2017 and then petered out in the central Caribbean Sea, no one could have suspected that the meager clump of clouds that remained would go on to become the second-costliest weather disaster in world history. But after crossing Mexico's Yucatan Peninsula into the Gulf of Mexico, Harvey was rejuvenated, rapidly intensifying into a ferocious Category 4 hurricane that hit Texas just north of Corpus Christi. Harvey's true mischief came after it stalled inland as a tropical storm for two days, dumping at least 40 inches of rain across a gigantic area from Houston to Port Arthur — larger than the entire state of Delaware. The storm total of 60.58 inches (1,534 mm) at Nederland, Texas, was the heaviest single amount ever recorded from a tropical cyclone or its remnants in the U.S. With damages of 164billion(2026USD)—mostlyfromflooding,HarveybecameahistoricalcatastropheexceededonlybyHurricaneKatrinaof2005.WhenallofHarvey’srainfallrunoffrushedtowardtheocean,itencounteredtheblockinginfluenceofseawaterbeingpushedinlandbythepersistentonshorewindsofthetropicalstorm,creatingasignificantcompoundfloodevent—coastalfloodingthatresultedfromacombinationofstormsurgeandriverrunoffunabletodrainintotheoceanbecauseofthestormsurgewaterspiledupagainstthecoast.Asimilarsetupcouldcauseanevenworsecatastropheinthefuture.Climatechangeiscausingmoreintense,slower−movinghurricanes,increasedrainfall,andhighersealevels.Buttraditionalriskassessmentmethodstypicallyconsideronehazardatatime—ignoringcompoundfloodevents—leadingtoanunderestimationofthedanger.Ifweincludeallthewaysclimatechangewilllikelyincreaseflooding,thefuturefloodriskalongsignificantportionsoftheU.S.GulfandAtlanticcoastsisnearlycertaintomakethemunlivablebylatethiscentury,evenunderamoderateglobalwarmingscenario.HowclimatechangeworsensthedangerA2023studylookingatthefloodingfromHarveynearPortArthur,Texas,foundthat19164 billion (2026 USD) — mostly from flooding, Harvey became a historical catastrophe exceeded only by Hurricane Katrina of 2005. When all of Harvey’s rainfall runoff rushed toward the ocean, it encountered the blocking influence of seawater being pushed inland by the persistent onshore winds of the tropical storm, creating a significant compound flood event — coastal flooding that resulted from a combination of storm surge and river runoff unable to drain into the ocean because of the storm surge waters piled up against the coast. A similar setup could cause an even worse catastrophe in the future. Climate change is causing more intense, slower-moving hurricanes, increased rainfall, and higher sea levels. But traditional risk assessment methods typically consider one hazard at a time — ignoring compound flood events — leading to an underestimation of the danger. If we include all the ways climate change will likely increase flooding, the future flood risk along significant portions of the U.S. Gulf and Atlantic coasts is nearly certain to make them unlivable by late this century, even under a moderate global warming scenario. How climate change worsens the danger A 2023 study looking at the flooding from Harvey near Port Arthur, Texas, found that 19% of the flood area occurred because of compound flooding. Under a global warming scenario where a repeat of Harvey hits with an additional sea level rise of 0.57 meters (1.9 feet), accompanied by 18% more total rainfall — plausible in 2050 — this area would increase to 33%. A potential sea level rise of 1.6 meters (5.2 feet) and an additional 50% in total rainfall, plausible by 2100, would cause the compound flooding area to rise to 46%, increasing the number of structures impacted by about a factor of 23 compared to 2017, causing tens of billions in additional damage. Figure 1. Storm-total rainfall from Hurricane Harvey, August 24-31, 2017. Harvey dumped over 40 inches (yellow colors) in Houston, with isolated amounts over 50 inches (pink colors) south of Houston and northwest of Port Arthur. Image credit: NOAA . There are three main ways climate change can increase flood risk along the U.S. Atlantic and Gulf coasts: An increase in the frequency of more intense hurricanes and ones moving more slowly at landfall, which will dump more rain Increased heavy rainfall because a warmer atmosphere holds more water vapor Sea level rise The relative importance of these three factors in a future warmer climate will vary depending upon the location, according to a 2022 study . This study found that across the Gulf of Mexico and Florida coastlines, the increase in rainfall was expected to be the largest driver. For parts of the Southeast and mid-Atlantic, the increase in the number of intense or slow-moving hurricanes would predominate. And along the upper mid-Atlantic and New England coastlines, sea level rise will dominate the future compound flood risk. Figure 2. The main driver of compound flooding on the U.S. coast. Across the Gulf of Mexico and Florida coastlines, the increase in rainfall is the largest driver (yellow colors), while the increase in storm frequency (of more intense, slow-moving storms) has the largest impact for parts of the Southeast and mid-Atlantic (blue). Along the upper mid-Atlantic and New England coastlines, sea level rise causes the most impact (green). Locations with no clear main driver are labeled NA (gray). (Image credit: Gori et al., Tropical cyclone climatology change greatly exacerbates US extreme rainfall–surge hazard , Nat. Clim. Chang. 12 , 171–178 (2022), https://doi.org/10.1038/s41558-021-01272-7 , open access) Sea level rise has already led to a massive increase in flood risk Sea level rise from all causes – for example, human-caused climate change, natural tectonic processes, and subsidence from groundwater pumping — has already led to a massive increase in the risk of damaging coastal flooding from storm surges alone, according to a 2026 study, Human-driven sea-level rise has quadrupled the frequency of coastal sea-level extremes since 1900 . Relative sea level rise from all causes made a 100-year coastal flood in 1900 into a one-in-five-year flood or less by 2005 in Key West, Jacksonville, Atlantic City, and Maine. Because sea level rise is accelerating, the odds of coastal flooding will increase even faster than the increases already observed since 1900. Flood risks are growing Charleston, South Carolina: What was a one-in-10-year coastal flood in 1901 occurred 17 times in 2025 . Galveston, Texas: What was a one-in-10-year flood in 1904 occurred nine times in 2024. Atlantic City, New Jersey : What was a one-in-10-year coastal flood in 1911 occurred 10 times in 2024 . Miami, Florida: What was a one-in-10-year coastal flood in 1931 occurred 14 consecutive days during the "king tides" of October 2025. Key West, Florida : What was a one-in-10-year coastal flood in 1913 occurred an astonishing 26 out of 27 days during the "king tides" of October 2025 ; what was a one-in-100-year flood in 1913 has occurred three times in the past 10 years . Data: NOAA Dramatic rises in compound flood risk are coming A return period refers to how often we can expect a weather event of a given severity to occur. For example, we use rainfall statistics from NOAA to compute how often a flood with a 1% chance of occurring in a given year will recur — which is defined as a one-in-100-year storm, with a return period of 100 years. A 2022 paper, Tropical cyclone climatology change greatly exacerbates US extreme rainfall-surge hazard , studied the odds of a truly extreme compound flood event — a one-in-100-year storm surge occurring at the same time as a one-in-100-year rainfall event. Historically, the return period of such an event was about once every 200-500 years along the coastlines of the Gulf of Mexico and southeast Atlantic (up to the Chesapeake Bay), shifting to once every 1,000 years or even less frequently along the New England coastline. But under an extreme global warming scenario for the year 2100, these odds would generally (with some exceptions, see Fig. 4) increase by seven- to 36-fold in the South and 30- to 195-fold to the north — a massive rise in extreme flood risk. Although this result was for an extreme global warming scenario , the strong signal found implies that a significant increase in extreme flood risk would occur even in a moderate global warming scenario. Figure 3. The return period in years in 2005 for what was a one-in-100-year flood in 1900 because of relative sea level rise. Data is plotted from the 2026 paper, Human-driven sea-level rise has quadrupled the frequency of coastal sea-level extremes since 1900 . For example, a 100-year coastal flood in 1900 in Jacksonville, Florida, and Atlantic City, New Jersey, was a one-in-two-year flood by 2005 (red circles with the number "2" in them). This change in flood risk is for sea level rise alone — additional increases in flood risk because of changes in precipitation are not included. The greatest rises in risk were to the north, because climate change is expected to bring greater increases in extreme precipitation closer to the poles. This was also the finding of a 2020 study, More meteorological events that drive compound coastal flooding are projected under climate change , which predicted that the greatest increases in compound flood threat should occur north of 40°N latitude. Figure 4. The change in return period for an extreme compound flood, defined as a one-in-100-year storm surge occurring at the same time as a one-in-100-year rainfall event, under an extreme global warming scenario. Left side of table: the return period in the historical climate (1980-2005). Right side: return period in the 2070-2100 period under an extreme global warming scenario, using the median value from eight different climate models. The return period increases by a factor of 14 to 265 for these nine cities. Data taken from the supplemental materials in: Gori et al., Tropical cyclone climatology change greatly exacerbates US extreme rainfall–surge hazard , Nat. Clim. Chang. 12 , 171–178 (2022). https://doi.org/10.1038/s41558-021-01272-7 . Main cause of future increased compound coastal flood risk: more intense and slower-moving hurricanes The model used in the 2022 study projected that the top 10% of most intense hurricanes would, along the majority of the U.S. coast, increase in intensity by 15-30% and move 20-30% slower in the future compared to the historical period. “The increase in storm intensity coupled with the decrease in translation speed drives an increased likelihood to observe both extreme rainfall and extreme storm tide in the future,” the authors wrote. A substantial inland compound flood risk along the Gulf of Mexico coast Rivers draining into the Gulf of Mexico have seen large increases in their maximum streamflow in recent decades (commonly 20-40% increases), making them susceptible to increased compound flooding. A 2021 paper found long-term increases in the frequency of compound storm surge and heavy rainfall flooding along the rivers of the northeastern Gulf of Mexico. Surprisingly, these compound flood events were largest a good distance inland, near the limit of where tidal influences stopped — not at the coast where compound events are usually expected. A 2026 study focused on North and South Carolina also found a considerable expansion of the threat of compound flooding inland in a future warmer climate. A Hurricane Sandy-like compound flood event: five times more likely by 2100 ? Hurricane Sandy in October 2012 caused devastating surge-driven flooding across heavily populated coastal areas in New York City, resulting in more than 91 billion (2026 USD) in damages. A 2024 paper, Climate Change Contributions to Increasing Compound Flooding Risk in New York City , found that a Sandy-like event can be expected about once every 150 years in the present climate. But climate change — through sea level rise and an increase in hurricane strength and rainfall — can be expected to make a similar storm about a one-in-65-year event by 2050, and a one-in-30-year event by 2100, under an emissions scenario slightly higher than the trajectory humanity is currently on. Increased compound flood threat from hurricanes earlier in the season A 2022 paper, Earlier onset of North Atlantic hurricane season with warming oceans , found that initial threshold dates of continental U.S. named storm landfalls have trended earlier by two days per decade since 1900. Modeling work suggests that the length of hurricane season will continue to increase because of climate change. A 2017 study found that a hurricane season that was two months longer (May-December) would increase the number of flood-risk days by 28-180% along rivers in four Southeast U.S river basins. Figure 5. Predicted water levels at the Carrollton gage on the Mississippi River in New Orleans as of July 10, 2019. The river was running high, at 16 feet above sea level, and the city’s levees protect the city to a height of 20 feet. The storm surge from Hurricane Barry was predicted to reach that level on July 13. The last time water levels that high were observed at this point on the Mississippi was in the Great Flood of 1927. Image credit: NOAA . As I wrote in a 2019 post, New Orleans’ Achilles Heel: A Hurricane Storm Surge During a Mississippi River Flood? , a trend toward earlier hurricanes increases the risk of storm surge moving up the Mississippi River that could overwhelm the levees in New Orleans, since the river tends to run high in late spring and early summer. This situation was feared in July 2019, when Hurricane Barry sent a storm surge up the river when the river was already running high from early-summer runoff (Fig. 5). Fortunately, Barry ended up delaying its intensification into a hurricane until after it passed the mouth of the Mississippi, resulting in a storm surge that was not as high as initially forecast. Other compound hurricane threats Climate change is likely to make two other types of compound hurricane threats more severe. One of these was covered in my previous post, The emerging danger of post-hurricane heat waves (2026). In addition, more intense hurricanes with higher winds and heavier rains have the potential to create a double-whammy of high-end wind damage and extreme inland flooding simultaneously, overwhelming infrastructure and emergency preparedness and response efforts that could have handled one of these hazards alone, but not both together. A preprint of a 2026 paper that has not yet undergone peer review, Global Warming Amplifies Inland Compound Risks From Tropical Cyclones , found that when comparing the recent climate (1981-2020) with an extreme climate-change projection for later this century (2061-2100), the annual probability of compound wind and precipitation extreme hazards ranking in the 99th percentile globally increases by 61-115% within 100 kilometers of the coast, and further escalates by 92-204% in areas 100-500 kilometers inland. This inland amplification is driven by more intense landfalling hurricanes and the increased moisture available caused by the 7% increase in water vapor holding capacity of the air per degree Celsius of warming. Hurricane Helene’s impact in 2024 in western North Carolina can be regarded as a harbinger storm in this regard. Coastal areas becoming unlivable A 2020 paper, Sea-level rise exponentially increases coastal flood frequency , found that for the most susceptible sites around the U.S., the odds of a one-in-50-year coastal flood “are likely to double approximately every five years into the foreseeable future.” This finding took into account not just storm surges from hurricanes but also from more common coastal storms such as Nor'easters. According to the U.S. Army Corps of Engineers , most coastal engineering works in the U.S. are designed for return periods of 50 to 100 years, so the increase in flood risk at so many sites represents a drastic increase in vulnerability. And if high-end sea-level rise projections of one meter (3.28 feet) by 2100 come true, sea-level rise will likely cause "once-in-a-lifetime" coastal flooding events to occur nearly every day before 2100. ( NOAA's 2022 sea level rise forecast gives 50% odds that sea level rise along the contiguous U.S. coast by 2100 will exceed 0.7 meters.) Figure 6. The return period in years in 2050 for what was a one-in-100-year flood in 2005 because of relative sea level rise. Data is plotted using data from the 2020 paper, Sea-level rise exponentially increases coastal flood frequency , in combination with observed and predicted sea level rise from The Virginia Institute of Marine Science annual Sea Level Rise Report Cards . For example, a one-in-100-year coastal flood in 2005 in Key West, Florida, is predicted to recur every 0.04 years (two weeks) by 2050 (red circle with the number "0.04" in it). This change in flood risk is for sea level rise alone — additional increases in flood risk because of changes in precipitation are not included. The forecasts out to 2050 are generated using the observed acceleration trend fitted with a quadratic curve (since sea level rise is increasing exponentially, and a straight-line linear fit is not appropriate). Note that these forecasts are not based on a climate model and may be underestimated. If we now add in the massive additional increase in flood risk resulting from compound flooding, good luck trying to insure your home. The huge increase in climate change-induced flood risk from sea level rise, heavier rainfall, and stronger/slower-moving hurricanes is nearly certain to force abandonment of portions of the U.S. Gulf and Atlantic coasts by late this century, even under a moderate global warming scenario. A 2026 study, The Growth Effects of Natural Disasters: Evidence From A Novel Global Dataset Over 1970-2023 , found that a one-in-100-year flood reduces GDP by about 0.5%, so it is easy to see how the coast could quickly become unlivable if once-in-a-lifetime floods are occurring nearly yearly in low-lying regions. Indeed, hurricane flooding has already led to the unofficial abandonment of several U.S. communities, and a number of others are already at significant risk, which I will detail in a series of future posts (spoiler alert: Barrier islands are high on the list). https://bsky.app/profile/drjeffmasters.bsky.social/post/3mnhxhqjjtc2g The only recourse we will have is to spend vast amounts of money to defend the most important places and retreat from or abandon the rest. A society-shaking mass migration of millions of Americans away from the coast is inevitable in future decades because of increased climate change-induced flood risk. The trigger for the beginning of this exodus may be only a few years away. To understand what’s coming, I recommend reading my 2024 post, When will climate change turn life in the U.S. upside down? Related posts on sea level rise Book review: “On the Move” is a must-read account of U.S. climate migration (2024) Book review: “The Great Displacement” is a must-read (2023) Part one of my three-part sea level rise series: How fast are the seas rising? (2023) Part two of my three-part sea level rise series: Eight excellent books on sea level rise risk for U.S. cities (2023) Part three of my three-part sea level rise series: 30 great tools to determine your flood risk in the U.S. (2023) Bubble trouble: Climate change is creating a huge and growing U.S. real estate bubble (2023) How sea level rise contributes to billions in extra damage during hurricanes (2022) Bob Henson contributed to this post. This article first appeared on Yale Climate Connections and is republished here under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License .