Tropical cyclones and climate change

• 1. Introduction

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  Tropical cyclones and climate change share a relationship that scientists have been piecing together for decades, and the picture that has emerged is not a simple one. Between 1979 and 2017, the share of tropical cyclones reaching Category 3 or higher on the Saffir-Simpson scale grew across the globe. That means storms with wind speeds exceeding 178 kilometers per hour were becoming more common, not just more destructive. And the forces behind that shift are now entangled with human-induced warming in ways that carry real consequences for hundreds of millions of coastal people.

  What does warming actually do to a storm? Why does a two-degree rise in global temperature change which coastlines get hit, and how hard? And why, despite all the evidence of intensifying storms, do scientists still disagree about whether there will be more of them in the future? Those are the questions at the center of this story.

• 2. Fuel for the Storm

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  Warm, moist air is the engine of every tropical cyclone, and climate change is adding more of that fuel to the ocean's surface. Sea surface temperatures are rising, which means the raw energy available to a developing storm is growing. A study published in 2012 suggested that sea surface temperatures can serve as a proxy for measuring the potential intensity of tropical cyclones, because the storms are so sensitive to the temperature of the water beneath them.

  The physics here traces back to a well-established thermodynamic principle called the Clausius-Clapeyron relation. That relation tells us that for every one degree Celsius of warming, the atmosphere's theoretical maximum water vapor content rises by roughly seven percent. More vapor in the air means more rain, and it means storms can sustain themselves more vigorously once they form.

  The Arabian Sea offers one of the most striking regional examples. Between 1982 and 2019, the number of cyclones in that basin increased by 52 percent. The number of very severe cyclones rose even more sharply, up by 150 percent over the same period. The total duration of cyclones in the Arabian Sea grew by 80 percent, while the duration of very severe cyclones grew by 260 percent. These are not marginal changes. They point toward a basin that has shifted its behavior substantially within living memory.

• 3. Intensity and Rapid Change

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  Rapid intensification is among the most dangerous features of modern tropical cyclones. A storm can jump from tropical storm to major hurricane within a day, far faster than evacuation orders can be carried out. Climate change has been driving an observed increase in this phenomenon in the Atlantic basin, with the proportion of storms undergoing rapid intensification nearly doubling over the years 1982 to 2009.

  Projections suggest the trend will continue. With two degrees Celsius of warming, an additional 13 percent of tropical cyclones are expected to reach Category 4 or 5 strength. A 2020 study of storms at tropical storm strength and above found that human-induced warming had already increased extreme three-hourly storm rainfall rates by 10 percent and extreme three-day accumulated rainfall by 5 percent. For hurricane-strength storms specifically, those figures rose to 11 percent and 8 percent.

  Storms are also lingering longer after landfall than they did in the past. Once a cyclone crosses the coast, it now decays more slowly, threatening communities further inland that historically sat outside the main danger zone. The 2020 Atlantic hurricane season was exceptionally active and broke numerous records for storm frequency and intensity, serving as a vivid illustration of the direction the data has been pointing.

• 4. Rainfall, Surge, and Flooding

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  The World Meteorological Organization stated in 2017 that the volume of rainfall from Hurricane Harvey was very likely increased by climate change. That assessment sits within a broader scientific picture: every climate model assessed in a 2019 review paper projected future increases in rainfall rates from tropical cyclones.

  A 2019 study found that climate-linked increases in evaporation and atmospheric water-holding capacity had already raised rainfall from hurricanes Katrina, Irma, and Maria by 4 to 9 percent. Future increases of up to 30 percent were projected for rainfall from Atlantic hurricanes. Meanwhile, storm surge is being compounded by sea level rise. Between 1923 and 2008, storm surge incidents along the United States Atlantic coast showed a positive upward trend. A 2017 study examined the combined effects of storm surge, river flooding, and coastal inundation, and projected that these compounding flood hazards will increase under continued warming.

  Natural buffers such as marshes, mangroves, and coral reefs can absorb some of this impact, acting as barriers to coastal erosion and surge. A 2015 study examining risk mitigation options in Freeport, Texas found that incorporating natural ecosystems into planning could reduce flood heights and lower the cost of engineered defenses. Despite that evidence, built defenses remain the default choice for most government agencies.

• 5. Where Storms Travel

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  Tropical cyclones are not staying where they have historically been. There is a documented poleward expansion in the latitude at which storms reach their maximum intensity, a shift that may be connected to climate change. In the North Pacific specifically, there may also have been an eastward expansion of storm tracks. Cyclones moving into higher latitudes reach colder water and different atmospheric conditions than the tropical environments they formed in.

  Between 1949 and 2016, tropical cyclones globally showed a slowdown in translation speed, meaning they moved across the ocean surface more slowly. A slower storm drops more rain on a single location and maintains its destructive winds over an area for longer. How much of this slowdown is attributable to climate change remains uncertain; not all climate models reproduce the feature.

  Research following the 2015 eastern and central Pacific hurricane season, which produced a record number of storms including three simultaneous Category 4 hurricanes, found that greenhouse gas forcing enhances subtropical Pacific warming. That warming is projected to increase the frequency of extremely active seasons in that region. A separate 2011 study linked intensified Atlantic hurricanes to a northward shift and amplification of African easterly waves, the atmospheric disturbances that seed many Atlantic storms.

• 6. The Frequency Question

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  One of the most contested questions in tropical cyclone science is whether climate change will produce more storms overall. The current answer from research is: probably not more, and possibly fewer. A majority of climate models project a decrease in the total global frequency of tropical cyclones under future warming. A 2020 paper comparing nine high-resolution climate models found robust frequency decreases in the Southern Indian Ocean and the Southern Hemisphere more generally, while producing mixed signals for the Northern Hemisphere.

  One factor that complicates the picture is sea surface temperature thresholds. A study published in 2015 concluded that tropical cyclone formation is possible with sea surface temperatures below 26 degrees Celsius, and that a cooler climate might actually produce more storms. In a warmer world, rising carbon dioxide levels combined with higher Southern Hemisphere sea surface temperatures are projected to reduce cyclone frequency in that region.

  Observations of actual storm counts have shown little change globally in recent decades, with increases in the North Atlantic and central Pacific partially offset by significant decreases in the southern Indian Ocean and western North Pacific. The divergence between basins matters enormously for which regions face growing risk and which might face relatively stable or even declining cyclone activity.

• 7. Regional Risks and Responses

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  Typhoons in the northwestern Pacific intensified by 12 to 15 percent on average since 1977, according to research based on records from Japan and Hawaii. In some regions, the strongest observed typhoons doubled or tripled in intensity. The populations most exposed to this shift include those in China, Japan, Korea, and the Philippines. Researchers attributed the trend to warming ocean waters, noting that decreasing vertical wind shear around China has created more favorable conditions for intense storms, partly as a consequence of the weakening East Asian summer monsoon.

  In the Pacific island nations, the Framework for Resilient Development in the Pacific has been established to coordinate disaster response and climate adaptation across the region. Countries including Tonga and the Cook Islands have developed Joint National Action Plans on Climate Change and Disaster Risk Management under that framework, identifying vulnerable areas, setting policy goals, and establishing timelines for actions ranging from reforestation to levee construction to renewable energy investment.

  In the United States, a 2018 study found no significant trend in landfalling hurricane frequency or intensity since 1900, but found that growth in coastal populations and regional wealth has been the dominant driver of rising hurricane-related damage costs. The National Flood Insurance Program has drawn criticism for creating incentives to rebuild in flood-prone areas, which researchers argue undermines long-term adaptation to hurricane and sea level rise risk.

• 8. Public Perception Shifts

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  Polling after major Atlantic hurricanes has tracked a measurable shift in American public opinion. In 2005-39 percent of Americans surveyed said they believed climate change contributed to hurricane intensity. By September 2017, following a season that included hurricanes Harvey, Irma, and Maria, that figure had climbed to 55 percent. The destruction wrought by Katrina, Wilma, and Sandy in the early 21st century sparked substantial media interest in the relationship between climate change and hurricanes.

  The picture outside the United States is more varied. After Typhoon Meranti struck in 2016, risk perception in China did not measurably increase, though researchers observed a clear rise in support for personal and community action on climate change. In Taiwan, people who had lived through a typhoon did not express greater anxiety about climate change specifically, but researchers found a positive correlation between anxiety about typhoons and anxiety about climate change more broadly.

  The gap between what scientists observe and what the public absorbs remains a live issue in climate policy. Rapidly intensifying storms that are hard to forecast, like those whose proportion nearly doubled in the Atlantic between 1982 and 2009, offer little warning time for coastal residents, which means public understanding of the underlying trends carries direct stakes for how communities prepare.