Getting hit with one hurricane is bad enough, but new research from Princeton Engineering shows that back-to-back versions may become common for many areas in coming decades.
Driven by a combination of rising sea levels and climate change, destructive hurricanes and tropical storms could become far more likely to hit coastal areas in quick succession, researchers found. In an article published Feb. 27 in the journal Nature Climate Change, the researchers said that in some areas, like the Gulf Coast, such double hits could occur as frequently as once every three years.
Climate change is both making hurricanes more dangerous and, over time, increasing the chances of them hitting the same site twice, according to research published Monday in the journal Nature Climate Change.
Researchers from Princeton’s Department of Civil and Environmental Engineering analyzed Atlantic tropical cyclones dating back to the mid-20th century to project toward the end of the 21st. Their analysis of nine East Coast locations found that, from 1949 to 2018, tropical cyclone hazards increased everywhere but Charleston, S.C., and Pensacola, Fla. The duration of those hazards also increased everywhere except Charleston.
Most people associate hurricanes with high winds, intense rain and rapid flooding on land. But these storms can also change the chemistry of coastal waters. Such shifts are less visible than damage on land, but they can have dire consequences for marine life and coastal ocean ecosystems.
We are oceanographers who study the effects of ocean acidification, including on organisms like oysters and corals. In a recent study, we examined how stormwater runoff from Hurricane Harvey in 2017 affected the water chemistry of Galveston Bay and the health of the bay’s oyster reefs. We wanted to understand how extreme rainfall and runoff from hurricanes influenced acidification of bay waters, and how long these changes could last.
Our findings were startling. Hurricane Harvey, which generated massive rainfall in the Houston metropolitan area, delivered a huge pulse of fresh water into Galveston Bay. As a result, the bay was two to four times more acidic than normal for at least three weeks after the storm.
This made bay water corrosive enough to damage oyster shells in the estuary. Because oyster growth and recovery rely on many factors, it is hard to tie specific changes to acidification. However, increased acidification certainly would have made it harder for oyster reefs damaged by Hurricane Harvey to recover. And while our study focused on Galveston Bay, we suspect that similar processes may be occurring in other coastal areas.
Vast quantities of water
Scientists predict that climate change will make hurricanes stronger and increase the amount of rain they produce over the next several decades. Changes in ocean chemistry, caused by runoff from these storms, are becoming an increasing threat to many marine ecosystems, especially coastal reefs built by oysters and corals.
Coastal estuaries like Galveston Bay, where rivers meet the sea, are some of the most productive ecosystems in the world. Galveston Bay is the largest bay on the Texas coast and one of the largest in the U.S.; it covers about 600 square miles, roughly half the size of Rhode Island. Its extensive oyster reefs provide about 9% of the national oyster harvest.
Hurricane Harvey, the wettest tropical cyclone in U.S. history, made landfall on the Texas coast as a Category 4 hurricane on Aug. 26, 2017. Harvey stalled at the coast for four days, sitting over both land and ocean.
Maintaining contact with warm Gulf of Mexico waters fueled the storm with both energy and rainfall, allowing it to persist and drop extreme amounts of rain directly onto Houston and surrounding areas – up to 50 inches in four days. All of that rain and floodwater had to go somewhere, and much of it flowed into Galveston Bay.
Climate change and ocean acidification
The ocean acidification issues that we study are a well-known effect related to climate change. Human activities, mainly burning fossil fuel, emit carbon dioxide into the atmosphere. The ocean absorbs about one-third of these emissions, which alters ocean chemistry, making seawater more acidic.
Acidification can harm many forms of marine life. It is especially dangerous for animals that build their shells and skeletons out of calcium carbonate, such as oysters and corals. As seawater becomes more acidic, it makes these structures harder to build and easier to erode.
Oysters fuse together as they grow, creating large rocklike underwater reefs that protect shorelines from wave erosion. These reefs provide habitat for other creatures, such as barnacles, anemones and mussels, which in turn serve as food sources for many fish species.
Rising atmospheric CO₂ levels are acidifying oceans worldwide. As our study shows, local events like tropical cyclones can add to global acidification.
Stormwater from Harvey caused extreme coastal acidification
The main cause of the unprecedented acidification that occurred after Hurricane Harvey was the excessive amount of rainfall and runoff that entered Galveston Bay. To help manage large-scale flooding in the Houston area, the city released large volumes of water from reservoirs for more than two months after Harvey. These releases extended the time during which stormwater entered Galveston Bay and increased its acidity.
Scientists use the pH scale to measure how acidic or basic (alkaline) water is. A pH value of 7 is neutral; higher values are basic, and lower values are acidic. The pH scale is logarithmic, so a decrease of one full unit – say, from 8 to 7 – represents a tenfold increase in acidity.
Rainwater is more acidic than either river water or seawater, which pick up minerals from soil that are slightly basic and can balance out absorbed carbon dioxide from the atmosphere. Rainwater’s pH is around 5.6, compared with between 6.5 and 8.2 for rivers and about 8.1 for seawater.
Galveston Bay contains a mix of fresh water from rivers and salty seawater from the Gulf of Mexico – oysters’ preferred habitat. We collected water samples in the bay two weeks after Harvey and found that the bay was made up almost entirely of river water and rainwater from the storm.
Since rainwater, river water and seawater all have different chemistries, we were able to calculate that rainwater made up almost 50% of the water in the bay. This means that acidic rainwater from Harvey replaced the basic seawater within the bay after the storm. The average bay water pH had dropped from 8 to 7.6, a 2.5-fold increase in acidity. Some zones had pH even as low as 7.4 – four times more acidic than normal.
This extreme acidification lasted for more than three weeks. Bay waters became corrosive not only to more sensitive larval and juvenile oyster shells, but to adult oyster shells as well. Scientists had predicted that increasing CO₂ could cause this scale of coastal acidification but did not expect to see it until around the year 2100.
The fresh water from Harvey also caused a severe oyster die-off in the bay because oysters need slightly salty water to survive. Harvey struck in the middle of oyster spawning season, and acidification may have slowed reef recovery by making it harder for young oysters to form new shells. Officials at the Texas Parks and Wildlife Department have told us that four years later, in late 2021, some Galveston Bay oyster reefs still showed very low additions of new oysters.
Other coastal areas at risk
Only a few studies, including ours, have analyzed how tropical cyclones affect coastal acidification. In our view, however, it is highly possible that other storms have caused the kind of extreme acidification that we detected in the wake of Harvey.
We reviewed the 10 wettest tropical cyclones in the U.S. since 1900 and found that nine, including Harvey, caused large amounts of rain and flooding in coastal areas with bay or estuary ecosystems. Other storms didn’t produce as much rainfall as Harvey, but some of the affected bays were much smaller than Galveston Bay, so less rain would have been needed to replace seawater in the bay and cause a similar level of acidification to what Harvey produced.
We think that this likely has already occurred in other places struck by hurricanes but went unrecorded because scientists weren’t able to measure acidification before and after the storms. As climate change continues to make tropical cyclones larger and wetter, we see storm-induced acidification as a significant threat to coastal ecosystems.
Tropical cyclones have been growing stronger worldwide over the past 30 years, and not just the big ones that you hear about. Our new research finds that weak tropical cyclones have gotten at least 15% more intense in ocean basins where they occur around the world.
That means storms that might have caused minimal damage a few decades ago are growing more dangerous as the planet warms.
Warmer oceans provide more energy for storms to intensify, and theory and climate models point to powerful storms growing stronger, but intensity isn’t easy to document. We found a way to measure intensity by using the ocean currents beneath the storms – with the help of thousands of floating beachball-sized labs called drifters that beam back measurements from around the world.
Why it’s been tough to measure intensity
Tropical cyclones are large storms with rotating winds and clouds that form over warm ocean water. They are known as tropical storms or hurricanes in the Atlantic and typhoons in the Northwest Pacific.
A tropical cyclone’s intensity is one of the most important factors for determining the damage the storm is likely to cause. However, it’s difficult to accurately estimate intensity from satellite observations alone.
Intensity is often based on maximum sustained surface wind speed at about 33 feet (10 meters) above the surface over a period of one, two or 10 minutes, depending on the meteorological agency doing the measuring. During a hurricane, that region of the storm is nearly impossible to reach.
For some storms, NOAA meteorologists will fly specialized aircraft into the cyclone and drop measuring devices to gather detailed intensity data as the devices fall. But there are many more storms that don’t get measured that way, particularly in more remote basins.
Our study, published in the journal Nature in November 2022, describes a new method to infer tropical cyclone intensity from ocean currents, which are already being measured by an army of drifters.
How drifters work
A drifter is a floating ball with sensors and batteries inside and an attached “drogue” that looks like a windsock trailing under the water beneath it to help stabilize it. The drifter moves with the currents and regularly transmits data to a satellite, including water temperature and location. The location data can be used to measure the speed of currents.
Since NOAA launched its Global Drifter Program in 1979, more than 25,000 drifters have been deployed in global oceans. Those devices have provided about 36 million records over time. Of those records, more than 85,000 are associated with weak tropical cyclones – those that are tropical storms or Category 1 hurricanes or typhoons – and about 5,800 that are associated with stronger tropical cyclones.
That isn’t enough data to analyze strong cyclones globally, but we can find trends in the intensity of the weak tropical cyclones.
Here’s how: Winds transfer momentum into the surface ocean water through frictional force, driving water currents. The relationship between wind speed and ocean current, known as Ekman theory, provides a theoretical foundation for our method of deriving wind speeds from the drifter-measured ocean currents.
Our derived wind speeds are consistent with wind speeds directly measured by nearby buoy arrays, justifying the new method to estimate tropical cyclone intensity from drifter measurements.
Evidence beneath the storms
In analyzing those records, we found that the ocean currents induced by weak tropical cyclones became stronger globally during the 1991-2020 period. We calculated that the increase in ocean currents corresponds to a 15% to 21% increase in the intensity of weak tropical cyclones, and that intensification occurred in all ocean basins.
In the Northwest Pacific, an area including China, Korea and Japan, a relatively large amount of available drifter data also shows a consistent upward trend in the intensity of strong tropical cyclones.
We also found evidence of increasing intensity in the changes in water temperatures measured by satellites. When a tropical cyclone travels through the ocean, it draws energy from the warm surface water and churns the water layers below, leaving a footprint of colder water in its wake. Stronger tropical cyclones bring more cold water from the subsurface to the surface ocean, leading to a stronger cooling in the ocean surface.
The 2022 Atlantic hurricane season officially ended on Nov. 30 with 14 named storms and eight hurricanes. It isn’t clear how rising global temperatures will effect the number of tropical cyclones that form, but our findings suggest that coastal communities need to be better prepared for increased intensity in those that do form and a concurrent rise in sea level in the future.
Then a year like 2022 comes along, with no major hurricane landfalls until FionaandIan struck in late September. The Atlantic hurricane season, which ends Nov. 30, has had eight hurricanes and 14 named storms. It’s a reminder that small sample sizes can be misleading when assessing trends in hurricane behavior. There is so much natural variability in hurricane behavior year to year and even decade to decade that we need to look much further back in time for the real trends to come clear.
Fortunately, hurricanes leave behind telltale evidence that goes back millennia.
Two thousand years of this evidence indicates that the Atlantic has experienced even stormier periods in the past than we’ve seen in recent years. That’s not good news. It tells coastal oceanographers like me that we may be significantly underestimating the threat hurricanes pose to Caribbean islands and the North American coast in the future.
The natural records hurricanes leave behind
When a hurricane nears land, its winds whip up powerful waves and currents that can sweep coarse sands and gravel into marshes and deep coastal ponds, sinkholes and lagoons.
Under normal conditions, fine sand and organic matter like leaves and seeds fall into these areas and settle to the bottom. So when coarse sand and gravel wash in, a distinct layer is left behind.
Imagine cutting through a layer cake – you can see each layer of frosting. Scientists can see the same effect by plunging a long tube into the bottom of these coastal marshes and ponds and pulling up several meters of sediment in what’s known as a sediment core. By studying the layers in sediment, we can see when coarse sand appeared, suggesting an extreme coastal flood from a hurricane.
With these sediment cores, we have been able to document evidence of Atlantic hurricane activity over thousands of years.
Others, including from AtlanticCanada, North Carolina, northwestern Florida, Mississippi and Puerto Rico, are lower-resolution, meaning it is nearly impossible to discern individual hurricane layers deposited within decades of one another. But they can be highly informative for determining the timing of the most intense hurricanes, which can have significant impacts on coastal ecosystems.
It’s the records from the Bahamas, however, with nearly annual resolution, that are crucial for seeing the long-term picture for the Atlantic Basin.
Why The Bahamas are so important
The Bahamas are exceptionally vulnerable to the impacts of major hurricanes because of their geographic location.
In the North Atlantic, 85% of all major hurricanes form in what is known as the Main Development Region, off western Africa. Looking just at observed hurricane tracks from the past 170 years, my analysis shows that about 86% of major hurricanes that affect the Bahamas also form in that region, suggesting the frequency variability in the Bahamas may be representative of the basin.
A substantial percentage of North Atlantic storms also pass over or near these islands, so these records appear to reflect changes in overall North Atlantic hurricane frequency through time.
By coupling coastal sediment records from the Bahamas with records from sites farther north, we can explore how changes in ocean surface temperatures, ocean currents, global-scale wind patterns and atmospheric pressure gradients affect regional hurricane frequency.
As sea surface temperatures rise, warmer water provides more energy that can fuel more powerful and destructive hurricanes. However, the frequency of hurricanes – how often they form – isn’t necessarily affected in the same way.
The secrets hidden in blue holes
Some of the best locations for studying past hurricane activity are large, near-shore sinkholes known as blue holes.
Blue holes get their name from their deep blue color. They formed when carbonate rock dissolved to form underwater caves. Eventually, the ceilings collapsed, leaving behind sinkholes. The Bahamas has thousands of blue holes, some as wide as a third of a mile and as deep as a 60-story building.
They tend to have deep vertical walls that can trap sediments – including sand transported by strong hurricanes. Fortuitously, deep blue holes often have little oxygen at the bottom, which slows decay, helping to preserve organic matter in the sediment through time.
Cracking open a sediment core
When we bring up a sediment core, the coarse sand layers are often evident to the naked eye. But closer examination can tell us much more about these hurricanes of the past.
I use X-rays to measure changes in the density of sediment, X-ray fluorescence to examine elemental changes that can reveal if sediment came from land or sea, and sediment textural analysis that examines the grain size.
To figure out the age of each layer, we typically use radiocarbon dating. By measuring the amount of carbon-14, a radioactive isotope, in shells or other organic material found at various points in the core, I can create a statistical model that predicts the age of sediments throughout the core.
So far, my colleagues and I have published five paleohurricane records with nearly annual detail from blue holes on islands across the Bahamas.
Each record shows periods of significant increase in storm frequency lasting decades and sometimes centuries.
The records vary, showing that a single location might not reflect broader regional trends.
For example, Thatchpoint Blue Hole on Great Abaco Island in the northern Bahamas includes evidence of at least 13 hurricanes per century that were Category 2 or above between the years 1500 and 1670. That significantly exceeds the rate of nine per century documented since 1850. During the same period, 1500 to 1670, blue holes at Andros Island, just 186 miles (300 kilometers) south of Abaco, documented the lowest levels of local hurricane activity observed in this region during the past 1,500 years.
Spotting patterns across the Atlantic Basin
Together, however, these records offer a glimpse of broad regional patterns. They’re also giving us new insight into the ways ocean and atmospheric changes can influence hurricane frequency.
While rising sea surface temperatures provide more energy that can fuel more powerful and destructive hurricanes, their frequency – how often they form – isn’t necessarily affected in the same way. Some studies have predicted the total number of hurricanes will actually decrease in the future.
The compiled Bahamian records document substantially higher hurricane frequency in the northern Caribbean during the Little Ice Age, around 1300 to 1850, than in the past 100 years.
Records from sites farther north tell us more about the climate. That’s because changes in ocean temperature and climate conditions are likely far more important to controlling regional impacts in such areas as the Northeastern U.S. and Atlantic Canada, where cooler climate conditions are often unfavorable for storms.
A warning for the islands
I am currently developing records of coastal storminess in locations including Newfoundland and Mexico. With those records, we can better anticipate the impacts of future climate change on storm activity and coastal flooding.
In the Bahamas, meanwhile, sea level rise is putting the islands at increasing risk, so even weaker hurricanes can produce damaging flooding. Given that storms are expected to be more intense, any increase in storm frequency could have devastating impacts.
When storms like Hurricane Ian make landfall, the first things they hit often are barrier islands – thin ribbons of sand that line the U.S. Atlantic and Gulf coasts. It’s hard to imagine how these narrow strips can withstand such forces, but in fact, many of them have buffered our shores for centuries.
Many barrier islands have been developed into popular tourist destinations, including Florida’s Sanibel Island and South Carolina’s Pawleys Island, both of which suffered heavy damage from Hurricane Ian. Islands that have been preserved in their natural state can move with storms, shifting their shapes over time. But many human activities interfere with these natural movements, making the islands more vulnerable.
Islands on the move
Barrier islands are made of sandy, erodible soil and subject to high-energy wave action. They are dynamic systems that constantly form and reform. But this doesn’t necessarily mean the islands are disappearing. Rather, they migrate naturally, building up sand in some areas and eroding in other areas.
New islands can form out in the ocean, either because local sea level drops or tectonics or sediment deposition raises the ocean floor. Or they may shift laterally along the shore as currents carry sediments from one end of the island toward the other. On the East Coast, barrier islands usually move from north to south because longshore currents transport sand in the same direction.
And over time many barrier islands move landward, toward the shore. This typically happens because local sea levels rise, so waves wash over the islands during storms, moving sand from the ocean side to the inland side.
How longshore drift moves sediment along a beach.
1=beach. 2=sea 3=longshore current direction 4=incoming waves 5=swash 6=backwash USGS
Building on shifting sands
Building hard infrastructure such as homes, roads and hotels on barrier islands interrupts their lateral migration. Needless to say, beach communities want their dunes to stay in place, so the response often is to build control structures, such as seawalls and jetties.
This protects buildings and roads, but it also disrupts natural sand transportation. Blocking erosion up-current means that no sediments are transported down-current, leaving those areas starved of sediment and vulnerable to erosion.
Many sandy tourist beach towns along the East Coast also turn to beach nourishment – pumping tons of sand from offshore – to replace sand lost through erosion. This does not interrupt natural sand transportation, but it is a very expensive and temporary fix.
For example, since the 1940s Florida has spent over US$1.3 billion on beach nourishment projects, and North Carolina has spent more than $700 million. This added sand will eventually wash away, quite possibly during the next hurricane to hit the coast, and have to be replaced.
What kind of protection?
In some cases, however, leaving barrier islands to do their own natural thing can cause problems for people. Some cities and towns, such as Miami and Biloxi, are located behind barrier islands and rely on them as a first line of defense against storms.
And many communities depend on natural resources provided by the estuaries and wetlands behind barrier islands. For example, Pamlico Sound – the protected waters behind North Carolina’s Outer Banks – is a rich habitat for blue crabs and popular sport fish such as red drum.
Unmanaged, some of these islands may not move the way we want them to. For example, a storm breach on a barrier island that protects a city would make that city more vulnerable.
Here in Mississippi, a string of uninhabited barrier islands off our coast separates Mississippi Sound from the Gulf of Mexico. Behind the islands is a productive estuary, important wetlands and cities such as Biloxi and Gulfport.
Because the Mississippi River has been dredged and enclosed between levees to keep it from spilling over its banks, this area does not receive the sediment loads that the river once deposited in this part of the Gulf. As a result, the islands are eroding and disappearing.
To slow this process, state and federal agencies have artificially nourished the islands to keep them in place and preserve the cities, livelihoods and ecological habitats behind them. This project filled a major breach cut in one island by Hurricane Camille in 1969, making the island a more effective storm buffer for the state’s coast.
When to retreat?
Geologically, barrier islands are not designed to stay in one place. But development on them is intended to last, although critics argue that climate change and sea level rise will inevitably force a retreat from the shore.
Reconciling humans’ love of the ocean with the hard realities of earth science is not easy. People will always be drawn to the coast, and prohibiting development is politically impractical. However, there are some ways to help conserve barrier islands while maintaining areas for tourism activities.
First, federal, state and local laws can reduce incentives to build on barrier islands by putting the burden of rebuilding after storms on owners, not on the government. Many critics argue that the National Flood Insurance Program has encouraged homeowners to rebuild on barrier islands and other coastal locations, even after suffering repeated losses in many storms.
Second, construction on barrier islands should leave dunes and vegetation undisturbed. This helps to keep their sand transportation systems intact. When roads and homes directly adjacent to beaches are damaged by storms, owners should be required to move back from the shoreline in order to provide a natural buffer between any new construction and the coastline.
Third, designating more conservation areas on barrier islands will maintain some of the natural sediment transportation and barrier island migration processes. And these conservation areas are popular nature-based tourism attractions. Protected barrier islands such as Assateague, Padre and the Cape Cod National Seashore are popular destinations in the U.S. national park system.
Finally, development on barrier islands should be done with change in mind and a preference for temporary or movable infrastructure. The islands themselves are surprisingly adaptable, but whatever is built in these dynamic settings is likely sooner or later to be washed away.
Anthony Grande moved away from Fort Myers three years ago in large part because of the hurricane risk. He has lived in southwest Florida for nearly 19 years, had experienced Hurricanes Charley in 2004 and Irma in 2017 and saw what stronger storms could do to the coast.
Grande told CNN he wanted to find a new home where developers prioritized climate resiliency in a state that is increasingly vulnerable to record-breaking storm surge, catastrophic wind and historic rainfall.
What he found was Babcock Ranch — only 12 miles northeast of Fort Myers, yet seemingly light years away.
Babcock Ranch calls itself “America’s first solar-powered town.” Its nearby solar array — made up of 700,000 individual panels — generates more electricity than the 2,000-home neighborhood uses, in a state where most electricity is generated by burning natural gas, a planet-warming fossil fuel.
When Hurricane Ian hit Florida, it was one of the United States’ most powerful hurricanes on record, and it followed a two-week string of massive, devastating storms around the world.
A few days earlier in the Philippines, Typhoon Noru gave new meaning to rapid intensification when it blew up from a tropical storm with 50 mph winds to a Category 5 monster with 155 mph winds the next day. Hurricane Fiona flooded Puerto Rico, then became Canada’s most intense storm on record. Typhoon Merbok gained strength over a warm Pacific Ocean and tore up over 1,000 miles of the Alaska coast.
Major storms hit from the Philippines in the western Pacific to the Canary Islands in the eastern Atlantic, to Japan and Florida in the middle latitudes and western Alaska and the Canadian Maritimes in the high latitudes.
A lot of people are asking about the role rising global temperatures play in storms like these. It’s not always a simple answer.
It is clear that climate change increases the upper limit on hurricane strength and rain rate and that it also raises the average sea level and therefore storm surge. The influence on the total number of hurricanes is currently uncertain, as are other aspects. But, as hurricanes occur, we expect more of them to be major storms. Hurricane Ian and other recent storms, including the 2020 Atlantic season, provide a picture of what that can look like.
Ourresearch has focused on hurricanes, climate change and the water cycle for years. Here’s what scientists know so far.
Hurricanes are powered by the release of heat when water that evaporates from the ocean’s surface condenses into the storm’s rain.
A warmer ocean produces more evaporation, which means more water is available to the atmosphere. A warmer atmosphere can hold more water, which allows more rain. More rain means more heat is released, and more heat released means stronger winds.
These are basic physical properties of the climate system, and this simplicity lends a great deal of confidence to scientists’ expectations for storm conditions as the planet warms. The potential for greater evaporation and higher rain rates is true in general for all types of storms, on land or sea.
That basic physical understanding, confirmed in computer simulations of these storms in current and future climates, as well as recentevents, leads to high confidence that rainfall rates in hurricanes increase by at least 7% per degree of warming.
Storm strength and rapid intensification
Scientists also have high confidence that wind speeds will increase in a warming climate and that the proportion of storms that intensify into powerful Category 4 or 5 storms will increase. Similar to rainfall rates, increases in intensity are based on the physics of extreme rainfall events.
Damage is exponentially related to wind speed, so more intense storms can have a bigger impact on lives and economies. The damage potential from a Category 4 storm with 150 mph winds, like Ian at landfall, is roughly 256 times that of a category 1 storm with 75 mph winds.
Whether warming causes storms to intensify more rapidly is an active area of research, with some models offering evidence that this will probably happen. One of the challenges is that the world has limited reliable historical data for detecting long-term trends. Atlantic hurricane observations go back to the 1800s, but they’re only considered reliable globally since the 1980s, with satellite coverage.
Within the last two weeks of September 2022, both Noru and Ian exhibited rapid intensification. In the case of Ian, successful forecasts of rapid intensification were issued several days in advance, when the storm was still a tropical depression. They exemplify the significant progress in intensity forecasts in the past few years, although improvements are not uniform.
There is some indication that, on average, the location where storms reach their maximum intensity is moving poleward. This would have important implications for the location of the storms’ main impacts. However, it is still not clear that this trend will continue in the future.
Storm surge: Two important influences
Storm surge – the rise in water at a coast caused by a storm – is related to a number of factors including storm speed, storm size, wind direction and coastal sea bottom topography. Climate change could have at least two important influences.
Stronger storms increase the potential for higher surge, and rising temperatures are causing sea level to rise, which increases the water height, so the storm surge is now higher than before in relation to the land. As a result, there is high confidence for an increase in the potential for higher storm surges.
Speed of movement and potential for stalling
The speed of the storm can be an important factor in total rainfall amounts at a given location: A slower-moving storm, like Hurricane Harvey in 2017, provides a longer period of time for rain to accumulate.
There are indications of a global slowdown in hurricane speed, but the quality of historical data limits understanding at this point, and the possible mechanisms are not yet understood.
Frequency of storms in the future is less clear
How the number of hurricanes that form each year may change is another major question that is not well understood.
There is no definitive theory explaining the number of storms in the current climate, or how it will change in the future.
Besides having the right environmental conditions to fuel a storm, the storm has to form from a disturbance in the atmosphere. There is currently a debate in the scientific community about the role of these pre-storm disturbances in determining the number of storms in the current and future climates.
Natural climate variations, such as El Niño and La Niña, also have a substantial impact on whether and where hurricanes develop. How they and other natural variations will change in the future and influence future hurricane activity is a topic of active research.
How much did climate change influence Ian?
Scientists conduct attribution studies on individual storms to gauge how much global warming likely affected them, and those studies are currently underway for Ian.
However, individual attribution studies are not needed to be certain that the storm occurred in an environment that human-caused climate change made more favorable for a stronger, rainier and higher-surge disaster. Human activities will continue to increase the odds for even worse storms, year over year, unless rapid and dramatic reductions in greenhouse gas emissions are undertaken.
Heat is the fuel that makes hurricanes big, powerful and rainy. As humans burn fossil fuels and release huge amounts of carbon dioxide and other greenhouse gasses, the amount of heat trapped on Earth rises steadily. The air gets hotter, and the ocean water gets hotter. When a baby hurricane forms in the Atlantic, all that heat is available to help the storm grow.
That’s what happened to Ian. When the storm first formed, it was relatively weak. But as it moved over very hot water in the Caribbean and Gulf of Mexico, it grew very quickly.
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