Climate Change

What makes 2017 Atlantic hurricane season unprecedented?

Tropical cyclones like hurricanes and typhoons are changing character due to global warming. Not only are they becoming more frequent, wind speeds are also intensifying much quickly, putting the lives of coastal communities at grave risk and causing widespread devastation.

By Subhojit Goswami, Akshit Sangomla
Published: Thursday 05 October 2017
Vessels that sank during Hurricane Irma in St John, US Virgin Islands on September 16 (Photo: Reuters)

Flood of hurricanes

The series of hurricanes that ravaged the Atlantic in September are a dress rehearsal of a stormy future. Hurricane Harvey produced the second largest rainfall event in US history, dumping about 1,316 mm of rain on southeast Texas. This is more than 150 per cent of average rainfall the state gets in a year. It claimed over 70 lives and devastated large parts of Texas. Then Hurricane Irma created a new record by maintaining a maximum wind speed of 298 kmph for 37 hours and killing over 60 people. A dozen Caribbean island nations were also flattened by hurricane Irma. Hurricane Maria was the first category 4 storm to hit Puerto Rico in nearly 90 years. The island’s governor Ricardo Rossello said, “We have not experienced an event of this magnitude in our modern history.”

The warning came eight months ago. In December 2016, a climate science report by 13 US federal agencies observed, “There is some danger, in the form of evoking complacency, in placing too much emphasis on the recent absence of a specific subset of hurricanes.” They were referring to the fact that the country had not witnessed a major hurricane landfall for 11 years. Harvey and Irma signalled the end of “hurricane drought”.

Tropical cyclones are known as hurricanes in the Atlantic or Eastern Pacific; typhoons in the western North Pacific; and, simply cyclones in the South Pacific and Indian Ocean. They are part of the process through which the Earth’s atmospheric system maintains its heat balance, transferring it from the tropics towards the poles and into space. When the air above the sea surface warms up to about 27°C or more, it rises into the atmosphere and cold air from the surrounding areas takes its place. This creates a downward spiral of air into the centre and an upward wind due to pressure differences between the ocean surface and the outer atmosphere. The wind carries water vapour along with it and takes the form of a swirl. This water vapour then forms clouds, which make a circular formation, and suck up more warm air and moisture from the ocean below, adding to their size.

Such a formation can be several hundred kilometres wide with a depression, called an eye, at its centre. The eye remains at low pressure and calm, with strong winds from all around flowing into it. The eye wall, located just outside the eye, causes the most damaging winds. Depending on its intensity, the wall pushes water inland faster than it can drain off. This rapid rise of water—several metres in just a few minutes—is the storm surge. It is the this storm surge that was whipped up by Irma’s eye wall and damaged 90 per cent of homes in Florida Keys. On September 20, when hurricane Maria triggered a storm surge, Puerto Rico witnessed a torrent of water.

A man walks among the debris caused by Hurricane Irma in the Dominican Republic on September 7 (Photo: Reuters)


The US, however, was lucky not to experience storm surges of such intensity between 2006 and 2016 as it was not hit by a Category 3 or higher “major hurricane” during these years. On May 25, forecasters at the National Oceanic and Atmospheric Administration (NOAA) predicted an above-normal hurricane season with 70 per cent likelihood of 11 to 17 named storms (wind speed of 63 kmph or higher), out of which 5 to 9 could become hurricanes (winds speed of 119 kmph or higher), including two to four major hurricanes (see ‘Categories of carnage’). Half way into 2017 hurricane season that lasts from June to November, 13 named storms, seven hurricanes and four major hurricanes have already been recorded. What is unusual is the development of two Category 4 and two Category 5 hurricanes in a month, and three major hurricanes passing through the same region in three weeks, as Maria, Irma and Jose have in the Caribbean. And this hurricane season is far from over.

“The season has the potential to be extremely active, and could be the most active since 2010,” the NOAA said in an August 9 press release. It is difficult to predict how the second-half of the hurricane season will play out, but it is well poised to surpass 2005’s record of 15 hurricanes and most number of Category 5 storms (Emily, Katrina, Rita and Wilma). Gerry Bell, lead seasonal hurricane forecaster at NOAA, noted that one of the factors pointing to an above-normal season include much warmer waters across the tropical Atlantic than what climate models had previously predicted.

Bell’s claim is endorsed by Kerry Emanuel, professor of Atmospheric Science at the Massachusetts Institute of Technology (MIT), who says tropical Atlantic is exhibiting high “thermal potential”, which means water can rapidly evaporate into the atmosphere. The greater the speed, the more favourable conditions are for hurricanes to form, and the more powerful they can become, adds Emanuel.

This “thermal potential” due to abnormally warm waters in the Atlantic coincides with scientific concern over rising greenhouse gases in the atmosphere and elevated global temperature. The surface temperature of water in the Atlantic was above 28°C this September. But Irma gained strength when it came across more warm water—about 32°C—on its path to Florida. The same was the case with Hurricane Harvey. It intensified rapidly because the sea surface temperature in Gulf of Mexico was about 4°C above normal. In fact, in the last 35 years, the total rise in temperature is about 0.32°C.

A 2013 study by the researchers at the Niels Bohr Institute in Copenhagen predicted a 10-fold increase in frequency of high-intensity hurricanes if the climate becomes 2°C warmer. “We find that 0.4°C warming of the climate corresponds to doubling of the frequency of extreme storm. If the temperature rises an additional degree, the frequency will increase by 3-4 times and if the global climate becomes 2°C warmer, there will be about 10 times as many extreme storm surges,” explains Aslak Grinsted, climate scientist at Niels Bohr Institute.

Source: National Hurricane Center, USA

“Our analysis of historical data on hurricanes in the last five decades suggests that we are already on track to see intense hurricanes more frequently,” he says. Not only has the number of hurricanes increased from 49 during 1970-79 to 75 in 2000-09, the share of major hurricanes has also seen a jump. Category 4 and 5 hurricanes consisted about 32 per cent of the total number of hurricanes in 1970-79. After two decades, the share of major hurricanes increased to 47 per cent. With almost two-and-a-half hurricane seasons still left in the current decade, the total number of hurricanes has already crossed 54, with 40 per cent of them being major hurricanes.

The jump in the number of hurricanes that we have seen in 2017 is partly attributed to the absence of wind shear: the change in the speed and direction of winds according to altitude. Wind shear distorts the shape of a hurricane, causing it to dissipate. It renders the system incapable of drawing in warm and moist air from the ocean, and thus, it fails to intensify. But in the case of Irma, there was no wind shear, which allowed the hurricane to grow to its enormous size and intensity.

This absence of wind shear could be due to the Atlantic Multi-Decadal Oscillation (AMO), a natural cycle of warming and cooling observed in the North Atlantic over the last 1,000 years. The warm phase of the cycle decreases the wind shear, leading to the intensification of hurricanes.

To make matters worse, even El Niño is stuck in neutral mode this year, thus improving prospects of Atlantic hurricanes. When El Niño is active, ocean surface temperatures in the equatorial Pacific become warmer than normal. This warming leads to more wind shear. The delay in the onset of El Niño, and the resulting absence of wind shear is likely to increase the chances of these cyclones forming right up to the end of hurricane season.

As if these reasons were not enough, rising sea level due to global warming is also exacerbating the impact of these storms. According to NASA, the global average sea level has increased by 10 to 20 cm since 1870. However, the annual rate of increase over the past 20 years has been roughly 0.3 cm. The Intergovernmental Panel on Climate Change (IPCC), in its 2013 report, claimed that the oceans could rise between 28 and 98 cm by 2100, which is enough to inundate several cities along the US East Coast. Vertical increase in sea level leads to increased horizontal reach of the storm surge. Hurricane Sandy in 2005 is an example where sea-level rise extended the reach of the storm by 70 sqkm, affecting 83,000 additional individuals living in New Jersey and New York.


This correlation between rising sea level and increasing storm surge does not augur well for the coastal communities around the world. Take the case of the US. The already crowded US coast will see population growth from 123 million to nearly 134 million by 2020, if current population trends continue, claimed a report by NOAA.

More than 39 per cent of the US population is concentrated in counties directly on the shoreline, the report revealed. “Population density in shore-line counties is more than six times greater than the corresponding inland counties. And the projected growth in coastal areas will increase population density at a faster rate than the country as a whole,” says Kristen Crossett of NOAA’s National Ocean Service.

As many as 13.1 million people along the US coasts could be displaced by the end of this century. By then, nearly 70 per cent of them will be residing in southeastern US, with half of that concentrated in Florida alone. These findings emerge from a 2016 joint study by researchers at the University of Georgia and Stetson University in Florida. According to NOAA’s National Ocean Service, more than 1,540 single-family housing units are permitted for construction every day in coastal counties. In fact, coastal counties are home to 53 per cent of the nation’s population, yet they account for only 17 per cent of US land area, excluding Alaska.

Apart from the projected increase in coastal population, it is the “risk of an increased frequency of poorly anticipated” high-intensity hurricane landfalls that could lead to higher rate of injuries and casualties. Emanuel, in his paper published in August 2016, observed that as number of storms that intensify rapidly just before landfall could increase substantially by the end of this century, forecasting this rapid intensification is problematic.

For example, in 2015, hurricane Patricia intensified from category 1 (139 kmph) to category 5 (333 kmph) within 24 hours. During this same period, the National Hurricane Center had predicted intensification by only 56 kmph. While it takes 2-3 days on an average for a category 1 hurricane to reach category 5, hurricane Maria took just 15 hours. “We used a modelling approach to simulate many US landfall events in the climate of the late 20th and late 21st centuries. We find a large increase in the incidence of rapid intensification before landfall that is caused by global warming,” says Emanuel.

A highway in Houston, Texas, submerged by the flood waters of hurricane Harvey on August 27 (Photo: Reuters)

In fact, over the past 30 years (1984 to 2014), storm speeds have increased on average by 1.3 metres per second—or 3 miles per hour— according to a joint study by James Elsner at Florida State University and Namyoung Kang, deputy director of the National Typhoon Center in South Korea. Elsner believes that higher wind speed is perhaps not related only to higher sea surface temperature as upper-level air temperatures might also be playing a role.

“Higher sea surface temperature doesn’t come alone. It is likely to be accompanied by higher pressure anomaly capping over the thermo-dynamically unstable tropical troposphere. It results in a blasting effect that could be added on to the sea surface temperature’s direct contribution,” says Kang.

Scientists are also faced with another important question: why did Hurricane Harvey linger on Texas longer than it should have? “It just sat, and it sat close enough to the Gulf of Mexico that it could just keep pulling in all this moisture, and then it dropped all that rain in Houston,” says Phil Klotzbach of Colorado State University in an interview with the Pacific Standard. They are now trying to understand whether this slowness had something to do with the “episodes in which the jet-stream—the high-flying river of air that meanders around the higher-to-mid latitudes—gets locked into one place for an extended period”, perhaps making the storm system lazy.

These tropical cyclones are not just lingering, but they are also migrating towards the poles. James Kossin, a scientist at the US National Oceanic and Atmospheric Administration’s Center for Weather and Climate in Madison, Wisconsin says that in the last three decades, tropical cyclones, except hurricanes, have moved towards the poles in both hemispheres. What prevented hurricanes from north/south migration is the large decadal variation in the Atlantic. “The greatest poleward shifts have been observed in the western North Pacific, which has increased the hazard exposure across a large region, including Taiwan and Japan. Regions in the subtropics are the ones that would see their exposure increase,” says Kossin.

Source: EPA

But the Trump administration has resisted talking about climate change. “To have any kind of focus on the cause and effect of the storm; versus helping people, or actually facing the effect of the storm, is misplaced,” says Scott Pruitt, an administrator with the US Environmental Protection Agency (EPA). Besides skirting questions from media on the link between hurricanes and global warming, White House officials have been suggesting that “hurricane systems are cyclical and not tied to global warming”. Their scepticism may suffer a quake as NOAA predicts sea-level rise in south Florida by as much as 86 cm by 2050 (see ‘Learning from Harvey’).

Research released by NOAA in August 2017 predicts that anthropogenic warming by the end of the 21st century is likely to increase intensity of tropical cyclones globally by 2 to 11 per cent on an average. It will likely cause tropical cyclones to bring substantially higher volume (about 10-15 per cent) of rainfall than current ones. Under such circumstances, coastal communities need support to mitigate climate disasters and deal with the aftermath.

However, the Trump administration is gearing up to vote on bills that would cripple the ability of the EPA to address health and environmental impacts due to climate disasters (see ‘Unsafe cuts’). If Trump’s proposals find support in the Congress, the EPA would face the biggest cut of any federal agency in the 2018 budget. Several programmes, aimed at helping communities deal with climate-related challenges, may soon cease to exist.


The 2017 hurricane season has been the worst for Caribbean countries. Hurricane Irma, alone, affected more than 10 Caribbean countries and it came with a record-breaking price tag—about $13 billion. On September 20, Puerto Rico was hit by Hurricane Maria. For a country in a dire financial condition, rebuilding the decimated power infrastructure and rehabilitating dozens of communities would plunge it deeper into a fiscal crisis. Dominica, one of the poorest countries in the Caribbean, saw about 95 per cent of roofs damaged or destroyed by Maria.

Earlier, hurricane Irma had completely flat-tened Antigua and Barbuda, and for the first time in 300 years, there’s not a single person living on the island. “We’re a US $1 billion economy facing a US $250 million problem,” Ronald Sanders, the Antigua and Barbuda ambassador to the United States, said while referring to the cost of rebuilding the island that has been reduced to rubble. While back-to-back hurricanes are not uncommon in the Caribbean, the intensity of hurricanes such as Maria and Irma was not seen for decades.

What makes it worse is that many Caribbean countries have an impoverished economy. Haiti is a classic example. In 2016, Hurricane Matthew hit US, Canada, Haiti and its neighbouring nations in the Caribbean. But despite hitting each of the nations hard, Haiti faced the greatest destruction with more than 470 deaths. Even hours before landfall, many Haitians were unaware of the impending disaster because the poorest country in the Americas, with 2.5 million people living in extreme poverty, does not have disaster pre-paredness mechanisms.

One of the markers of lack of preparedness in the Caribbean islands is their electricity grids. They are exposed, centralised and powered by fossil fuels. When an island is hit by storms and power plants are shut down, the entire island goes dark. Damage to vulnerable seaports also cuts off access to neighbouring nations. For these low-lying countries, which are already heavily indebted, the risk of losing wealth in climate disasters is ultimately the last nail in the coffin.

In the upcoming UN climate talks, the Caribbean nations are planning to urge the richer countries to play a bigger role in helping region bolster its defense mechanism. There is a need to speed up access to capital, especially the $10-billion Green Climate Fund, set up to help developing countries to tackle climate change. “It takes too long for these resources to move, given the urgency of the need,” says William Warren Smith, President of Caribbean Development Bank. After the Atlantic hurricane season ends, the focus must shift to addressing this growing resilience gap.

Data source: NASA and NOAA

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