The 2017 hurricane season: A series of monster storms
The satellite images were scary: Hurricane Irma, for a long time a Category 5 storm (the highest category), blew across the Caribbean islands towards Florida last week. Just the eye of the storm was larger than many of the islands that Irma hit.
Over the last few years, we had been seeing mostly weaker hurricanes that often stayed in the Atlantic, or if they did reach land, usually spared heavily populated areas. And yet recently not one but several superstorms have occurred, some of which have reached the coasts of the United States or Mexico. A few facts:
Why are there so many severe storms occurring all of a sudden?
September is usually the month with the most hurricanes, since the water in the North Atlantic and Gulf of Mexico is at its warmest. An additional factor this year is that vertical wind shear, i.e. the difference between the winds in the upper troposphere and those at the ocean surface, has been very weak over the tropical North Atlantic. This allows tropical cyclones to emerge and begin their rotational movement more easily.
In addition, water temperatures on the surface of the tropical North Atlantic are currently higher than average. This allows more water to evaporate, and the higher concentration of water vapour in the atmosphere in turn fuels the severity of the storms. These two factors are therefore contributing to the creation of more and stronger cyclones.
Has the 2017 hurricane season been unusual?
As of early September (8 September 2017), there have been 11 tropical cyclones in the tropical North Atlantic in 2017. Six of those reached hurricane force, of which three became major hurricanes featuring wind speeds of at least 178 km/h. This means that there have already been roughly as many storms as the long-term average for entire years since 1950 (11.5/6.3/2.6).
There is a natural cycle of water temperatures in the North Atlantic that consists of colder and warmer phases, each of which lasts several decades. Hurricane activity is significantly greater in the warm phases, and one such warm phase began in the North Atlantic in the mid-nineties. The average frequency of tropical storms during warm phases is 14.8 storms, of which 7.6 are hurricanes and 3.5 are major hurricanes.
Some of this year’s storms themselves were unusual: Hurricane Harvey moved slowly across the ocean as a Category 4 storm, collecting enormous amounts of moisture, before making landfall in Texas on 26 August. It then stayed at almost the same place for a long period, collecting water vapour all the while. That is what shattered the rainfall records: more than 1000 litres of rain per square metre fell across the eastern coast of Texas (with more than 1300 litres in some locations), a phenomenon which, statistically, occurs roughly once in a thousand years. The result was severe flooding.
Irma has been one of the strongest hurricanes since record-taking began, and it spent the longest time with wind speeds of the highest category (5; >250 km/h), before it was downgraded one level on 8 September.
The most costly cyclone season so far was 2005, with overall losses of approximately US$170bn. The value of losses caused by this year’s storms cannot yet be estimated, but will certainly rank among the highest in recent years.
Is climate change playing a role?
Cyclone activity in the Atlantic varies greatly depending on natural climatic fluctuations. Yet ocean temperatures in the Atlantic have been increasing steadily over the last few decades due to climate change as well, and this may be driving stronger storms. On the whole, although scientists do not expect climate change to increase the total number of tropical storms in the second half of the century, a greater proportion of such storms are predicted to be extremely strong. In addition, global warming is contributing to an increase in water vapour in the atmosphere, which means that tropical cyclones will have greater rainfall potential.
However, single events or even seasons cannot themselves be directly attributed to climate change. The latter’s effects can only be measured with long-term statistics.
What needs to be done?
These recent storms are making it clear how important preventative measures are. Infrastructure, buildings, transportation routes and industrial facilities along the exposed Mexican, US and Caribbean coasts need to be fortified, and early-warning systems and evacuation plans improved, in order to mitigate future losses and save lives.
The recent natural catastrophes in the USA and the Carribean as well as in India demonstrate all too painfully the significant gap in cover for natural hazard losses. Greater insurance density is important, as it helps to alleviate the financial consequences of a catastrophe for more people.