Sep 21, 2017

Hurricanes: A perfect storm of chance and climate change?

Puerto Rico
Matt McGrath
BBC News
The swirling edges of Hurricane Maria seen from space as it slammed into Puerto Rico. Image: NASA
The swirling edges of Hurricane Maria seen from space as it slammed into Puerto Rico. Image: NASA

The succession of intense and deadly tropical cyclones that have barrelled across the Atlantic in recent weeks have left many people wondering if a threshold of some sort has been crossed. Is this chain of hurricanes evidence of some significant new frontier in our changing climate?

The answer is mostly no, but with worrying undertones of yes.

The first thing to note about this season is that it shows the power of science and weather forecasting.

Every year, the US National Oceanic and Atmospheric Administration (Noaa) puts out a hurricane forecast for the season that runs from 1 June to 30 November for the north Atlantic, Caribbean and the Gulf of Mexico.

In August, Noaa updated its predictions, stating that there would be 14-19 named storms and of these, 5-9 would become hurricanes.

To date, we've had seven cyclones, with four that have gained category three status or stronger. So this season is unusual but not unprecedented. The bigger picture shows that between 1981 and 2010 the average was six hurricanes per season.

What has happened this year is that a number of natural variable factors have come together and helped boost the number and power of these cyclones. In the background, climate change has loaded the dice.


Most researchers who study extreme events like hurricanes agree that climate change is most likely making the impacts of these events much worse.

Rising temperatures lead to warmer air holding more moisture, which causes more intense downpours in a hurricane. The oceans have risen thanks to thermal expansion and glacier melt and this works to increase the dangers posed by storm surges.

"In terms of the factors that control the genesis and the intensification of these hurricanes, a number of these point to the fact that they will undoubtedly be slightly more severe due to the extra heat content in the ocean due to the long-term warming of the climate," said Richard Allan.

Kerry Emanuel strongly agrees.

"The warming of the climate has increased the underlying probabilities of very heavy rain events like happened in Harvey and very high category hurricanes like Irma.

"It is just not sensible to say either storm was caused by climate change, but the underlying probabilities are going up."