Cyclone Pam: Untangling the complex science on tropical storms and climate change

Winds of up to 300 kilometres per hour tore through the South Pacific island nation of Vanuatu over the weekend, hitting the heavily populated capital of Port Vila on Saturday morning.

Aid agencies say Cyclone Pam could be one of the worst disasters ever to hit the region, the BBC reports. The death toll currently stands at eight and is expected to rise as rescuers reach the more remote islands.

Speaking at a disaster preparedness conference in Japan, Vanuatu’s President Baldwin Lonsdale said he thought climate change was contributing to the rise in extreme weather.

With aid finally reaching the storm-stricken nation, Carbon Brief looks at how climate change is altering how often this part of the world bears the brunt of such a destructive force.

Wind speed in Cyclone Pam at 1900 GMT on 16th March 2015. Winds of more than 200 km per hour (red) are still being recorded as the cyclone continues its path from Vanuatu down the east coast of New Zealand. Images courtesy of Cameron Beccario via earth.nullschool.net.

Storm severity

A cyclone is a tropical storm. Tropical storms are given different names depending on which ocean they form in. They are called hurricanes in the north Atlantic and northeast Pacific, typhoons in the northwest Pacific, and cyclones in the Indian Ocean.

Vanuatu frequently experiences cyclones. The cyclone season runs from December to April when the weather in the region is hot and wet. Tropical storms derive energy from the warmth of the ocean and convert it into wind strength.

While strong storms aren’t unusual for the region, Cyclone Pam was exceptional. Prof Kevin Trenberth, expert in climate change and extreme weather at the US National Centre for Atmospheric Research, tells Carbon Brief:

In the large area around Vanuatu the sea surface temperatures were one to two degrees Celsius above normal … So the atmosphere all around there has some 10 to 20% more moisture in it than a comparable storm in the 1970s would have had.

Sea surface temperature anomaly for 16th March 2015, compared to 1981-2011 daily average. Blue is lower than average, red is higher. Images courtesy of Cameron Beccario via earth.nullschool.net.

Warmer-than-average ocean temperatures in the region undoubtedly increased Cyclone Pam’s size and strength. The force exerted on buildings and structures when cyclones make landfall increases disproportionately with wind speed.

Some, but not all, of the extra warming can be pinned on human activity, Trenberth adds. Much of it is a result of a weak El Niño event in the Pacific Ocean. He says:

[A]bout 0.6 degrees Celsius can be blamed on human-induced global warming and that means over one degree is “natural” and associated with a weak El Niño.

As well as climbing temperatures creating stronger winds, rising sea levels mean that when storm surges hit, the flooding impact is likely to be higher.

A study published today suggests sea level rise could cause at least 84 to 139 extra deaths per year from cyclone-related coastal flooding in the United States by 2100, for example. That’s without taking into account any changes in population or building of flood defences.

Leaders of low-lying Pacific island nations have repeatedly called for action on climate change in recent years, reports the Washington Post.

Media attention

The BBC and The Guardian report Lonsdale’s comments that Vanuatu has seen changing weather patterns, rising seas and heavier-than-usual rain, all of which are influenced by climate change. Lonsdale says:

We see the level of sea rise… the cyclone seasons, the warm, the rain, all this is affected. This year we have more than in any year… yes, climate change is contributing to this.

The Independent, on the other hand, mistakenly takes Lonsdale’s words to mean he attributes the storm’s very existence to climate change. Today’s article says:

The “monster” cyclone that hammered a tiny South Pacific archipelago over the weekend was caused by climate change, it was claimed today.

Scientific nuance

The science on climate change and extreme weather is nuanced, and sometimes hard to navigate. While a warming climate is influencing extreme events, by making them stronger or more frequent, it can’t be said to have “caused” an event on its own.

The Economic Times of India quotes Rachel Kyte, World Bank vice president and special envoy for climate, as saying:

I don’t think I would say climate change caused (Cyclone) Pam, but I would say the fact is in the past three or four years we’ve seen category fives coming with a regularity we’ve never seen before â?¦ It is indisputable that part of the Pacific Ocean is much warmer today than in previous years, so these storms are intensifying.

On the BBC’s Today Programme this morning, Tim Palmer, professor of climate dynamics and predictability at Oxford University explained how stronger storms are what scientists and models expect with climate change. He said:

I think it is entirely consistent to say that these incredibly intense tropical cyclones that we’ve seen – not just Pam that hit Vanuatu, but Haiyan, the one that hit the Philippines in the last winter – [are]… exactly this type of extreme cyclone predicted by the climate models to increase under climate change, under global warming, so I think it is entirely consistent to say that climate change has played a role. (2:28:40)

So, while you won’t see scientists pinning the blame for a particular event on climate change alone, you will see them saying climate change has increased the odds that an event will reach destructive proportions when it does hit.

The global picture

While we might expect that a warming ocean should mean stronger storms right across the globe, assessing a global trend over the past century is difficult. Good quality records of storm activity are mainly limited to the northern hemisphere.

But that doesn’t mean there aren’t records of greater cyclone activity in some parts of the world. A recent paper by Dr Jim Kossin from the US National Oceanic and Atmospheric Administration (NOAA) and colleagues concluded:

Dramatic changes in the frequency distribution of lifetime maximum intensity (LMI) have occurred in the North Atlantic, while smaller changes are evident in the South Pacific and South Indian Oceans, and the stronger hurricanes in all of these regions have become more intense.

Hurricane lifetime maximum intensity in the North Atlantic, eastern and western North Pacific, and South Pacific Oceans (far right). Source: Kossin et al. (2014).

The study found the intensity of cyclones of all strengths in the South Pacific has increased by 2.5 metres per second per decade, with the strongest 20% increasing by as much as 5 metres per second per decade.

It’s important to draw a distinction between cyclone strength and frequency. While theoretical and model experiments show warmer seas drive more intense storms in the future, the total number isn’t expected to increase. The Intergovernmental Panel on Climate Change  says:

Based on process understanding and agreement in 21st century projections, it is likely that the global frequency of occurrence of tropical cyclones will either decrease or remain essentially unchanged, concurrent with a likely increase in both global mean tropical cyclone maximum wind speed and precipitation rates.

Most studies on tropical cyclones have come to the same conclusion, but not all. A recent study by Kerry Emanuel suggests the number of tropical cyclones worldwide could exceed 100 per year by about 2070, compared to an average of 90 per year at the moment.

When it comes to extreme weather, questions of causation never come with 100% certain answers. But while there are many factors raising a country’s vulnerability, climate change suggests  we’re likely to see tropical cyclones get stronger and more destructive in future. And that probably means more scenes like we’ve seen in Vanuatu this weekend.

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UPDATE 16th March 14:45 This post has been updated to include new  research on the impact of rising sea levels on coastal flooding, published after going to press.

Main image: Cyclone Pam nears Vanuatu.