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Roz Pidcock

19.11.2013 | 2:45pm
ScienceQ & A: Untangling the science on climate change and tropical storms
SCIENCE | November 19. 2013. 14:45
Q & A: Untangling the science on climate change and tropical storms

As aid finally reaches the stricken Philippines, the question of whether or not typhoon Haiyan can be linked to climate change has received more coverage this weekend.

With some pretty nuanced conclusions in the scientific literature and some selective deployment of those conclusions by parts of the media, it’s become a tricky issue to navigate.

Here’s a guide to some of the questions about climate change and tropical storms, and the sometimes quite complicated answers.

Was Haiyan a tropical storm, a typhoon or a cyclone?

All of the above. Tropical storm events 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.

                           Cyclone Winds3 

With winds of nearly 315 kilometres per hour when it hit the Philippines, typhoon Haiyan is the strongest tropical storm on record to hit landfall. Source: Weather Underground

Does the IPCC say there’s no link between tropical storms and climate change?

No, it doesn’t.

Scientific understanding of tropical storms is that they derive energy from the warmth of the ocean and convert it into wind strength. Some of the heat trapped by greenhouse gases in the atmosphere is entering the oceans, causing them to warm.

The IPCC says in its most recent report [Chp 14, Section 14.6.3]:

“Shorter term increases [in tropical cyclone activity] such as those observed in the Atlantic over the past 30-40 years appear to be robust and have been hypothesised to be related, in part, to regional external forcing by greenhouse gasses and aerosols, but the more steady century-scale trends that may be expected from carbon dioxide forcing alone are much more difficult to assess given the data uncertainty in the available tropical cyclone records.”

In other words, while we might expect a warming ocean should mean stronger storms, assessing a global trend over the past century is difficult. Since about 1970 when the satellite record began, good quality records of storm activity are pretty much limited to the northern hemisphere.

As Jeff Masters of the Weather Underground blog explains to PBS news, natural variability is another complicating factor. He said:

“We don’t have good enough observations and they don’t go back in time far enough â?¦ these storms have a lot of natural ups and downs and we really haven’t been observing them very well to be able to tell if they’ve been changing or not.”

This is a nuanced conclusion, and you’d be forgiven for getting the wrong idea from some of the discussion in the media.

For example, on Thursday night, BBC’s Question Time featured Lord Nigel Lawson, head of climate skeptic thinktank the Global Warming Policy Foundation, telling the audience:

“The IPCC says there’s absolutely no link between climate change and tropical storms”

Lawson appears to be confused – we can’t find anything the IPCC has said which could be fairly paraphrased in this way. His decision to cite the IPCC might also seem strange, given that he recently described it as “a deeply discredited organisation”.

Climate skeptic journalist David Rose made a similar argument in last week’s Mail on Sunday, saying:

“The UN IPCC … does not agree tropical storms have become more intense or more frequent, but says the opposite”.

So, has tropical storm activity changed at all?

Yes. The IPCC notes some regional changes since the 1970s of a measurement called ‘power dissipation’, which combines of storm frequency, intensity and duration.

The latest IPCC report says evidence shows an upward trend is “virtually certain” in the North Atlantic. While trends are generally less clear in other oceans, the western North Pacific – the region where typhoon Haiyan hit – is also mentioned.

The report says [Chp 2, section 2.6.3]:

“Time series of cyclone indices such as power dissipation … that measures total wind energy by tropical cyclones, show upward trends in the North Atlantic and weaker upward trends in the western North Pacific since the late 1970s.”

A study by Kerry Emanuel in 2007 calculated a 100 per cent increase in power dissipation in the North Atlantic since the 1970s in response to rising sea surface temperature, and a 35 per cent increase in the western tropical Pacific.

                 USAID-DCHAPhilippines Typhoon Yolanda -Haiyan _Map

Fact sheet showing the path of Haiyan, wind speeds and the number of people affected. Source: USAID

Will we get an increasing number of tropical storms in the future?

Not necessarily. We could see fewer.

Kevin Trenberth, hurricane expert with the US National Centre for Atmospheric Research, tells us scientists expect there will be an increase in activity in future. But activity can mean several things: more storms, bigger storms, more intense storms, or longer lasting storms.

Theoretical and modelling experiments show warmer seas drive more intense storms, but it’s possible that we might not see as many overall, he says:

“[S]tudies show that with higher sea surface temperatures, the tropical storms are more intense and bigger. But such storms leave a stronger cold wake behind, making the environment less favorable for the next storm.”

On balance, the IPCC says it’s likely (at least 66 per cent chance) that the total number of storms worldwide will either decrease or remain essentially unchanged.

It’s more difficult to predict what might happen in specific oceans, the report notes. Trenberth explains:

“The reason is that more activity in one basin usually has an adverse effect in other basins owing to large-scale changes in the atmospheric winds.  For example, more activity in a warmer Pacific during El Niño years, means less in the Atlantic.”

Will future tropical storms be more damaging?

Yes, probably.

When talking about future trends, the distinction between the frequency and intensity of tropical storms is important.

While the IPCC doesn’t predict an increase in total number of tropical storms worldwide, it does say climate change is likely to make those that do occur more intense. The latest IPCC report says [Chp 14 page 4]:

“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.”

In other words, the science suggests warmer waters will affect tropical storm activity in the future by making storms stronger and rainfall heavier, though overall numbers may not change.

Climate change secretary Ed Davey, also appearing on last week’s Question Time, made this distinction between frequency and storm strength but seemed to get confused over past and future trends in tropical storms.

He said:

“I think it’s actually right that there is no evidence that climate change is increasing the frequency of tropical storms. What there is evidence of is that it is increasing the impact of the intensity of those storms.”

To be pedantic, it’s not clear what “increasing the impact of the intensity” means here. Generally though, this sounds like a statement about past global trends. In which case, Davey isn’t justified in saying there’s evidence the intensity of storms is increasing globally – the IPCC says there isn’t evidence to support this statement because of limited data.

Had Davey been talking about global trends expected in the future, or past regional trends in the North Atlantic or the western North Pacific, his comments would have been more accurate.

Video showing the damage caused by the storm surge as typhoon Haiyan made landfall in the Philippines.  Source:  Reuters

As this Telegraph piece from Geoffrey Lean explains, there’s another reason tropical storms might be increasingly damaging in the Philippines and surrounding areas. Lean says:

“There is, though, one far less discussed factor that appears to have aggravated Haiyan’s impact: rising sea levels do seem to have swelled the storm surge that caused most of the deaths.”

Sea levels have risen three times faster in the western North Pacific than the global average of about three millimetres per year, partly because of changes in ocean circulation and partly because water expands as it warms.

Sea level rise has pushed storm surge up by about 18 cm since accurate measurements began in the early 1990s, increasing the risk of damaging storm surges when storms make landfall. This is a small, but not insignificant, proportion of the five metre storm surge that hit the Philippines – and continuing sea level rise this century will see that contribution increase.

Storm _surge

An illustration of how the rise in sea level caused by a storm – the storm surge – relates to normal sea level and high tides. Rising sea levels due to climate change push the storm surge higher. Source: NOAA

And local human impacts can worsen the impact of these kind of disasters. The piece makes some other important points, that too much groundwater extraction is causing parts of the country to sink, and mangroves that once provided protection against storms are being decimated.

So was Haiyan down to climate change or not?

When it comes to extreme weather, questions of causation can never be answered with 100 per cent certainty. But scientists point out the environment all these storm events are occurring in is changing due to human activities.

It’s clear there are many factors raising the Philippines’ vulnerability to this sort of extreme weather event – and according to the latest science, climate change is there in the mix.

Nevertheless, it’s impossible to say whether this level of destruction could have been avoided in a world that wasn’t warming. Instead, scientists talk about climate change as increasing the odds of storms reaching Haiyan-scale proportions in the future.

As Prime Minister David Cameron told an audience in Sri Lanka’s capital Colombo, what’s needed now is action to reduce nations’ vulnerability. The Times reports Cameron as saying:

“I’ll leave the scientists to speak for themselves about the link between severe weather events and climate change. The evidence seems to me to be growing. As a practical politician, i think the sensible thing is to say: let’s take preventive and mitigating steps”.


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