Did Global Warming Cause Super Typhoon Haiyan? Is Climate Change To Blame For Killer Cyclones?

 

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Climate scientists make the general point that we should expect to see more intense storms in the future because that is genuinely what they expect to see, that is what the physics says should happen and that is what their computer climate models tell them will happen. But there has been no statistically significant increase in the frequency of more intense storms on a global scale even though the world has warmed significantly since the 1950s. Click to enlarge.

In the wake the catastrophic damage to the Philippines caused by Super Typhoon Haiyan, many people have sought to link this awful tragedy to climate change.

In an emotional outburst at a United Nations climate conference in Warsaw on Monday 11 November, the head of the Philippines delegation Yeb Sano blamed the “staggering” devastation on climate change and promised to stop eating until delegates made “meaningful progress”. Many mainstream media outlets have made a connection between global warming and this monster cyclone with Britain’s Mirror among the first to sound the alarm in a headline warning that “as climate change continues we should expect more devastating storms”. While Britain’s Guardian newspaper has used the disaster to call for climate change to be put “back on the politicians’ agenda”.

However, professional climate scientists have been more circumspect about attributing Haiyan to human activity – and with good reason: the fact is that the record does not support a connection between global warming and cyclones, even though the science suggests there should be. For example, on the BBC Radio 4 Today programme on 12 November, UK Meteorological Office climate attribution expert Dr Peter Stott made the point that we should expect to see more storms such as Haiyan as the world warms but avoided specifically attributing Haiyan to climate change.

Michel Jarraud secretary-general of the World Meteorological Organisation (WMO) was equally careful in comments to  the Warsaw climate change conference. In a report issued at the conference on Wednesday 13 November, Jarraud made the point that, although individual tropical cyclones cannot be directly attributed to climate change, rising sea levels made coastal populations more vulnerable to storm surges such as that caused by Haiyan and he also warned that we could expect to see more intense cyclones in the future as a result of global warming. But he did not blame Super Typhoon Haiyan on climate change.

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No obvious connection. Click to enlarge.

As the WMO pointed out in a major report published in July: “Owing to the naturally high internal variability of the climate system, however, it is still difficult to assess in a systematic way the degree and amount of climate-change influence on a single observed event,” it said.

This means that it is impossible for climate scientists to say that any one typhoon, however awful, has been caused by climate change. This is mainly due to the difficulty of what is called attribution – the ability to link the impact of climate change with specific extreme weather events.  Speaking to the Guardian, Oxford University climate expert Professor Myles Allen called for more resources to be spent on attribution studies that could calculate the impact that climate change has on cyclones. Allen also said that “there are physical arguments and evidence that there is a risk of more intense hurricanes”. And he is right, there are good arguments for expecting more intense hurricanes as the world gets warmer.

Climate scientists make the general point that we should expect to see more intense storms in the future because that is genuinely what they expect to see, that is what the physics says should happen and that is what their computer climate models tell them will happen. Research in Nature Geoscience and in the PNAS science journal as well as the research overview presented in the latest report from the Intergovernmental Panel on Climate Science (IPCC) all suggest that we should expect more intense cyclones. And this would make sense.  Simple physics suggests that as the world gets warmer then there will be more energy available for such violent storms; but with cyclones, as with so much else in climate science, the physics is far from simple.

There is no doubt that world is warmer now than it was in the middle of the 20th century and there should be every reason to expect this warmth to feed through into cyclone activity and yet there is also no doubt that tropical cyclone activity is declining notwithstanding Haiyan. Indeed, we have just had the warmest decade on record and yet global cyclone activity has been at an historic low, there is no statistically significant evidence of an increase in the intensity of cyclones and total cyclone energy in the atmosphere is declining.

Historically low cyclone activity

October was a particularly stormy month in the western pacific with seven typhoons but data shows that globally 2013 has been quiet in terms of the number of tropical cyclones and the total amount of cyclone energy expended in the atmosphere even though the WMO forecasts that this will be the seventh warmest year on record.

According to the WMO report released in Warsaw, as of early November 2013, there had been a total of 86 tropical cyclones worldwide in the year to date compared with an average of 89 storms over the period from 1981–2010.

The Western North Pacific typhoon season recorded 30 storms, including 13 typhoons, which is above the 1981−2010 average of 26. The Eastern North Pacific basin also had above-average hurricane activity in 2013. There were a total of 17 storms, eight of which intensified to hurricane status. But the North Indian Ocean had a below-average season with only two tropical cyclones compared with the 1981−2010 average of four. And in the North Atlantic, with the season officially ending on 30 November, there have been a total of 12 named storms, compared to the 1981‒2010 seasonal average of 12 storms.

Data also shows that: the total accumulated cyclone energy in the atmosphere in 2013 is running below the long term average; the number of hurricane-force tropical cyclones, where the maximum lifetime wind of the storm speed exceeds 64 knots, in the last year is below the long term average; the number of major hurricane strength tropical storms, where the maximum lifetime wind speed of the storm exceeds 96 knots, in the last year is below the long term average.

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Cyclone activity in 2013 is below the long term average, despite Haiyan. Click to enlarge.

Furthermore, the frequency of both hurricane-force tropical cyclones and major hurricane strength tropical cyclones has been in decline since the mid-1990s. Also, the global total accumulated cyclone energy in the atmosphere has been in a bumpy decline since the mid-1990s.

This decline in cyclone frequency and in the annual total accumulated cyclone energy that has taken place since the mid-1990s is at odds with what simple physics suggests and with popular perceptions about global warming and climate change. And, apparently, at odds with the predictions of climate scientists that global warming should lead to more intense hurricanes.

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Global cyclone energy is in decline despite global warming. Click to enlarge.

The fact is that neither the IPCC nor the WMO have been able to find convincing evidence of a connection between the significant increase in global mean surface temperatures that took place during the second half of the 20th century and changes in the intensity and frequency of cyclones – and these are two organisations that would have publicised such a link had they discovered it. Indeed, the IPCC says in its latest report into the science of climate change published in draft form in September that there is “low confidence” that changes in tropical cyclone activity can be attributed to human influence.

Most powerful storm to make landfall

Super Typhoon Haiyan is the most powerful storm to make landfall since Typhoon Tip in 1979 which was at that time the largest and most powerful storm ever recorded; indeed, it is quite possible that Haiyan may prove to be even more powerful than Tip. But that in itself is not proof that global warming is responsible.

Such storms are extremely rare but they are not outside of the range of what climate scientists call natural variability and the fact that there is 34 years between Tip and Haiyan. It would require a statistically significant increase in power of storms to enable climate scientists to even infer a connection with climate change. At the moment, no such evidence exists as the IPCC acknowledges in its science report: “Current datasets indicate no significant observed trends in global tropical cyclone frequency over the past century and it remains uncertain whether any reported long-term increases in tropical cyclone frequency are robust, after accounting for past changes

The IPCC warned in its report that we are “likely” to see an increase in the global mean tropical cyclone maximum wind speed – that is, the IPCC expects that storms will get more powerful. But we have not seen that show up in the global cyclone data so far despite significant warming in the second half of the 20th century that should, on the basis of the physics, drive storm intensity.

The IPCC is “virtually certain” that there has been an increase in the intensity of the strongest storms in the Atlantic and there is some evidence of an increase in the western North Pacific but it does not attribute this to climate change. And global data does not show that similar increases are happening elsewhere.

So the facts are that tropical cyclone activity is below average in 2013 and has been in long term decline and there is no statistically significant increase in the frequency of more intense storms on a global scale even though the world has warmed significantly since the 1950s. Haiyan was simply a very powerful cyclone.

This counter-intuitive phenomenon of a warming world with decreasing cyclone activity is yet more evidence that the climate system is more complicated than many, particularly in the media, suppose.

Headlines are easy. Climate science is harder.

Note

This is an updated version of a post published on 10 November.

Sources

You can see the data on cyclone activity in 2013 and some historical data assembled by Dr Ryan Maue on the Weather Bell website here

Read the WMO report “The Global Climate 2001-2010: A Decade Of Extremes” here

Read the draft IPCC AR5 physical science report “Climate Change 2013: The Physical Science Basis” here

Recent historically low global tropical cyclone activity by Ryan N. Maue, Article first published online: 20 JUL 2011, DOI: 10.1029/2011GL047711 here.

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