Climate Change and the conditions for increased hurricane and typhoon activity

After last week's devastating typhoon, I'm going to have a look at the connection between global warming and increased tropical cyclone activity (both hurricanes in the Atlantic, and typhoons in the Pacific). From a logical standpoint, it makes sense that if there is more energy in the ocean and atmosphere systems in the form of higher temperatures, then the available energy for these super typhoons will be higher. So you would expect that there would be both more frequent and stronger typhoons. Following an article in The Guardian on Typhoon Haiyan and its links to climate change, I'd like to look at some of the evidence for this linkage.

Tropical cyclones and climate change

When looking at tropical cyclones in relation to climate change, it is normal to look at both their intensity and their frequency. This IPCC SREX summary report states that there is low confidence in any increases in frequency or intensity of these events:

There is low confidence in any observed long-term (i.e., 40 years or more) increases in tropical cyclone activity (i.e., intensity, frequency, duration), after accounting for past changes in observing capabilities.

However, in a recent paper: Downscaling CMIP5 climate models shows increased
tropical cyclone activity over the 21st century  (Emanuel, 2013), a case is made that both the intensity and the frequency of tropical cyclones could increase over the next few decades. This paper uses 6 models from the CMIP5 ensemble of General Circulation Models (GCMs), and downscales them (embeds a model of finer resolution into the GCM) so as to be able to better resolve the storms themselves. From the paper:

One distinct advantage of our downscaling technique is that it captures the full spectrum of storm intensity

From this, they conclude that running the model at a higher resolution provides a better estimate for the overall storm intensity. The results that they found, namely that intensity and frequency will increase by 8-80% and 11-41% respectively, are interesting because they are at odds with a similar study carried out on an early generation and current generation models - CMIP3 and CMIP5 - in this paper: Dynamical Downscaling Projections of Twenty-First-Century Atlantic Hurricane Activity: CMIP3 and CMIP5 Model-Based Scenarios (Knutson et al., 2013). In this paper, the CMIP3 models showed that the intensity of storms would increase by 87%, but that the frequency of tropical cyclones would decrease by 27%. The more recent CMIP5 models found similar results: storms would increase by 39-45% and frequency would decrease by 20-23%.

In an earlier review article: tropical cyclones and climate change (Knutson et al, 2010), both current (in 2010) generation models run at normal resolution, and higher resolution models are considered. It finds that high-resolution models project that storms will become stronger by 2-11% by 2100, which is in qualitative agreement with the above papers, but the magnitude of the change is much smaller. It was written before both of the above papers, which could explain the discrepancy. The article goes on to say that for the current generation of models, storm frequency is projected to decrease by 6-34%. Whereas higher resolution models "project substantial increases in the frequency of the most intense cyclones". The article points out that the use of downscaling can have a noticeable effect on the results in terms of frequency of stronger storms:

There is a clear tendency among the models, particularly at higher resolution (60-km grid spacing or less), to project an increase in the frequency of the stronger tropical cyclones

Conclusions

There seems to be a fairly broad consensus that the intensity of the largest storms will increase over the next century. However, there seems to be considerable variation in the projections for frequency between different models on one hand, and different studies on the other. The role of downscaling appears to be important to the outcomes of these studies, with models run at a higher resolution tending to project higher frequencies of storms.

Typhoon Haiyan

I woke up this morning to news of Typhoon Haiyan and its aftermath. It made for sobering listening, and hearing first hand accounts of the destruction drove home quite how massive and powerful this typhoon was, along with the effect it has had on the Philippines. The death toll has reached almost 1000 people with over 10,000 feared dead, as well as many hundreds of thousands now finding themselves without shelter, let alone food, water or electricity. The storm surge (which now seems like a far more malevolent title for this blog than when I picked it) appears to have reached over 5 metres, which is comparable to the water heights seen in the 2011 Japanese Tsunami. This "super typhoon" was a category 5 cyclone with a maximum wind speed of gusts of 235mph, and was strong enough to deposit large ships hundreds of metres inland:

It is possibly the largest typhoon to make landfall, and it's the 13th typhoon to hit the Philippines this year. The government there has set aside almost $500 billion for the rehabilitation efforts.

Possible attribution of Typhoon Haiyan to climate change?

 

Linking individual events to climate change is extremely hard. It is also a contentious issue that can bring out heated discussion. However, with climate talks starting in Warsaw at the moment, and the envoy for the Philippines vowing to fast until meaningful progress has been reached, I think it is important that these questions are asked. The same envoy, Naderev Sano, issued an emotional appeal at last year's climate talks in Doha. His words seem remarkably prescient now:



In the next few posts I will look at these questions, and see whether it makes sense to link a particular event to climate change,  whether the frequency or intensity of these events might increase due to climate change and if their effects will be more pronounced in the future.

Finally, this extreme weather event is above anything else a humanitarian crisis. Many people have lost their lives and many more will need aid in the next few months. There are a number of charities who are collecting specifically for Typhoon Haiyan. You can donate using one of the links below:

• Oxfam 
• Unicef 
• Save the Children