The Great Hurricane Debate

I have talked about the past in my palaeotempestology series, and some more about how models can help us and what they are capable of representing at different scales. But what can we really say about the present day climate and recent changes, regarding tropical cyclones like the devastating Hurricane Katrina (Figure 1).

In this blog, I ask: What is the current thinking on tropical cyclones in a changing climate? 

Figure 1: Hurricane Katrina just before landfall. Source: NASA

When we talk about storms and global warming there is a lot of uncertainty. Although we cannot assign any particular storm to global warming, we can talk about a shift in the likelihood of certain types of storm. Figure 2 below, shows this concept (using temperatures). The graphs show a distribution of possible futures, with the central vertical line representing the most likely. The curves fall away on both sides represent a lower probability of occurrence and we head towards the extremes.

With climate change, what is normal now, is likely to be shifted one way or another (warmer in terms of global temperature) which will change the likelihood of our current perception of extremes. The phrase ‘new normal’ is an evocative way of emphasising the shift.  Below is an example of how different changes in probability distributions affect temperature, but any variable can be displayed in this ways, for example probability of occurrence of storms.

Figure 2: Different changes in probability distributions of temperature. Source: Kodora and Ganguly 2014.

Figure 2 is useful in explaining probability shifts on a normal (Poisson) distribution. Don't worry about the small text:

• The top graph (a) shows shifting the distribution of possible events to the left or right increasing the likelihood of the extremes (in the shallower edges of the curve) in the direction of shift.
• The middle graph (b) shows the effect of changing the maximum probability of occurrence (the average most frequently occurring conditions and its ‘fattening’ of the extremes (also known as ‘fat tails’), which represents a change in variability. 
• And the bottom graph (c) shows how changing the shape of the curve may affect average conditions but actually leaves the extremes largely untouched. 

This is how we need to think about possible climates in the future, and how distributions of storm intensity may change.

Dicing with extremes

Another way to put it would be using a dice analogy. On a ten sided die (yes, I do have a ten-sided die, as I used to play dungeons and dragons), let’s say 1 and 2 represent a category 1 hurricane and 9 and 10 are a category 5 hurricane. On a given day (obviously not real probabilities), if we assume global warming is shifting the intensity of storms to become more severe, all we are saying is that a storm in the future might be a category 5 storm if we perhaps roll an 8, 9 and 10 on that die, with a reduced chance of what we know as the category 1 storm, only occurring if we roll a 1.

So what’s the story for tropical cyclones?

There has been much debate over the years. Theoretical reasoning relies largely on the impacts of increasing temperatures affecting sea surface temperature, and extra water vapour in the air, which are a key factors in generating and develop tropical cyclones. However, the climate is not so simple. In a ‘Science: Perspectives’ piece by Kevin Trenberth in 2004, he explains how there is large variability in hurricane activity linked to ENSO, but as sea surface temperature and water vapour are increasing, they could enhance convection and therefore impact the intensity and rain-making potential of tropical cyclones. He also comments that trends in tracks and activity rates are harder to quantify.

It seems that there is much recent study on the changes in frequency, rather than the intensity, of storms. In a previous blog, I noted that climate models can represent the atmospheric conditions that encourage tropical cyclone genesis, however, intensity is more difficult due to the small scale features involved with storm development and convection that govern exactly how strong the wind becomes or the depth of pressure in the eye of the storm. This means that although we may be able to identity trends, we are unlikely to be able to quantify the changes, which limits application.

Through the Coupled Model Intercomparison Project, now on its fifth round of comparisons of the world’s biggest climate models (CMIP5), we are steadily making progress. Bellenger et al. 2014 describes how the latest models can capture modes of climate variability that influence tropical cyclone formation and evolution. The paper highlights an improvement in a previous cold-bias of sea surface temperatures in the Pacific Ocean, but generally not too much difference elsewhere, allowing the use of CMIP3 and CMIP5 models when assess ENSO. Climate models now also capture monsoon rains with high confidence (IPCC: WG1 Summary for Policy Makers). Historically, CMIP models have developed vortices that represent tropical cyclones, but they are generally too weak. (IPCC: WG1 AR4 Chapter 8).

Turning up the dial on Tropical Cyclone Intensity

The general consensus seems to be that tropical cyclones are not necessarily expected to increase in frequency, but they are likely to increase in severity. A shift in the intensity of storms towards the stronger wind speeds is a likely impact of global warming according to Holland and Bruyere (2013) as we can see from Figure 3.

Figure 3: Saffir-Simpson scale hurricane category proportion of total North Atlantic Tropical Cyclones (including Tropical Storms), changing through time (years indicated in the legend). Source: Holland and Bruyere (2013)

  

Back in 2005, Kerry Emanuel also discussed in trends tropical cyclone activity, and how their destructiveness has increased in the previous 30 years. A recent paper concurred with this finding from Estrada et al. (2015) who link US$2 to $12 billion of the losses incurred due to the busy 2005 hurricane season, to the effects of climate change. Tom Knutson (2004) also found similar results through studying the choice of climate models used to define the CO2-related warming, and the choice of parameterization schemes for convection in hurricanes. He found increases in tropical cyclone intensities linked to high CO2 environments (anthropogenically warmed simulations) in his model analysis.

Christopher Landsea, of the National Hurricane Centre in Miami, questioned many of Emanuel’s methods, in an article in Nature (2005). A debate ensued that caused a rift in the meteorological community. Landsea agreed with Will Gray in concluding that most of the variability on tropical cyclone frequency, especially intensity, is derived from natural variability, or at least that the observed data is not able to make a significant link to global warming. 

Gray preferred a theory linking hurricane intensity to the Thermohaline Circulation in the world’s oceans. An interesting article in the Wall Street Journal in 2006, highlights some of the awkward moments surrounding this passionate debate. Another more scientific angle from the guys at RealClimate.org, highlights where the different sides of the argument were formed regarding global warming’s effect on tropical cyclones.

Looking back now it seems apparent that the extra attention from two active hurricane seasons in a row, 2004 and 2005, may well have added fuel to the fire of the debate (Trenbeth 2005).

Poleward Bound?

IPCC synthesis of the past and future global changes in tropical cyclone frequency provide only low confidence (IPCC: Summary for Policy Makers), however there are regional patterns that have been elucidated.

Another interesting recent paper by James Kossin et al. 2014 found a slow poleward migration of tropical cyclone maximum lifetime intensity; a metric which is relatively insensitive to past data uncertainty. The trend is fairly small but linked to the last 30 years, and so I wonder if this is indeed another anthropogenic signal or part of natural variability. The main implication of such a poleward shift of the region affected by tropical cyclones, is that areas that have never experienced them before (and therefore are perhaps not built to withstand their destructive force), may become exposed in the future. And conversely, areas near the Equator that are currently in tropical cyclone-affected regions, may see a lower frequency of events.

Interesting stuff, and I look forward to more papers on this subject.

Latest models

Mizuta et al. (2012) showed how recent high resolution climate models (at 20 km resolution or so) have improved characterisation of intensity of tropical cyclones, at least to the extent to being able to examine distribution shifts within their own outputs, but still cannot tell us exactly how future storms will look in the year 2100. They are also able to represent variability in yearly activity rates with fairly low resolution (100 km) models (IPCC: WG1 AR5 Chapter 9). This is interesting when compared to the size of most tropical cyclones being only a few times bigger.

It’s an exciting time for climate modelling. I remember only 5 or so years ago as a forecaster that the global model resolution of the operational weather forecasting models was around 20 km. It’s amazing to think that this resolution is now being used to experiment with future climates over years and decades.

Conclusion

Recent findings echo the higher intensity theory, hence the inclusion of increases in tropical cyclone intensity in the late 21^st century described as: “More likely than not in the Western North Pacific and North Atlantic.” (IPCC: Summary for Policy Makers). The latest IPCC report also concludes that it is “virtually certain” that there have been increases in intense tropical cyclone activity in the North Atlantic since 1970, but low confidence that this is anthropogenic in origin.

An increase in intensity certainly makes sense to many in the field – more water vapour and higher sea surface temperatures in the system may not create more tropical cyclones, but may well allow them to become stronger. After all, tropical cyclones are only trying to redistribute heat to the poles, so more heat potential has to end up somewhere… right? But then, if global warming is affecting the poles more than the tropics then surely this counter acts the effect somewhat by reducing the gradient. Perhaps the global gradient has something to do with changes in frequency. This will have to be a future blog subject too!

Ultimately, most of the scientists studying tropical cyclones around the world agree that global warming is happening, and that is very likely to be anthropogenic in origin (IPCC: Summary for Policy Makers). Although some still contend that there may not be enough evidence to confidently maintain that the intensity of tropical cyclones is increasing globally, there is a strong signal to say tropical cyclones have already increased in intensity. Furthermore, there are strong hints that intensity will continue to increase in the future.

Hopefully, we’ll get to a point where the science is settled and we can get on with adapting to the consequences of our changing climate. It certainly would be better to have more study to prove the idea beyond reasonable doubt.