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.

Palaeotempestology: Lake sediment records

Digging in to sediment records

With continued debate among scientists on exactly how future climate change will affect storm frequency and severity, it seems logical to see if we can find out more about variability in storm activity from the past.

Lake sediments are extremely useful in studying past climates, for which we have no observational record (through conventional weather recording equipment). They provide a slice through time to look at the changes in lake chemistry and environmental activity affecting the make up of suspended particles in the lake that eventually settle at the bottom.

Radiocarbon dating, thickness of layers of different sediments, analysis of diatoms and inference from the occasional break in the record (a hiatus, perhaps due to the drying out of a lake), are various ways in which lake sediments can give us clues about the past.

Within this range of different approaches there are a few ways in which sediments from lakes can be used to look at past storm events. In my previous blog, I highlighted a paper by Dr Jeff Donnelly et al. in 2015 entitled “Climate forcing of unprecedented intense-hurricane activity in the last 2000 years”. It presents a history of storm events over the past two thousand years, using an analysis of sediment grain size in their collected samples, with a resolution of around 1 year. The work uses evidence gathered from field work during the project (and previous studies) to determine the presence of two distinct periods of higher activity in severe hurricanes for the west North Atlantic coastline of North America: one between 1400 and 1675 C.E.; and another period of high frequency storms further back in time between 250 and 1150 C.E.

The study location is a place called Salt Pond, in Massachusetts. It has a tidal inlet linking it to the ocean, making it full of brackish waters. This proximity to the ocean means that the pond is exposed to ‘overwash’ during storm surge events associated with large storms heading northwards along the Eastern seaboard of the United States. These salt water incursions occur when the storm surge level is higher than any natural or man-made defences. This ‘overwash’ leads to ‘coarse grain event beds’, and so these can be used as an indicator of severe storm activity. This process is vaidated using known hurricanes landfalls, which are represented in the sediment records and act as ‘anchors’ to verify that the samples are valid.

The study builds on a number of papers that were produced after the convening of a workshop on Altlantic palaeohurricane reconstructions in 2001 at the University of South Carolina. The workshop aimed to identify new opportunities in the field of palaeotempestology. A summary of the workshop can be found here. Dr Jeff Donnelly and colleagues studied a number of lakes in the Northeast of the US, in the states of New Jersey and New England, and so to learn a bit about the methodology, I dug into some of the papers in some more depth.

Getting your hands dirty

It seems the only way to get at clues available from sediment records is to get your hands dirty. I found an earlier paper by Donnelly at al. from 2001 which built a 700 hundred year sediment record of severe storms in New England. This paper (and a couple more in Boldt et al. 2010, Liuand Fearn, 2000) started to show me that each project strategy is subtly different. 

Various schemes are planned based on the conditions of the study sites, to find the best locations for sampling overwash areas in a consistent manner. The aim is to try to consistently capture the process by which more intense storms erode more sand from the coastal beach and bring this coarse sediment into the brackish lakes and ponds, larger storms being assumed to produce wider fans of overwash sand deposits, being thicker near the shore and thinner near the centre of the study lake. A range of
samples should be taken to try to represent the range of possible characteristics of past intense storms. Figure 1 (below) is a hypothetical diagram from Liu and Fearn (2000) to show various patterns of deposition. Note the radial patterns associated with the various directions of storm approach, with the larger fans associated with more intense storms.

Figure 1: Hypothetical coarse grain deposition fans in severe storm surge events. Source: Liu and Fearn, 2000 

The coarse sand creates a layer over the more usual organic-based deposits that settle on the bottom of a lake as a stratified layer. This happens most effectively in anoxic lake beds (lacking dissolved oxygen) since any mixing from plant of animal life will be minimal.

Having never been in the field to collect sediment samples, I found it interesting to see how Donnelly et al. (and other teams) maintained a consistent chronology in the sediment records. They took multiple samples and use the variety of methods above to build their chronology.

Markers in time

Isotopic radio carbon dating and stratigraphic markers used to mark certain control points to validate the data. Pollution horizons are useful in this respect, for example lead concentrations mark the beginning of the industrial revolution as it quickly made it's way into the water systems and lakes and then 'fixed' by anoxic sediments. The presence of lead pollution is an indicator of the late 1800's (Donnelly et al. 2001) and then another change occurs when lead was removed from gasoline in the 1970's and 1980's. This is a good example as it shows how these markers are useful for calibrating sediment records, in a way that is easily understood and recognised.

Pollen records can also mark certain points in history, for example the European colonisation of the eastern U.S. led to large scale clearance of the vegetation for farmland meaning that the pollen composition changes drastically (Russell et al. 1993).

Once these markers are established, previous storms are used to calibrate storm events, and then previous coarse grain even layers are identified and carbon dated.

Clear as mud?

So having learned a lot more about sediment analysis in relation to palaeotempestology, I now have a greater respect for what these cores of old mud and sand can tell us about the past. However, it does seem to me that there is still a large degree of uncertainty in the data when trying to discern an idea about individual storms. For example, what if two storm occur in quick succession as a cluster, before a sediment layer has had a chance to settle and ‘lock in’ the information? This may end up looking look like one larger or more intense storm, when actually it is the frequency of storms in that season which is varing. Donnelly et al. 2001 give an example from their study location of a lack of agreement between historical accounts of two intense storms in 1635 and 1638 which likely created overwash signatures, but in the sediment proxy data, only one event was indicated. This means that the estimated frequencies may have significant uncertainty.

Also, responses of lake or pond to overwash events may change over time due to changes in natural or man-made barriers. However, even with these uncertainties in mind, it is still clear that there is great value in understanding the past clues left behind by storms in our coastal lake sediments. 

Without any alternative information, the best that we can do is to piece together palaeotempestological proxies and glean snippets of information to build a longer record of storms.

It also provides grounds for comparison in using climate models to try to understand past variability,
another subject I intend to explore in a future blog.

For now, I’ll leave you with an informational video by Ocean Today in conjunction with the Smithsonian Institution and NOAA, just after Hurricane Sandy in 2012 which will hopefully make a clear demonstration of what overwash looks like and how the coastal beach material can be dragged in across to end up in lakes or ponds that lay close to the ocean to give us these markers of past events.

My next blog will be on the evidence that can be derived from coral cores.

The calm *blog* before the storm...

The standard advice about starting any writing endeavour is to write about what you know. Well, I’m here to use this blog to learn more about the complex issue of climate change, so that leaves me somewhat limited, but since no journey starts without a first step my starting point will be storms.

Now, I’m not talking about your everyday ‘it’s a bit blowy outside’ kind of storms, but instead I mean the big ones, the rare beasts that wreak havoc to those in their path, with impacts that spread way beyond those who are unlucky enough to be directly in the firing line.

In a changing climate, extreme storms are expected to also change in terms of their frequency and severity, but there is much uncertainty. It’s not just the storms themselves that are likely to change. In the future our exposure and vulnerability to their impacts will also evolve. How these factors interlink, and exactly how extreme storms are likely to change, are subjects of much debate and a key question in the field of climate change science. A warmer world is expected to change how often we see storms like Hurricanes Katrina which ravaged an ill-prepared New Orleans, or Hurricane Sandy which flooded much of the east coast of the U.S. including New York  (comparison before and after images on this link - worth a look). Then there are clusters of windstorms like Lothar and Martin that hit Europe just after Christmas in 1999 causing billions of Euros in insured losses, let alone the amount of uninsured damage, which may also change in frequency in the future (the subject of a future blog I'm sure). Storms like Super Typhoon Haiyan caused immeasurable suffering and loss in the Philippines in 2013. Tragic loss of life reached into the thousands, and the huge amount of damage was largely uninsured with government responses barely able to help those affected and international aid being a large source of the relief. This highlights the global differences in vulnerability to these extreme events. However, in the context of climate change, the often-asked questions are: “Can ‘storm x’ be attributed to climate change?” or “Are we likely to see more storms similar to ‘storm x’?” Not easy questions to address, but so in this blog I’ll be searching for the answers that are out there (if any), and look at various sides of the science in a quest to respond to these very reasonable questions.

Hurricane Katrina on its way towards New Orleans. Source: NASA

With those sobering thoughts in mind, I intend to use this blog as an exploration into what we know about tropical cyclones and their relationships to climate in the past, present and future. From palaeotempestology looking back thousands of years, to high resolution climate modelling out to 2100, I’ll be bringing together interesting facts, reviewing papers, and hopefully bringing out some entertaining and engaging moments along the way.

Perhaps just a quick word about me before we get going, for the sake of good manners. I’ve just started a part-time Masters degree in Climate Change (hence this blog), but also work full time coordinating and leading research projects for a large insurance/reinsurance broker. My background is in meteorology having worked for over a decade as a weather forecaster in the UK and in Bermuda (I wanted to forecast hurricanes).

I also have an interest in the communication of science having worked in the media side of forecasting at the start of my career, and studied at art college before moving on to a BSc in Environmental Science at the turn of the millennium. So basically, I’m a self-confessed weather geek, who likes paintings and films. I've contributed to my company's blog a few times, but never run my own site, so I'll hopefully build some good content on this page as I go along.

I'm always happy to receive comments about the weather and climate, especially severe storms, so feel free to reply on any posts and I'll respond as soon as I can.