This is a continuation of a previous blog post, A hurricane season of record-breaking storms – Part 1: Historical context
With every large storm, there are always questions concerning climate change. Can storms such as those we’ve seen in recent weeks be attributed to climate change? This is a difficult question as there are uncertainties to consider, but to a degree, yes, some storms or storm characteristics can indeed be related to climate change. Scientific research into the attribution of hurricane events to climate change provides a range of approaches, using statistical means and expressing changes in recurrence intervals (aka return periods) of different storm intensities.
There are limited conclusions that can be made from statistical analysis of observed data, due to the rare nature of extreme events and the relatively short record. An alternative approach provides a compelling argument based on experiments using high-resolution climate models. These models provide insights into tropical cyclone behaviour based on environments with and without elevated levels of greenhouse gases. The differences provide suggestions that some aspects of tropical cyclones are indeed worse due to global warming.
The rationale for this conclusion is largely thermodynamic, and hints at greater potential for high intensity storms over a wider area. This basically takes into account higher sea surface temperatures and atmospheric moisture in a future warmed climate providing the ingredients for hurricane development more readily.
There is also the influence of aerosols to consider over the observed records. Certain types of manmade aerosols have been shown to suppress hurricane activity, but clean air acts in the 1970’s and 1980’s have reduced the aerosols content in the air, which model studies show can lead to a rise in tropical storm frequency. Natural climate variability, such as ENSO, further clouds trends in observed and modelled future hurricanes, as well as an overall poleward shift of the location of peak storm intensity.
Changes in atmospheric circulation are also important, but are more difficult to properly represent with large-scale general circulation models used to investigate climate change. For example, increasing wind shear may lead to more inhibition, and therefore hurricanes may never reach their thermodynamic potential. Higher wind shear (e.g., larger differences between wind speeds aloft and at the surface) means more chance for hurricanes to get ‘knocked over,’ like a spinning top on a sloping surface. There’s also uncertainty about how atmospheric circulations will change: Research suggests increasing frequency of El-Niño-like conditions (i.e. more wind shear in the Atlantic and fewer hurricanes), although this is far from a foregone conclusion.
Despite the uncertainty, there is general consensus that present-day hurricanes may not necessarily be more frequent, but they are thought to be more intense when they do occur. This message has been consistent from the scientific community for nearly two decades, and it may be that it’s hitting home, too. A recent poll suggests that the majority of Americans now acknowledge that climate change makes hurricanes more intense. In short, it’s complicated, but there are important links that can be highlighted.
Recent storms and climate change
A single season is not adequate to identify long-term trends, but the characteristics of the storms we have seen recently fit with the rationale supporting more intense storms with climate change. Harvey, for example, was an extraordinary rain-bearer, and potentially represents a glimpse into the future. In a warmer climate, storms may be able to carry greater amounts of moisture, which would convert into rainfall as the storms move northwards and/or make landfall.
Rapid intensification or “explosive development” occurs when storms find all the conditions they need to thrive, and is defined as a drop in pressure of greater than 24mb in 24 hours. It’s not 100% clear as to how this particular phenomenon will change in the future, but recent research suggests it may become more common as hurricanes approach land in a warmer climate. These recent hurricanes have all exhibited rapid intensification, which creates further potential for damage and harm as it’s notoriously difficult to forecast and therefore reduces time for evacuation and preparedness. And it may drive enhanced losses as those in the path of exploding storms fail to receive warnings with enough time to brace for landfall or leave the area.
The high number of storms reaching major hurricane status (Category 3 or above) this year is also noteworthy and may be an example of the kind of season we are more likely to see under future warming scenarios. However, there is a long way to go to fully understand what a future hurricane climatology may look like, just as climate models are becoming more complex and sophisticated, the short length of the record is being addressed by the field of palaeotempestology (the subject of a future blog).
As part of ongoing efforts to evaluate, validate and build industry models that can help in the assessment of losses from storms such as the recent hurricanes, the Willis Research Network collaborates with a wide range of leading scientists around the world to provide new insights, data and applications, to be used for Willis Towers Watson’s client services. Willis Research Network experts from Columbia University, the National Center for Atmospheric Research, and Wharton Risk Management and Decision Processes Center will present some of their recent findings at the WRN Autumn Seminar (for more information check the WRN events page). To register to attend this event please email firstname.lastname@example.org.