With the continuing population increase in coastal areas, predicting fluctuations in tropical hurricane activity is of obvious importance to society and scientists alike. Concerns about the possible effects of global warming on tropical hurricane activity has motivated a number of theoretical, modelling and empirical studies, in the hopes that understanding the resulting trends will allow for the projection of how tropical storms will differ under various scenarios of climate change. Understanding how hurricane intensity is impacted allows for the potential mitigation, or at least preparation, for future natural disasters.
Following predictions about transient greenhouse gas changes made in the 1980s, global surface temperature has increased by ?0.2°C per decade (Hansen et al, 2006). Similarly, sea surface temperature has followed an upward trend over the last 50 years, with increases on average of 0.5°c between 1970 and 2004 (Elsner and Kocher, 2004). With current predictions seeing a continued increase in both surface and sea temperature, is it important we understand how these changes may impact tropical hurricane formation.
Our basic understanding of how tropical hurricanes form suggests that there could be a relationship between hurricane activity and rising sea surface temperatures (SST). It is widely acknowledged that SST >36°c is a requirement for tropical hurricane formation (Gray, 1968) and as SSTs are set to rise further, some tropical ocean basins may face an increasing number of more intense tropical hurricanes (Chu and Clark, 1999). This is because the higher SST and resulting higher specific humidity can contribute more energy during the storms formation, allowing for more destructive storms (Webster et al, 2005).
There is some evidence that tropical hurricanes have already been impacted (Elsner et al, 2008; Kossin et al, 2014) and common consensus has been that future projections for climate change indicate that anthropogenic warming will at least cause globally averaged intensity of tropical hurricanes to increase (Knutson et al, 2010), however there are also some who believe the links between SST and hurricane intensity are weak and explained by other factors.
Whether the characteristics of tropical hurricanes have changed, or will change in a warming climate, has been the subject of considerable investigation, often with conflicting results (Knutson et al, 2010). This difference in findings stems from the fact that trend detection is impeded by substantial limitations in the availability and quality of tropical hurricane global historical records (Knutson et al, 2010). The most comprehensive datasets were not primarily intended as datasets to be used for trend analysis and despite using the best techniques available (Walsh et al, 2015) the density of reporting ship traffic was relatively sparse during the early decades of the records, such that if storms from the modern era (post 1965) had hypothetically occurred during those earlier decades, a substantial number would not have been directly observed by the ship-based ‘observing network of opportunity’ (GFDL, 2017). After adjusting models for the missing storms, one study (Vecchi et al, 2008) found that the rising trend in tropical hurricane storm count and intensity was no longer statistically significant and only a small nominally positive upward trend from 1878-2006 was observed (GFDL, 2017). Similarly, a global analysis of tropical hurricane intensity trends from 1981-2006 found increases in the intensities of the strongest hurricanes (Knutson et al, 2010), however these results are again impeded by the short time period of the dataset.
Whilst some models show significant trends, the problems revolving around the current available dataset due to limited ship density in the pre-satellite era and the resulting missing storms mean scientists are still uncertain whether any changes in the past tropical hurricane activity can be attributed to climate change and rising SST or errors in the modelling and natural climate variability (Knutson et al, 2010).
Tropical hurricane projections rely heavily on dynamical models including global climate models, higher resolution global atmospheric models forced by SSTs from global climate models or even higher resolution regional downscaling models (Knutson et al, 2010), each method has strengths and weaknesses and when applied to the same model they can produce wildly different results (Tory et al, 2014). This causes a lack of consensus among the researchers. For example, two statistical models of hurricane activity vs SST, both of which performed comparably during the historical period, give dramatically different projections for late 21st century activity, with one predicting an increase of 300% in power dissipation by 2100 (Knutson et al, 2010), the other projecting a much smaller change which falls more in line with other dynamical models (Knutson et al, 2010). Other research produced by Bender et al. (2010) projected a significant increase (+90%) in the frequency of very intense (category 4 and 5) hurricanes when using the CMIP3/A1B 18-model average climate change projection. However, subsequent downscaled projections run by Knutson et al. (2013), whilst still showing an increase in category 4 and 5 hurricanes, was only marginally significant for the early 21st century (+45%) and the last 21st century (+39%) (GFDL, 2017).
The examples provided, whilst simplified for this paper, illustrate how different dynamical, statistical or downscaling techniques can offer substantially different projections in the response to tropical hurricanes and rising SST. In terms of general modelling approaches, all techniques can provide complimentary approaches and are worth pursuing however, each has its limitations and the results given must be interpreted with caution.
Large natural fluctuations in the frequency and intensity of tropical hurricanes greatly complicates both the detection of long-term trends and their attribution to rising sea surface temperature (Knutson et al, 2010 and it remains uncertain whether past changes in tropical hurricane activity has exceeded the variability expected from natural causes. A number of factors are known to naturally influence hurricane activity, including multi-decadal oscillations in oceanic thermohaline circulation, upper tropospheric high-pressure regions, dips in tropospheric vertical wind shear, atmospheric stability and equatorial shear winds that favour tropical hurricane development (Elsner, 2000, 2003; Goldenberg et al, 2001; Chu, 2004; Trenberth et al, 2005; Hoyos et al, 2006′ Gray, 2005) with only some of these seeming to correlate to rising SST.
Strong inter-annual variability in hurricane statistics and the influence of natural forces such as the El Nino and north Atlantic oscillation, make it difficult to discern any trends relative to rising sea surface temperatures (Webster et al, 2005). Ting et al (2015) indicated that the increase in hurricane intensity was dominated not by rising SST but by multi-decadal natural variability. Emmanuel (2005) similarly stated that tropical hurricanes do not respond directly to increasing SST but that the appropriate measure of their thermodynamic environment is the potential intensity, which depends not only on surface temperatures but on the whole temperature profile of the troposphere. Holland and Bruyere (2013) however, observed an increase in both global and basin hurricanes but could find no natural multi-decadal trends, relating all increases to anthropogenic effects arising from global changes (Walsh et al, 2015).
The large inter-annual variation in numbers of tropical storms indicates that other factors are important in potential storm intensity with current evidence suggesting that whilst rising sea surface temperatures do impact storm intensity, they have less effect than regional fluctuations and average tropospheric temperature (Emmanuel, 2005). The evidence strongly suggests that tropical storms not only depend on sea surface temperature but other variables as well, usually to a larger extent. The reduced number of hurricanes in the Atlantic during El Nino for example is related to increase vertical wind shear and to a subsidence effect in the regions of the Atlantic where tropical storms usually develop (Wu and Lau, 1992). Numerous studies have addressed the issue of changes In the global frequency and intensity of hurricanes in the warming world (Webster et al,2005) however, few models have included detailed data for the other variables that impact storm intensity. This fundamental uncertainty about which forces drive year to year variations in hurricane activity and how they are impacted by rising sea surface temperatures allows for further debate on the subject.
Tropical hurricanes are one of the most devastating natural disasters and with the population expanding in coastal regions, vulnerability to large storms is growing. Global warming has a pervasive influence on SST but the complex way it impacts tropical hurricanes is not yet fully understood. As the SST continues to warm, certain tropical ocean basins may face an increasing number of more intense tropical hurricanes (Chu and Clark, 1999). Holland and Webster (2007), like many others, suggested that an increase in the frequency of intense tropical hurricanes in the past 30 years is related to the increase in SST, but whether these changes in activity are actually related remains a controversial issue (Landsea, 2005) with many scientists believing that the understanding of these trends is incomplete and that the confidence in them is hampered by a lack of consistent data and the lack of understanding in natural seasonal variances.
It remains important to improve climate model simulation and there is considerable work that can be performed to produce more homogenous datasets of tropical hurricanes for climate analysis (Walsh et al, 2015) alongside the production of datasets that make allowance for under sampling of tropical hurricane counts in the pre-satellite era.
In conclusion, despite several models showing a correlation between SST and hurricane intensity in recent decades, it is premature to conclude that human activity and the resulting impacts, in particular rising SST, has caused a detectable change. However, there is a medium confidence in that future SST projections will increase the number of intense hurricanes despite an overall decrease in hurricane days.