How Does Climate Change Affect Atlantic Hurricanes? 2017 In Perspective
While no evidence exists that there will be more (or fewer) hurricanes or more hurricanes hitting our coasts, there is theoretical and statistical evidence that the strongest hurricanes are getting stronger as the oceans heat up due to global warming from the emission of greenhouse gases. The Atlantic ocean was very warm this year contributing to the duration and extent of storm intensification of the strongest hurricanes. The strength and position of the Bermuda High steered the hurricanes toward the United States.
What might happen to hurricanes in the future as the climate warms continues to be an active area of research. Over the past decade, however, scientists have begun to uncover clues as to the kinds of changes that are already occurring.
The theory of maximum potential intensity, which relates intensity to ocean heat, refers to a theoretical upper limit of hurricane strength given the proper environment [Emanuel 1988]. The upward trend is clearly seen by relating the wind speed inside a hurricane to a measure of the underlying ocean temperature.
This relationship using hurricanes that have occurred across the Atlantic ocean [Elsner, Trepanier, Strazzo, and Jagger 2012] is shown in Figure 1. Across regions where the ocean is colder, the fastest winds (limiting intensity) are weaker. In contrast, in regions where the ocean is warmer the fastest winds are much stronger. This relationship (sensitivity of limiting hurricane intensity to ocean warmth) is quantified as the slope of the best-fit line, which amounts to 8 (+/-1.2) m/s/K. Current climate models fail to reproduce this sensitivity indicating results from them about what hurricanes might be like in the future are unreliable [Strazzo, Elsner, Trepanier, and Emanuel 2013].
Figure 1. Scatter plot of the fastest hurricane winds versus ocean temperature. The fastest winds are based on a limiting intensity model for the peak strength of hurricanes occurring over twenty grid cells across the North Atlantic. The best-fit regression line is shown in blue and the 95% uncertainty band about the line is shown in gray. Vertical bars contain the 80% of the estimates based on a bootstrap re-sampling algorithm [From Elsner et al. 2012].
The sensitivity of hurricane intensity to ocean temperature suggests that stronger hurricanes are likely as the ocean warms. This is indeed happening. For each hurricane we record its lifetime highest wind speed, then look at the relationship between these fastest winds and ocean temperature [Elsner, Kossin, and Jagger 2008].
Figure 2 shows this relationship as regression trends that increase with increasingly strong hurricanes. Since El Nino also plays a role in modulating Atlantic hurricanes, a variable that tracks its monthly changes is included in the regression [Elsner and Jagger 2013]. At lower quantiles (weaker winds) the trend line is near zero but at higher quantiles (stronger winds) there is an upward trend (greater sensitivity of intensity to ocean heat). Hurricanes are not always in ideal environments for strengthening, but when the strongest ones are they are sensitive to warming oceans as dictated by theory.
Figure 2. Trends in life time fastest winds. Coefficient of the sea-surface temperature term of a quantile regression of life time maximum wind speed for Atlantic hurricanes controlling for El Ni~no through the Southern Oscillation Index (SOI) [From Elsner and Jagger 2013].
The theory is silent about the number of hurricanes and indeed there is no significant upward or downward trend in Atlantic hurricane frequency since accurate record keeping has been in place. So metrics that are influenced by the hurricane frequency, like Accumulated Cyclone Energy (ACE) used widely by the National Oceanic and Atmospheric Administration (NOAA), are problematic for examining climate change effects.
Some scientists have suggested that this upward trend in hurricane energy is part of the natural variability in the Atlantic known as the Atlantic Multidecadal Oscillation (AMO) rather than global warming [Goldenberg et al. 2001]. A test of these two competing hypothesis suggests that global warming causes the AMO since it is found that preceding values of global temperature predict the AMO and not conversely [Elsner 2007].
Others have noted hurricane property losses have not gone up recently and the numbers of hurricanes hitting the United States have declined over the past century and a half. Historically, however, when a hurricane hits the coast the best predictor of the magnitude of property loss is ocean temperature with warmer oceans leading to greater loss amounts [Jagger, Elsner, and Burch 2011].
Citations and Additional Reading
Elsner, J. B. (2007). “Granger causality and Atlantic hurricanes”. In: Tellus A 59.4, pp. 476-485. URL: http://myweb.fsu.edu/jelsner/PDF/Research/Elsner2007.pdf.
Elsner, J. B, J. P. Kossin and T. H. Jagger (2008). “The increasing intensity of the strongest tropical cyclones”. In: Nature 455.7209, pp. 92-95. URL: http://myweb.fsu.edu/jelsner/PDF/Research/ElsnerKossinJagger2008.pdf.
Elsner, J. B. and T. H. Jagger (2006). “Prediction models for annual US hurricane counts”. In: Journal of Climate 19.12, pp. 2935-2952. URL: http://myweb.fsu.edu/jelsner/PDF/Research/ElsnerJagger2006a.pdf.
Elsner, J. B, J. C. Trepanier, S. E. Strazzo and T. H. Jagger (2012). “Sensitivity of limiting hurricane intensity to ocean warmth”. In: Geophysical Research Letters 39.17. ISSN: 1944-8007. URL: http://myweb.fsu.edu/jelsner/PDF/Research/ElsnerTrepanierStrazzoJagger2012.pdf.
Elsner, J. B. and T. H. Jagger (2013). “Hurricane Climatology: A Modern Statistical Guide Using R”. Oxford University Press, USA. ISBN: 9780199827633. URL: https://www.amazon.com/Hurricane-Climatology-Modern-Statistical-Guide/dp/019982763X
Emanuel, K. A. (1988). “The maximum intensity of hurricanes”. In: Journal of the Atmospheric Sciences 45.7, pp. 1143-1155.
Goldenberg, S. B., C. W. Landsea, A. M. Mestas-Nunez and W. M. Gray (2001) “The recent increase in Atlantic hurricane activity: Causes and implications”. In: Science, 293, pp. 474-479.
Jagger, T. H, J. B. Elsner and R. K. Burch (2011). “Climate and solar signals in property damage losses from hurricanes affecting the United States”. In: Natural Hazards 58.1, pp. 541-557. URL: http://myweb.fsu.edu/jelsner/PDF/Research/JaggerElsnerBurch2011.pdf.
Strazzo, S, J. B. Elsner, J. C. Trepanier and K. A. Emanuel (2013). “Frequency, intensity, and sensitivity to sea surface temperature of North Atlantic tropical cyclones in best-track and simulated data”. In: Journal of Advances in Modeling Earth Systems 5.3, pp. 500-509. URL: http://myweb.fsu.edu/jelsner/PDF/Research/StrazzoElsnerTrepanierEmanuel2013.pdf.