Huge Dangers, Huge Unknowns

Hurricane Sandy's enormous $65 billion price tag put that great storm in third place for the most expensive weather-related disaster in U.S. (and world) history. Indeed, when we look at the list of most expensive U.S. weather-related disasters since 1980, hurricanes dominate, claiming six of the top ten spots. Drought is also a formidable presence, accounting for three of the other top-ten budget-busting disasters. Thus, a critical question for society is: how will our dangerous uncontrolled experiment of dumping massive amounts of heat-trapping carbon dioxide into the air affect droughts and hurricanes? For droughts, the answer is relatively straightforward--in general, dry parts of the world like the Southwest U.S. and Southern Europe are predicted to get drier. The worst droughts will get more extreme, wherever they occur, since it will be hotter. But for hurricanes, the uncertainties are greater. Since hurricanes are heat engines that extract heat energy from the oceans to power themselves, hurricane scientists are confident that the very strongest storms will get stronger by the end of the century, when Earth's land and ocean temperatures are expected to warm to levels unmatched since the Eemian Era, 115,000 years ago. But hurricanes are fussy creations, and are sensitive to wind shear and dry air. Although the strongest storms should get stronger when "perfect storm" conditions are present, these "perfect storm" conditions may become less frequent in the future, due to the presence of higher wind shear and/or more dry air. Indeed, the climate models used to formulate the 2007 IPCC report suggested that we might see a decrease in the total number of hurricanes in the Atlantic (and worldwide) later this century. However, the latest set of models used to formulate the 2013 IPCC report, due out in September, show that the total number of hurricanes both globally and in the Atlantic may increase, according to a paper published in July 2013 by one of our top hurricane scientists, Dr. Kerry Emanuel of MIT. Hurricane intensity may also increase, leading to a global increase of 40% of Category 3 and stronger hurricanes by 2100. However, the uncertainties in this research are great. We really don't know how global warming will affect the number of hurricanes and their intensity.


Figure 1. The list of most expensive U.S. weather-related disasters since 1980 is dominated by hurricanes.

Climate models and hurricane frequency
Hurricane scientists are in agreement that we have not been able to positively detect changes in the frequency of hurricanes in the Atlantic since reliable storm records began in 1944, when the Hurricane Hunters began collecting data. A landmark 2010 review paper, "Tropical Cyclones and Climate Change", authored by ten top hurricane scientists, including Dr. Emanuel, concluded that the U.S. has not seen any long-term increase in landfalling tropical storms and hurricanes, and that "it remains uncertain whether past changes in tropical cyclone frequency have exceeded the variability expected through natural causes." Based in part on modeling studies using climate models run for the 2007 IPCC report, the scientists concluded that "it is likely that global mean tropical cyclone frequency will either decrease or remain unchanged owing to greenhouse warming." For example, one of the modeling studies the review paper quoted, Knutson et al. (2008), "Simulated reduction in Atlantic hurricane frequency under twenty-first-century warming conditions", projected a decrease in Atlantic tropical storms by 27% and hurricanes by 18% by the end of the century. An important reason that their model predicted these decreases was due to a predicted increase in wind shear. As I explain in my wind shear tutorial, a large change of wind speed with height over a hurricane creates a shearing force that tends to tear the storm apart. The amount of wind shear is critical in determining whether a hurricane can form or survive.

But a July 2013 study by Dr. Emanuel, "Downscaling CMIP5 climate models shows increased tropical cyclone activity over the 21st century", challenges this result. Dr. Emanuel argues that tropical cyclones are likely to become both stronger and more frequent as the climate continues to warm. This increase is most likely to occur in Western North Pacific, with smaller increases in the Atlantic. Dr. Emanuel took output from six newer higher-resolution climate models used to formulate the 2013 IPCC report, and used the output to drive a high-resolution hurricane model. The simulations found that the global frequency of tropical cyclones will increase by 11% to 40% by 2100, with intensity increases as well. The combined effects produced a global increase in Category 3 and stronger hurricanes of 40%. The behavior of these strongest hurricanes is critical, since they do most of the damage we observe. Over the past century, Category 4 and 5 hurricanes made up only 6% of all U.S. landfalls, but accounted for 48% of all U.S. damage (if normalized to account for increases in U.S. population and wealth, see Pielke et al., 2008.)


Figure 2. Projected changes in tropical cyclone track density during the 2006-2100 period compared to the 1950-2005 period, using output from six climate models included in the 2013 IPCC report. The global frequency of tropical cyclones is predicted to increase by 11% to 40%, with the largest changes occurring in the Northwest Pacific off the coast of Japan. Smaller increases are predicted for the Atlantic and near Australia. Image credit: Kerry Emanuel, "Downscaling CMIP5 climate models shows increased tropical cyclone activity over the 21st century",  Proceedings of the National Academy of Sciences, July 8, 2013, doi: 10.1073/pnas.1301293110.

However, a study by Knutson et al. (2013), using the same latest-generation climate models as used by Emanuel (2013), but using the output from the models to drive a different high-resolution hurricane model, found a 20% decrease in Atlantic tropical storms and hurricanes by 2100. Two other 2013 studies by Villarini et al. and Camargo, also using output from the 2013 IPCC models, found essentially no change in Atlantic tropical cyclones. The reason for the differences, lies, in part, with how much global warming is assumed in the studies. Dr. Emanuel's study, which found an increase in tropical cyclone activity, assumed a worst-case warming situation (RCP 8.5), following the emissions path humanity is currently on. The Knutson et al. study, which found a decrease of 20% in Atlantic tropical cyclones, used a scenario (RCP 4.5) where it was assumed humans will wise up and cause about half of the worst-case greenhouse warming. The study found found "marginally significant" increases in Atlantic Category 4 and 5 hurricanes of 39% - 45% by 2100. Dr. Emanuel ends his paper with the statement, "the response of tropical cyclones to projected climate change will remain uncertain for some time to come."


Figure 3. Expected change in Atlantic Category 4 and 5 hurricanes per decade expected by the year 2100, according to Knutson et al. (2013), "Dynamical Downscaling Projections of 21st Century Atlantic Hurricane Activity: CMIP3 and CMIP5 Model-based Scenarios." This research used the latest generation of climate models from the 2013 IPCC report, and found "marginally significant" increases in Atlantic Category 4 and 5 hurricanes of 39% - 45% by 2100.

Commentary
Keep in mind that hurricane damages are currently doubling every ten years without the effect of climate change, according to Pielke et al., 2008. This is primarily due to the increasing population along the coast and increased wealth of the population. The authors theorize that the Great Miami Hurricane of 1926, a Category 4 monster that made a direct hit on Miami Beach, would have caused about $150 billion in damage had it hit in 2005. Thus, by 2015, the same hurricane would do $300 billion in damage, and $600 billion by 2025. This is without considering the impact that accelerating sea level rise will have on storm surge damages. Global sea level rise over the past decade has been about double what it was in the 20th century, and the rate of sea level rise is expected to increase further in the coming decades. Storm surge does the majority of damage in major hurricanes, and storm surges riding on top of higher sea levels are going to do a lot more damage than we are used to seeing in the coming decades. If we toss in the (controversial) increases in Category 3 and stronger storms like Dr. Emanuel suggests may occur, the hurricane damage math gets very impressive. We can also add onto that the relatively non-controversial increase in tropical cyclone rainfall of 20% expected by 2100, which will sharply increase damages due to fresh water river flooding. There is evidence we may already be seeing an increase in heavy precipitation events associated with tropical cyclones in the U.S. A 2010 study by Kunkel et al., "Recent increases in U.S. heavy precipitation associated with tropical cyclones", found that the number of Southeast U.S. tropical cyclone heavy precipitation events, defined as 1-in-5-year events, more than doubled between 1994 - 2008, compared to the long-term average from 1895 - 2008.


Figure 4. Time series of the 15-year running average (plotted at the end point of the 15-yr blocks) of a Tropical Cyclone Heavy Precipitation Index (red) and 15-year running average of U.S. landfalling hurricanes (blue). Note that there has been no long-term increase in U.S. landfalling hurricanes, but there has been a sharp increase in extreme rainfall events associated with landfalling tropical cyclones--the kind of rainfall events most likely to cause damaging flooding. Image credit: Kunkel et al. (2010), "Recent increases in U.S. heavy precipitation associated with tropical cyclones", Geophysical Research Letters.

It is essential that we limit coastal development in vulnerable coastal areas, particularly along barrier islands, to reduce some of the astronomical price tags hurricanes are going to be causing in the future. Adoption and enforcement of strict building standards is also a must, as well as more reforms to the government's National Flood Insurance Program (NFIP), which subsidizes development in high-risk coastal regions that private insurers won't touch. NFIP is now $25 - 30 billion in the red, thanks to Hurricane Katrina and Hurricane Sandy. Reform of NFIP is already underway. In 2012, before Sandy hit, Congress passed the Biggert-Waters Flood Insurance Reform Act, which requires people with NFIP policies to pay large premium increases of about 25% per year over the next five years. Naturally, this move has caused major controversy.

References
Camargo, S., (2013), "Global and regional aspects of tropical cyclone activity in the CMIP5
models," J. Climate.

Emanuel, K.A., 2013, "Downscaling CMIP5 climate models shows increased tropical cyclone activity over the 21st century", PNAS, July 8, 2013, doi: 10.1073/pnas.1301293110

Knutson et al., 2010, "Tropical Cyclones and Climate Change", Nature Geoscience 3, 157 - 163, Published online: 21 February 2010 | doi:10.1038/ngeo779

Knutson et al., 2013, Dynamical Downscaling Projections of 21st Century Atlantic Hurricane Activity: CMIP3 and CMIP5 Model-based Scenarios, Journal of Climate 2013 ; e-View
doi: http://dx.doi.org/10.1175/JCLI-D-12-00539.1

Pielke, R.A, et al., 2008, "Normalized Hurricane Damage in the United States: 1900 - 2005," Natural Hazards Review, DOI:10.1061/ASCE1527-6988(2008)9:1(29)

Villarini, G, and G.A. Vecchi, 2012, "Twenty-first-century projections of North Atlantic tropical storms from CMIP5 models," Nature Clim. Change 2:604–607.

Related posts
Global warming and the frequency of intense Atlantic hurricanes: model results, my 2010 blog post.

Climate Central's analysis of the new 2013 Kerry Emanuel paper.

Goodbye, Miami: Jeff Goodell's sobering 2013 article in Rolling Stone on the challenges Miami faces due to sea level rise and hurricanes.

by Jeff Masters