Policy Peril Segment 3 – Hurricanes (updated 8/19/09)

Is global warming making hurricanes more destructive? Did global warming contribute to the devastation of New Orleans by Hurricane Katrina? Would Kyoto-style energy rationing help avert future weather-related catastrophes?

Well, just ask Al Gore! In An Inconvenient Truth, Gore claims there’s a “strong new emerging consensus” that global warming is increasing the duration and intensity of hurricanes (AIT, p. 81), he depicts New Orleans as a global warming victim (pp. 94-95), and the threat of increasingly powerful storms is a major part of the alleged “climate crisis” that Gore proposes to solve by restricting our access to carbon-based energy.

Gore’s message is not subtle. The movie poster for An Inconvenient Truth shows a hurricane spinning out of the smokestack of a coal-fired power plant.


As noted in previous posts, I am blogging on excerpts from CEI’s film, Policy Peril: Why Global Warming Policies Are More Dangerous Than Global Warming Itself. Our film (DVD, actually) provides skeptical perspectives on Gore’s “science” and “solutions.”

Today’s excerpt is on hurricanes. To watch it, click here. There’s more in Policy Peril on hurricanes, so if you want to pick up the thread where this snippet leaves off, fast foreward to about 7 minutes and 30 seconds into the film, which you can access here.

The indented section immediately below presents the text of today’s video excerpt, along with the charts appearing in the clip, and links to the supporting scientific papers:

Narrator: What about hurricanes? Gore says there is a “strong new emerging consensus” that global warming is increasing the intensity and duration of major tropical cyclones [hurricanes].

Dr. Patrick Michaels (Cato Institute): I can find a whole bunch of papers that say yes, a bunch that say no, a bunch of papers that say, “I don’t know.”

Narrator: Here’s a study Al Gore will never cite. Phil Klotzbach of Colorado State University measured changes in hurricane strength in the world’s six major hurricane basins from 1986 to 2005. There’s an increase in the North Atlantic, a decrease in the Northeast Pacific, and not much change anywhere else.


To persuade us that hurricanes are becoming stronger, Gore reports that economic damages from hurricanes increased dramatically in recent decades. He shows this on a graph similar to this one.


Figure description: U.S. hurricane damages, 1900-2005, not adusted for changes in population, wealth, and the consumer price index. Source: Pielke, Jr. et al. 2008. Normalized hurricane damages in the United States: 1900-2005. Natural Hazards Review Volume 9, Issue 1, pp. 29-42. 

But the graph is misleading. Consider this fact, more people today live in just two Florida counties, Dade and Broward, than lived in all 109 coastal counties from Texas to Virginia in 1930. There’s tons more stuff in harm’s way than there used to be. No wonder damages are bigger!

Dr. Michaels:  If you take a look at hurricane damages and adjust for population levels, for property values–things you have to adjust for–for inflation, you find there is no significant increase in the damage rate. In fact, the 1926 Florida Hurricane is the record holder by farby I believe about $50 billion more damage than Katrina in today’s dollars. So, there’s just no link here.


Figure description: U.S. hurricane damages, 1900-2005, if all hurricane strikes had hit the same locations but with today’s population, wealth, and consumer price index. Source: Pielke, Jr. et al. 2008.

A study on hurricane damages in China comes to the same conclusion as did the Pielke team. From 1983 to 2006, the researchers found no long-term trend in economic losses due to hurricanes once changes in population, the consumer price index, and, most importantly, GDP are taken into account. See the figure below.


Source: Zhang, Q. et al. 2009. Tropical cyclone damages in China 1983-2006. Bulletin of the American Meteorological Society, April 2009.

Let’s take a closer look at some of Dr. Michaels’s statements.

Michaels (Pat to his friends) says he can find a “bunch of papers” on all sides of the debate on the possible influence of global warming on hurricanes–the point being that Al Gore’s “strong new emerging consensus” does not in fact exist. 

Pat, his research associate Paul C. Knappenberger, and Dr. Robert Davis of the University of Virginia provide a partial list of “skeptical” references on pp. 33-34 of their comment on EPA’s Advance Notice of Proposed Rulemaking: Regulating Greenhouse Gases under the Clean Air Act (ANPR). I reproduce that list below, plus other studies of skeptical bent, provide links to the scientific papers (or their abstracts) along with pertinent graphic materials, and summarize key finding in bold italics. I also include links to Web-based commentary by Dr. Michaels or the Center for the Study of Carbon Dioxide and Global Change.

Key papers affirming the existence of a trend towards stronger hurricanes include:

  • Emanuel, K. 2005. Increasing destructiveness of tropical cyclones over the past 30 years. Nature 436: 686-688. Hurricane strength, a combination of wind speed and duration, which Emanuel calls the “power dissipation index,” increased by 50% since the mid-1970s, and is highly correlated with sea-surface temperature.
  • Webster, P. et al. 2005. Changes in Tropical cyclone number, duration, and intensity in a warming environment. Science 309: 1844-1846. The number and percentage of major (Category 4 & 5) hurricanes increased from 1970 to 2004.
  • Trenberth, K.E. et al. 2007. Observations: Surface and Atmospheric Change [chapter 3 of Climate Change  2007: The Physical Science Basis. Working Group I Report, Fourth Assessment Report, Intergovernmental Panel on Climate Change]. The intensity of tropical cyclones has increased since 1970.

Papers disputing the global existence and/or magnitude of a trend towards stronger hurricanes include:

  • Landsea, C.W. et al. 2005. Hurricanes and global warming. Nature 438: E11-13. Emanuel mishandled data and his methodology is flawed.
  • Landsea, C.W. et al. 2006. Can we detect trends in extreme tropical cyclones. Science 313: 452-454. The apparent trend towards more powerful hurricanes is a consequence of improved monitoring in recent years of non-landfalling hurricanes. [For Pat Michaels’s review, click here.]
  • Klotzbach, P.J. 2006. Trends in global tropical cyclone activity over the past twenty years (1986-2005). Geophysical Research Letters 33 doi: 10.1029/2006GL025881. From 1986 to 2005, there was an increase in hurricane strength (“accumulated cyclone energy”) in the North Atlantic, a decrease in the Northeast Pacific, and not much change in the other four hurricane basins. [For Pat Michaels’s review, click here.]
  • Swanson, K.L. 2007.  Impact of scaling behavior on tropical cyclone intensities. Geophysical Research Letters 34 doi: 10.1029/2007GL030851. There is no statistically significant correlation between sea surface temperatures and average tropical cyclone intensity in either the Atlantic or western Pacific Ocean from 1950 to 2005. [For Pat Michaels’s review, click here.]

In their comment on EPA’s ANPR, Michaels, Knappenberger, and Davis also list other papers that “do not draw as close a linkage between anthropogenic climate changes and increasing hurricane frequency and/or intensity” as the IPCC purports to find in the scientific literature. Studies of this stripe include:

  • Briggs, W.M. 2008. On the changes in the number and intensity of North Atlantic tropical cyclones. Journal of Climate 21: 1387-1402. There is “almost no evidence that distributional mean of individual storm intensity, measured by storm days, track length, or individual storm power dissipation index, has changed (increased or decreased) through time.” [For Pat Michaels’s review, click here.]
  • Knutson, T.R. et al. 2008 . Simulated reduction in Atlantic hurricane frequency under twenty-first century warming conditions. Nature Geosciences doi:10.1038/ngeo202. Global warming will decrease Atlantic hurricane frequency by so much more than it will increase average hurricane strength that cumulative Atlantic hurricane power in the 21st Century will decrease by 25%.
  • Wang, C. & Lee, S.K. 2008. Global warming and United States landfalling hurricanes. Geophysical Research Letters 35(1): L02708. Warming of the Atlantic Ocean is associated with an increase in vertical wind shear, which in turn coincides with a “weak but robust” downward trend in U.S. landfalling hurricanes. See the figure below. [For Pat Michael’s review, click here.]


Figure description: Number of U.S. landfalling hurricanes from 1851 to 2006 (red bars), the black straight line is the long-term trend, the blue line is the seven-year running mean (from Wang & Lee 2008). 

  • Kossin, J.P. & Vimont, D.J. 2007. A more general framework for understanding  Atlantic hurricane variability and trends. Bulletin of the American Meteorological Society 88(11): 1767-1781. A large part of the variability of Atlantic hurricane intensity, duration, and frequency can be explained by interannual and multidecadal shifts in a natural oscillation known as the Atlantic Meriodonal Mode (AMM). [For Pat Michaels’s review, click here.]
  • Landsea, C.W. 2007. Counting Atlantic tropical cyclones back to 1900. EOS: Transactions of the American Geophysical Union 88. Improved monitoring in recent years is responsible for most, if not all, of the observed trend in increasing frequency of tropical cyclones.
  • Latif, M., Keenlyside, N., & Bader, J. 2007. Tropical sea surface temperature, vertical wind shear, and hurricane development. Geophysical Research Letters 34(1) L01710. From 1870 to 2003, there is no sustained long-term trend in Atlantic hurricane activity; a key variable controlling wind shear and, thus, Atlantic hurricane strength is the warmth of the topical North Atlantic relative to the warmth of the tropical Indian and Pacific Oceans. [For a review by the Center for the Study of Carbon Dioxide and Global Change, click here.]
  • Nyberg, J. et al.  2007. Low hurricane activity in the 1970s and 1980s compared to the past 270 years. Nature 447(7145): 698-701. The average frequency of North Atlantic hurricanes decreased gradually from the 1760s until the early 1990s, reaching anomalously low values during the 1970s and 1980s. The period of enhanced activity since 1995 is not unusual compared to other periods of high hurricane activity “and thus appears to represent a recovery to normal hurricane activity rather than a direct response to sea surface temperature.” [For Pat Michaels’s review, click here.]
  • Vecchi, G.A. & Soden, B.J. 2007a. Increased tropical Atlantic wind shear in model projections of global warming. Geophysical Research Letters 34: 1029/2006GL028905. A suite of state-of-the-art climate model experiments project “substantial” increases in tropical Atlantic and East Pacific wind shear over the 21st Century–changes “comparable to or larger than” model-projected changes in other factors affecting hurricanes; hence, the models “do not suggest a strong anthropogenic increase” in hurricane activity in those basins. [For a review by the Center for the Study of Carbon Dioxide and Global Change, click here.]
  • Vecchi, G.A. & Soden, B.J. 2007b. Effect of remote sea surface temperature change on potential tropical cyclone intensity. Nature 450(7172): 1066-1070. Changes in hurricane intensity due to natural climate variations (such as the warming of one ocean basin relative to another) “may be larger than the response to the more uniform patterns of greenhouse-gas-induced warming.” [For a review by the Center for the Study of Carbon Dioxide and Global Change, click here.]
  • Vecchi, G.A., Swanson, K.L, and Soden, B.J. 2008. Whither Hurricane Activity? Science 322: 687-689. Science is currently unable to decide, due to insufficient data, which factor chiefly controls Atlantic hurricane intensity–the absolute sea surface temperature (SST) of the basin’s main development region, or the region’s SST relative to other tropical ocean basins. If absolute SST is the key factor, then global warming should increase Atlantic hurricane activity. If relative SST is the key factor, then the future should exhibit “little long term trend” in hurricane intensity. [For Pat Michaels’s review, click here; for a review by the Center for the Study of Carbon Dioxide and Global Change, click here.]
  • Besonen, M.R. et al. 2008. A 1000-year, annually resolved record of hurricane activity from Boston, Massachusetts. Geophysical Research Letters 35, L14705, doi: 1029/2008GL039950. Analysis of sediment cores from Lower Mystic Lake shows “centenniel-scale variations in  frequency [of category 2-3 hurricanes in the Boston area] over the past millennium . . . with a period of increased activity between the 12th-16th centuries and decreased activity during the 11th and 17th-19th centuries.” In other words, there is considerable natural variability on centenniel scales, and Boston got hit with hurricanes long before the “greenhouse” era of SUVs and coal-fired power plants. [For Pat Michaels’s review, click here.]
  • Mock, C.J. 2008. Tropical cyclone variations in Louisiana, U.S.A., since the late 18th century. Geochemistry, Geophysics, Geosystems, 9, Q05Vo2, doi: 10.1029/2007GC001846. “Parts of the early and mid-19th century exhibit greater tropical cyclone and hurricane activity [in Lousiana] than at any time within the last few hundred years.” Indeed, “If a higher frequency of major hurricanes occurred in the near future in a similar manner as the early 1800s or in single years such as in 1812, 1831, and 1860, it would have devastating consequences for New Orleans, perhaps equalling or exceeding the impacts such as in hurricane Katrina in 2005.” In short, there is no long-term trend in hurricanes in the vicinity of New Orleans, as the figure below shows. [For Pat Michaels’s review, click here; for a review by the Center for the Study of Carbon Dioxide and Global Change, click here.]


Figure description: Upper graph shows number of Louisiana tropical cyclones along with centered 10-year running sums; lower graph shows number of Lousiana hurricanes along with centered 10-year running sums. Green dots show major hurricanes.

  • Chenoweth, M. and D. Devine. 2008. A document-based 318-year record of tropical cyclones in the Lesser Antilles, 1690-2007. Geochemistry, Geophysics, Geosystems 9, Q08013, doi: 10.1029, 2008GC002066. Newspaper accounts, ship logbooks, meteorological journals, and other documents were used to create a new database on hurricane intensity in the islands that wrap around the eastern end and southern fringe of the Caribbean sea. Applying a new methodology, the researchers found no evidence that hurricane activity is increasing over three centuries of recorded events. [For Pat Michaels’s review, click here.]
  • Klotzbach, P.J. and W.M. Gray. 2008. Multidecadal variability in North Atlantic tropical cyclone activity. Journal of Climate 21, 3929-3935. From 1878 to the present, the ups and downs of hurricane activity in the North Atlantic show “remarkable agreement” with changes in the Atlantic Multidecadal Oscillation (AMO), which shifts back and forth between  positive (warm) and negative (cool) phases. See figure below. [For Pat Michaels’s review, click here.]


Figure description:  Annually averaged Atlantic basin hurricanes (H), hurricane days (HD), major hurricanes (MH), and major hurricane days (MHD) for the top 20 AMO years (blue bars) and bottom 20 AMO years (red bars).

  • Kuleshov, Y. et al. 2008. On tropical cyclone activity in the Southern Hemisphere: Trends and the ENSO connection. Geophysical Research Letters 35, L14508, doi: 10.1029/2007GL032983. For the 1981/82 to 2005/2006 hurricane seasons, there are no apparent trends in the total numbers and cyclone days in the Southern Hemisphere (South Indian Ocean and South Pacific Ocean. [For Pat Michaels’s review, click here.]
  • Chan, J.C.L. 2007. Decadal variations of intense typhoon occurrence in the western North Pacific. Proceedings of the Royal Society. A, 464: 249-272. From 1960 to 2005, the number of category 4 and 5 hurricanes in the North Pacific undergo a strong multidecadal (16-32 years) variation, largely due to El Nino-Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO), but exhibit no long-term trend. “Thus, at least for the WNP (western North Pacific], it is not possible to conclude that these variations in intense typhoon activity are attributable to the effect of global warming.” [For Pat Michael’s review, click here; for a review by the Center for the Study of Carbon Dioxide and Global Change, click here.]
  • Englehart, P.J. et al. 2008. Defining the frequency of near-shore tropical cyclone activity in the eastern North Pacific from historical surface observations (1921-2005). Geophysical Research Letters 35, L03706, doi: 10.1029/2007GL032546. Long-term tropical cyclone frequency off the Pacific coast of Mexico exhibits a significant “negative” trend. See the figure below. [For Pat Michaels’s review, click here.]


Figure description: Hurricane frequency off the Pacific coast of Mexico (from Englehart 2008)

To wrap up, no consensus has been reached about the possible influence of global warming on hurricanes. Consider this joint statement by 120 members of the World Meteorological Organization:

The possibility that greenhouse gas induced global warming may have already caused a substantial increase in some tropical cyclone indices has been raised (e.g., Mann and Emanuel, 2006), but no consensus has been reached at this time.

A considerable body of science, ably reviewed by Dr. Michaels, the Center for the Study of Carbon Dioxide and Global Change, and the Nongovernmental International Panel on Climate Change, indicates that natural multi-decadal climate oscillations are now and will remain the dominant driver of Atlantic hurricane behavior in the 21st Century.

Some final thoughts. First, Gore’s prophesy of an era of increasingly more violent and/or frequent hurricanes no longer seems as plausible as it did in the aftermath of Katrina and Rita and the active hurricane seasons of 2004 and 2005.

Ryan Maue, a Ph.D. candidate in Florida State University’s Department of Meteorology, in eye-popping charts of accumulated cyclone energy (ACE) in the Northern hemisphere and globally, shows that we have entered a period that might be described as a “hurricane depression.” ACE in the Northern hemisphere is now at its lowest level in 30 years:


Figure description: May-June-July ACE, 1970 – 2009. The three month ACE for 2009 is the second lowest since 1970.

Globally, tropical cyclone energy is also near record lows over the past 50 years.


Second, even if global warming does make hurricanes stronger, that would not be a valid reason to pursue Kyoto-style energy rationing. As Bjorn Lomborg points out, carbon suppression policies would have no measurable effect on hurricane behavior for many decades (if ever), yet would cumulatively cost trillions of dollars. The possible influence of global warming on hurricanes is simply one more reason to undertake proven, cost-effective measures such as improving building codes, evacuation routes, early warning systems, and emergency response capabilities. Indeed, it is another reason, as Kerry Emanuel, Peter Webster, and eight other leading hurricane experts affirm in a joint statement, for governments to stop subsidizing development in high-risk areas.

Finally, the best hurricane protection strategy for developing countries is economic growth. In 1955, a Category 5 hurricane called Janet slammed into Chetumal, on Mexico’s Yucatan Penninsula, killing 600 people. On August 21, 2007, another Category 5, Hurricane Dean, hit the same spot and killed no one. It may be the first time in history when a Category 5 hit a populated area and everyone survived. What changed between 1955 and 2007? Not the weather. The big difference, Dr. Michaels observes, is that Mexico today is much wealthier than it was in the 1950s. Storm warning information is now widely available, there are better roads for evacuation, and emergency response programs are better funded. Unfortunately, Al Gore’s agenda to reduce global CO2 emissions 50% by 2050 can succeed only if poor countries accept emission limitations that stifle their economic development–a topic we’ll discuss later in this series.

Finally, New Orleans was not a global warming victim, as Gore insinuates. Katrina was the worst natural disaster in U.S. history not because of any extra power the storm allegedly got from global warming (Katrina was only a Category 1 hurricane by the time it hit New Orleans), but because the federal government failed to construct adequate flood defenses for a city built below sea level in a known hurricane corridor. My colleague, John Berlau, chronicles this sad tale in his 2006 book, Eco-Freaks.