Teaching and Learning Guide for: The Current Debate on the Linkage between Global Warming and Hurricanes
Authors: Shepherd J. Marshall , Knutson Thomas 
Published: June 2008

I IntroductionHurricanes are one of nature's most awe‐inspiring natural systems that are just one manifestation of the generic tropical cyclone; a nonfrontal synoptic scale low‐pressure system that occurs over tropical or subtropical waters, with organized convection (i.e. thunderstorm activity) and a distinct cyclonic surface wind circulation. These storms typically form some distance from the equator in environments with: (i) warm ocean surface waters (>26.5 °C); (ii) a potentially unstable atmosphere; (iii) a moist mid‐troposphere; (iv) a preexisting near surface disturbance, such as a tropical easterly wave, with established spin and convergence; and (v) a small amount of vertical wind shear. The suffering and economic losses caused by Hurricane Katrina and the parade of tropical storms and hurricanes that affected the United States in 2004–2005 has energized global debate on the cause of anomalous hurricane activity and the extent to which such activity is in fact anomalous in the long (century‐scale) term. The question of whether global warming is impacting hurricane frequency and/or intensity also has become a topic of debate in the wider community of decision‐makers, stakeholders, citizens, and scientists. Within the research community, an especially important ongoing debate concerns the relative roles that anthropogenic forcing (e.g. global warming) and natural variability have on the observed changes, such as the apparent recent increase in intense hurricane activity in the Atlantic basin. Shepherd and Knutson (2007) review arguments and perspectives on this issue.II Readings and Weblinks1. Socioeconomic impacts of hurricanesHurricanes and related storms around the world significantly impact on the structure of society through the effects they have on national and regional economies, public health, and policy. Recently, the US National Science Board (2007) published a document entitled ‘Hurricane Warning: The Critical Need for a National Hurricane Research Initiative’ (available at http://www.nsf.gov/nsb/), which provides a thorough, albeit US‐centric, perspective on the implications of hurricanes to the socioeconomic–political fabric of our society. Other starting points for research on the socioeconomic impacts and hurricanes are:Pielke, R. A. Jr (2007). Future economic damage from tropical cyclones: sensitivities to societal and climate changes. Proceedings of the Philosophical Transactions of the Royal Society (accepted)Pielke, R. A. Jr, and Pielke, R. A. Sr (1997a). Hurricanes: their nature and impacts on society. London: John Wiley & Sons, p. 279.Pielke, R. A. Jr, and Pielke, R. A. Sr (1997b). Societal vulnerability to hurricanes: considerations on the use of long‐range forecasts. In: Diaz, H. F. and Pulwarty, R. S. (eds) Hurricanes, climatic change and socioeconomic impacts. Berlin, Germany: Springer, pp. 147–184.2. Hurricane structure and genesisStarting points for obtaining fundamental information on the evolution, structure, and characteristics of hurricanes are:Wu, L., Braun S. A., Halverson, J., and Heymsfield, G. (2006). A numerical study of Hurricane Erin (2001). Part I: model verification and storm evolution. Journal of the Atmospheric Sciences 63, pp. 65–86.Frank, W. M., and Ritchie, E. A. (1999). Effects of environmental flow upon tropical cyclone structure. Monthly Weather Review 127, pp. 2044–2061.Molinari, J., and Vollaro, D. (2000). Planetary‐ and synoptic‐scale influences on Eastern Pacific tropical cyclogenesis. Monthly Weather Review 128, pp. 3296–3307.Websites that provide a wealth of information, data, and images include:

The official website of the US National Hurricane Center website (http://www.nhc.noaa.gov)
The National Oceanic and Atmospheric Administration (NOAA) Hurricane Research Division Frequently Asked Question website (http://www.aoml.noaa.gov/hrd/tcfaq/tcfaqHED.html)
The NASA Hurricane Resource website (http://www.nasa.gov/mission_pages/hurricanes/main/index.html)
The NOAA Hurricane Portal website (http://hurricanes.noaa.gov/)
The NASA Earth Observatory website (http://earthobservatory.nasa.gov/Library/Hurricanes/hurricanes_3.html)

3. Climate change linkage to tropical cyclonesShepherd and Knutson (2007) provide a current review of the subject and in 2006 the World Meteorological Organization (WMO) issued a statement on ‘Tropical Cyclones and Climate Change’ that assesses the state of knowledge of the impact of anthropogenically induced climate change on tropical cyclones. Both the summary and comprehensive statements, which are good places to begin an in‐depth study of the subject, can be found at:

Summary WMO Statement website (http://www.wmo.ch/web/arep/press_releases/2006/iwtc_summary.pdf)
Comprehensive WMO Statement website (http://www.wmo.ch/web/arep/press_releases/2006/iwtc_statement.pdf)

The number of intense hurricanes over the period 1975 to 2004 (Courtesy of http://www.earthobservatory.nasa.gov as adapted from Webster, P. J., et al. Changes in tropical cyclone number, duration, and intensity in a warming environment.

Science

309, pp. 1844–1846).

Basin
1975–1989
1990–2004

East Pacific
36
49

West Pacific
85
116

North Atlantic
16
25

Southwestern Pacific
10
22

North Indian
1
7

South Indian
23
50

The following key papers also provide different perspectives on the debate.Curry, J. A., Webster, P. J., and Holland, G. J. (2006). Mixing politics and science in testing the hypothesis that greenhouse warming is causing a global increase in hurricane intensity. Bulletin of the American Meteorological Society 87 (8), pp. 1025–1037.Elsner, J. B. (2006). Evidence in support of the climate change‐Atlantic hurricane hypothesis. Geophysical Research Letters 33, p. L16705; doi:10.1029/2006GL026869.Emanual, K. (2005). Increasing destructiveness of tropical cyclones over the past 30 years. Nature 436, pp. 686–688.Emanual, K., R. Sundararajan, and J. Williams (2008). Hurricanes and global warming: Results from downscaling IPCC AR4 simulations. Bulletin of the American Meteorological Society 89, 347–377.Klotzbach, P. J. (2006).Trends in global tropical cyclone activity over the past twenty years (1986–2005). Geophysical Research Letters 33 (10), p. L10805; doi:10.1029/2006GL025881.Knutson, T. R., and Tuleya, R. E. (1999). Increased hurricane intensities with CO2‐induced global warming as simulated using the GFDL hurricane prediction system. Climate Dynamics 15 (7), pp. 503–519.Landsea, T. R. (2005). Hurricanes and global warming. Nature 438, pp. E11–E12.Lau, K. M., and J. M. Kim (2007). How nature foiled the 2006 hurricane forecasts. Eos Trans. AGU, 88(9), 105–107.Pielke, R. A. Jr, et al. (2005). Hurricanes and global warming. Bulletin of the American Meteorological Society 86, pp. 1571–1575.Trenberth, K. (2005). Uncertainty in hurricanes and global warming. Science 308, pp. 1753–1754.4. Observing tropical cyclones and their environmentTropical cyclones and their associated environment (e.g. sea surface temperature, rainfall structure, wind field) are inherently difficult to observe because they are primarily ocean‐based systems that pose extreme hazards for in situ measurements. The following papers serve as an introduction to the methods, techniques, and platforms that are used to observe tropical cyclones and their environment:Scharroo, R., Smith, W. H. F., and Lillibridge, J. L. (2005). Satellite altimetry and the intensification of Hurricane Katrina. Eos, Transactions, American Geophysical Union 86 (40), p. 366.Velden, C., Daniels, J., Stetner, D., et al. (2005). Recent innovations in deriving tropospheric winds from meteorological satellites. Bulletin of the American Meteorological Society 86 (2), pp. 205–223.Lonfat, M., Marks, F. D., and Chen, S. S. (2004). Precipitation distribution in tropical cyclones using the tropical rainfall measuring mission (TRMM) microwave imager: a global perspective. Monthly Weather Review 132 (7), pp. 1645–1660.Aberson, S. D. (2003). Targeted observations to improve operational tropical cyclone track forecast guidance. Monthly Weather Review 131 (8), pp. 1613–1628.National Science Board. (2007). Hurricane warning: the critical need for a national hurricane research initiative. http://www.nsf.gov/nsb/III Topics for Further DiscussionThe following discussion questions are derived from Shepherd and Knutson's (2007) article.

Introductory
Advanced

IV Connecting Key Topics in Shepherd and Knutson (2007) with Lecture Topics

Introductory
Arguments for a link between global warming and hurricane activity
The role of observational studies
Atlantic SSTs and hurricanes: trends or cycles?

V Further Resources and Practical Exercises

Hurricane resource links
Student activitiesThe following activities are fun and instructive. Activity one is a simple introductory activity that allows a student print a hurricane tracking chart (Atlantic Basin only) and plot the progress of storms. The second activity will allow students to conduct more advanced research on tropical cyclone characteristics, genesis, and intensity using the self‐instruction guide in the link. The third activity will allow students to become familiar with and design climate change experiments using a real educational climate model (e.g. the student should attempt to design and run a double carbon dioxide simulation using Educational Global Climate Model (EdGCM) and discuss the results).
Introductory http://www.nhc.noaa.gov/AT_Track_chart.pdf
Advanced