W4400: Survival Guide – Hurricanes Prepared by Bali White and Emily Firth December 2006 I background information

“Though there is evidence both for and against the existence of a detectable anthropogenic signal in the tropical cyclone climate record to date, no firm conclusion can be made on this point”.

World Meteorological Organization (WMO) International Workshop on Tropical Cyclones,

Costa Rica, November 2006.

Despite extensive hurricane-related research conducted in the past few decades, a significant level of uncertainty still exists in the link, if any, between global warming and future hurricane projections. In recent times there has been increasing media attention and public interest in this discussion, most likely due to the number of recent high-impact tropical cyclone events around the globe (see Section I) which have left devastating socio-economic impacts in their wake but also due to a general increase in public awareness of human-induced global warming.

There is now significant evidence that tropical sea surface temperatures (SSTs) over most tropical basins have increased in magnitude by between 0.25 – 0.5 degrees C during the past several decades (WMO, 2006) and more substantial future warming is anticipated due to greenhouse forcing (IPCC, 2001).

Global Climate Models (GCMs) have been used to simulate hurricane activity under various conditions and with specific parameters to explore the relationship between hurricane activity and atmospheric and ocean physics. Projections of future hurricane trends have been made by comparing the results with observations and datasets of hurricanes over the past few decades, yet the confidence placed in the results is limited by a number of inherent uncertainties.

The uncertainties and limitations that make future projections difficult include:

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  Strong year-to-year and multi-decadal variability in SSTs and hurricane activity. It is not clear whether observed SST variability is attributable to natural physical phenomena such as the El-Nino Southern Oscillation (ENSO) and the Atlantic Multi-decadal Oscillation (AMO) which both have significant impacts on SST and cyclone activity, or due to anthropogenic activities and increases in greenhouse gas emissions, or resulting from a combination of both (WMO, 2006).
  
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  Significant changes in tropical cyclone wind-speed monitoring over the last few decades.
  
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  Variations in the methods used to monitor and record tropical cyclone events, both historically and regionally.
  
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  Temporal limitations, with reliable data from satellite information available only for the past 30 years.
  

Recently published papers report a significant positive correlation between tropical SST and tropical cyclone intensity over the past 30 years. Using a satellite derived dataset, Webster et al (2005) articulated this trend as a two-fold global increase in the number and proportion of hurricanes reaching categories 4 and 5 since 1970; while Emanuel (2005) reported a near doubling of Power Dissipation Index (PDI)¹ in the North Atlantic and North Pacific over the past 30 years.

It is expected that increases in intensity will be reflected in both maximum wind speeds and storm-related precipitation (IPCC, 2001; Webster et al, 2005; Emanuel, 2005; Curry et al, 2006).

2. Regions of hurricane origin likely to remain unchanged

Based on model simulations and projected climate scenarios under increased greenhouse gas concentrations, the regions of hurricane formation and impact are not expected to change significantly (WMO, 2006).

In its Third Assessment Report, the IPCC stated that there is currently no published evidence for significant long term increase in hurricane frequency. This is supported by the conclusion reached by Webster et al (2005) that ‘careful analysis of global hurricane data shows that, against a background of increasing SST, no global trend has yet emerged in the number of tropical storms and hurricanes’.

With the current level of uncertainty surrounding future hurricane trends based on global warming projections, the general consensus is that a more extensive global data record and a stronger understanding of the role of hurricanes in atmospheric and ocean physics is required. As articulated by Curry et al (2006) in their review of the politics and scientific debate surrounding hurricane activity and global warming, ‘progress on this topic requires multidisciplinary collaboration that includes hurricane researchers and forecasters, climate researchers and modelers, and oceanographers to address this complex scientific problem’.

IV Useful websites and resources

HURDAT: Atlantic basin hurricane database www.aoml.noaa.gov/hrd/hurdat

Intergovernmental Panel on Climate Change (IPCC) Third Assessment Report. Climate Change 2001: The Scientific Basis. http://www.grida.no/climate/ipcc_tar/wg1/index.htm

Wikipedia: The Free Encyclopedia www.wikipedia.org/wiki/Hurricanes

Curry, J. A., P. J. Webster, and G. J. Holland, 2006. Mixing politics and science in testing the hypothesis that greenhouse warming is causing a global increase in hurricane intensity, Bull. Amer. Met. Soc., 87, 1025-1037.

Emanuel, KA, 2005. Increasing destructiveness of tropical cyclones over the past 30 years, Nature, 433: 686-688.

IPCC, 2001. Climate Change 2001: The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change [Houghton, J.T., Y. Ding, D. J. Griggs, M. Noguer, P. J. van der Linden, X. Dai, K. Maskell, and C. A. Johnson (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA 881pp.

Knutson, TR, RE Tuleya and Y Kurihara, 1998. Simulated increase of hurricane intensities in a CO2-warmed climate, Science, 279 (5353): 1018-1020.

Webster, P.J., G. J. Holland, J. A. Curry, H.-R. Chang, 2005. Changes in tropical cyclone number, duration and intensity in a warming environment, Science, 309: 1844-1846.