GLOBAL WARMING AND TROPICAL CYCLONE INTENSITY (Large changes already occurring or small changes decades from now?) Based on “Statement on Tropical Cyclones and Climate Change” endorsed at Sixth WMO International Workshop on Tropical Cyclones in San Jose, Costa Rica on 30 November 2006 John McBride (Australia BoM) plus eight other scientists were the primary authors, and the Statement was endorsed by researchers and forecasters from 34 countries/regions Summary of conclusions intended for public release and the complete statement with references (only peer-reviewed publications were considered) are available on the Tropical Meteorology Research Program link at

BACKGROUND – PREVIOUS ASSESSMENTS Two previous International Workshops on Tropical Cyclones have resulted in statements on anthropogenic climate change and tropical cyclones. IWTC-IV resulted in Hendersen-Sellers et al. (1998 BAMS) Little if any increase in global number of tropical cyclones Small (less than 10%) increase in maximum intensity IPCC statement on climate change highlights increasing global sea-surface temperatures with increase of 0.5  C relative to long-term mean.

Source: NASA/GISS

CONTROVERSY BEGINS WITH NEW PAPERS DURING 2005 Emanuel: Substantial increase in the power (cube of the maximum wind speed integrated over life of cyclone) over the Atlantic and western North Pacific basins during the last 50 years

Emanuel (2005) Original PDI from Emanuel (2005) Revised PDI from Landsea ( updated)

CONTROVERSY BEGINS WITH NEW PAPERS DURING 2005 Webster et al.: Substantial (nearly 100%) increase throughout the globe in the proportion of the most severe (categories 4 and 5 on the Saffir-Simpson scale) tropical cyclones from 1975 to 2004, with a compensating decrease in the weaker systems.

Webster et al. (2005)

ARE LARGER ECONOMIC LOSSES NECESSARILY DUE TO GLOBAL WARMING? Large increases in economic damage and disruption have occurred in recent decades. Increasing coastal populations (and increasing insured values) have led to increased vulnerability. Normalizing the U. S. landfall economic losses by increasing population, increasing wealth, and sensitivity leads to the conclusions: –Storms of equivalent normalized losses have occurred in the past –No strong trend in normalized economic losses can be detected over the period (~ 100 years). Whereas losses of human lives have decreased in developed countries, large losses of lives will continue in developing countries as a result of population growth and societal factors. CONCLUSION 3. The recent increase in societal impact from tropical cyclones has largely been caused by concentrations of population and infrastructure in coastal regions.

COUNTER-ARGUMENTS TO CONTROVERSIAL ARTICLES (primarily questioning tropical cyclone data reliability) Landsea et al. (2006): Numerous tropical cyclones may have been missed in earlier record, even in the Atlantic basin.

Open Atlantic Ocean Differences 1933 Hurricane Season 2005 Hurricane Season

Satellite era - 58% strike land

Pre-satellite era - 77% strike land

COUNTER-ARGUMENTS TO CONTROVERSIAL ARTICLES (primarily questioning tropical cyclone data reliability) Landsea et al. (2006): Numerous tropical cyclones may have been missed in earlier record, even in the Atlantic basin. Early aircraft reconnaissance did not always penetrate to the center – rather center fixes were obtained at outer radii from radar images Kamahori et al. (2006): JMA typhoon data base – substantial decrease in number of category 4 and 5 typhoons. Satellite intensity estimates via Dvorak technique have changed as resolution has improved and interpretation procedures evolved Klotzback (2006): used consistent satellite imagery and found only a small (~10%) trend in category 4 and 5 tropical cyclones around the globe. Kossin et al. (in review) have consistently re-done satellite record

A globally consistent satellite-based reanalysis of hurricane intensity trends… Excellent agreement in the Atlantic and East Pacific! New record Old record Kossin et al. (in review)

Terrible agreement everywhere else! Kossin et al. (in review)

COUNTER-ARGUMENTS TO CONTROVERSIAL ARTICLES (primarily questioning tropical cyclone data reliability) Landsea et al. (2006): Numerous tropical cyclones may have been missed in earlier record, even in the Atlantic basin. Early aircraft reconnaissance did not always penetrate to the center – rather center fixes were obtained at outer radii from radar images Kamahori et al. (2006): JMA typhoon data base – substantial decrease in number of category 4 and 5 typhoons. Satellite intensity estimates via Dvorak technique have changed as resolution has improved and interpretation procedures evolved Klotzback (2006): used consistent satellite imagery and found only a small (~10%) trend in category 4 and 5 tropical cyclones around the globe. Kossin et al. (in review) have consistently re-done satellite record CONCLUSION 4. Tropical cyclone wind-speed monitoring has changed dramatically over the last few decades, leading to difficulties in determining accurate trends.

COUNTER-ARGUMENTS TO THE BASIS FOR AN INCREASING LONG-TERM TREND Chan (2006) extended Webster et al. (2006) for western North Pacific to earlier years and found recent trend is part of a large multi-decadal variation. Goldenberg et al. (2001) argue same for Atlantic. Mann and Emanuel (2006): dispute a multi-decadal oscillation exists and attribute decadal changes in Atlantic sea-surface temperatures to variations in radiative forcing (solar activity, volcanic and man-made aerosols, and greenhouse gasses).

In Atlantic, the frequency of storms is well correlated with tropical Atlantic SSTs Emanuel (2006); Mann and Emanuel (2006)

COUNTER-ARGUMENTS TO THE BASIS FOR AN INCREASING LONG-TERM TREND Chan (2006) extended Webster et al. (2006) for western North Pacific to earlier years and found recent trend is part of a large multi- decadal variation. Goldenberg et al. (2001) argue same for Atlantic. Mann and Emanuel (2006): dispute a multi-decadal oscillation exists and attribute decadal changes in Atlantic sea-surface temperatures to variations in radiative forcing (solar activity, volcanic and man-made aerosols, and greenhouse gasses). CONCLUSION 5. There is an observed multi-decadal variability of tropical cyclones in some regions whose causes, whether natural, anthropogenic or a combination, are currently being debated. This variability makes detecting any long-term trends in tropical cyclone activity difficult.

COUNTER-ARGUMENTS BASED ON INCONSISTENCY WITH THEORY Knutson and Tuleya (2004): have used a regional numerical model to estimate the increase in maximum intensity associated with greenhouse warming of the ocean

Summary of simulations: Hurricanes are more intense for warmer climate conditions … Note: Min. central pressures are averages over day 5 of integrations. Source: Knutson and Tuleya, J. Climate, 2004.

COUNTER-ARGUMENTS BASED ON INCONSISTENCY WITH THEORY Knutson and Tuleya (2004): have used a regional numerical model to estimate the increase in maximum intensity associated with greenhouse warming of the ocean CONCLUSION 6. It is likely that some increase in tropical cyclone peak wind speed and rainfall will occur if the climate continues to warm. Model studies and theory project a 3-5% increase in wind speed per degree Celsius increase of tropical sea-surface temperatures. Webster et al. (2005): intensity increase is five times larger than the Knutson and Tuleya study would suggest. Emanuel (2005): Intensity increase is eight times larger than the Knutson and Tuleya study would suggest. CONCLUSION 7. There is an inconsistency between the small changes in wind speed projected by theory and modeling versus large changes reported by some observational studies.

WILL GLOBAL WARMING LEAD TO MORE GLOBAL TROPICAL CYCLONES? Some recent medium- and higher-resolution climate models suggest fewer tropical cyclones in a warmer climate. Some tropical cyclone regions have more cyclones, and some have less cyclones.

Future changes in the frequency of tropical storms Ocean basin modelreference GlobalNAWNPENPNISISWP T106 JMA 10y ~120km Sugi et al T42 NCAR CCM2 10y Tsutsui N144 HadAM3 15y McDonald et al T106 ~120km CCSR/NIES/FRCGC Hasegawa and Emori T106 ~120km JMA 10y Yoshimura & Sugi 05 fewer T63 ECHAM5-OM Bengtsson et al ~20km MRI/JMA Oouchi et al Ratio (%) of number of storms in global warming experiment to number in control experiment Red = significantly more tropical storms in the future simulation Blue = significantly fewer tropical storms in the future simulation Summary: fewer tropical storms globally in the future simulations, sign of regional changes varies between model and basin © Crown copyright 2006.

WILL GLOBAL WARMING LEAD TO MORE GLOBAL TROPICAL CYCLONES? Some recent medium- and higher-resolution climate models suggest fewer tropical cyclones in a warmer climate. Some tropical cyclone regions have more cyclones, and some have less cyclones. Due to coarse resolution, climate models produce only weak tropical cyclone-like circulations; the cyclogenesis process in these models may (likely) differ from reality. CONCLUSION 8. Although recent climate model simulations project a decrease or no change in global tropical cyclone numbers in a warmer climate, there is a low confidence in this projection. In addition, it is unknown how tropical cyclone tracks or areas of impact will change in the future.

PRESENT UNDERSTANDING OF TROPICAL CYCLONES AND ANTHROPOGENIC CLIMATE CHANGE CONCLUSION 1. Although 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 at this time. Counter arguments to the controversial articles of Emanuel (2005) and Webster et al. (2005) have demonstrated that the tropical cyclone data base does not support the conclusions of those studies. CONCLUSION 2. No individual tropical cyclone can be directly attributed to climate change. My Restatement: Given the present understanding of the expected intensity and rainfall response to the 0.25   C increase in tropical sea-surface temperatures, and given our present ability to observe the winds and precipitation in tropical cyclones, any unusual wind or precipitation in an individual tropical cyclone can not be attributed to global warming. My statement: It is incorrect to state that 7% of the rainfall in Hurricane Katrina can be attributed to global warming.

WHY FURTHER RESEARCH IS REQUIRED Just because the existing tropical cyclone record does not support the proposed substantial increases in the Emanuel and the Webster et al. studies, one should not conclude that significant natural climate variability is not occurring. Unusual tropical cyclones are occurring. Multi-decadal variability in tropical cyclone numbers exists in the Atlantic and in the western North Pacific El Nino Southern Oscillation affects tropical cyclone activity in ways that are not completely understood. CONCLUSION 10. If the projected rise in sea level due to global warming occurs, then the vulnerability to tropical cyclone storm surge flooding would increase.