More Intense Tropical Cyclones: Likely Impact of Global Warming

Meterologists from around the world gathered in Cape Town, South Africa, this week.

At the World Meterological Conference a report was tabled summarizing information on the impact of global warming on cyclones including hurricanes and typhoons.*

Titled Statement on Tropic Cyclones and Climate Change, two of the nine authors are from the Australian Bureau of Meterology.

Julian Heming from the United Kingdom Met office gave the following summary in a media release:

“The main conclusion we came to was that none of these high-impact tropical cyclones could be specifically attributed to global warming. Whilst there is no conclusive evidence that climate change is affecting the frequency of tropical cyclones worldwide, there is an ongoing debate as to whether it is affecting their intensity.

The report is unusual, in so much as these type of documents associate with climate conferences are often written in such a way that they exaggerate the likely impact of global warming. This report seems to represents a middle ground and acknowledges there is no consensus on the issue of increased intensity.** It does acknowledges a potential impact from global warming and the likely nature of this impact – more severe hurricanes.

The report also includes comment that:

1. While demographic trends [more people living in more hurricane prone coastal environmentals] are the dominant cause of increasing damage by tropical cyclones, any significant trends in storm activity would compound such trends in damage.

2. Projected rises in global sea level are a cause for concern in the context of society’s vulnerability to tropical cyclones. In particular for the major cyclone disasters in history the primary cause of death has been salt-water flooding associated with storm surge.

3. A robust result in model simulations of tropical cyclones in a warmer climate is that there will be an increase in precipitation [rainfall] associated with these systems (for example, Knutson and Tuleya, 2004). The mechanism is simply that as the water vapor content of the tropical atmosphere increases, the moisture convergence for a given amount of dynamical convergence is enhanced. This should increase rainfall rates in systems (viz tropical cyclones) where moisture convergence is an important component of the water vapor budget. To date no observational evidence has been found to support this conclusion; so no quantitative estimate can be given for the anticipated rainfall increase without further research.

I interprete this last point to mean that as it gets warmer it is likely to get wetter?

The report includes the following summary of tropical cyclone activity during 2004 and 2005 and notes that a number of high-impact tropical cyclones events occurred during this period:

1. Ten fully developed tropical cyclones made landfall in Japan in 2004, causing widespread damage.

2. Southern China experienced much below-normal tropical cyclone landfalls and subsequently suffered a severe drought.

3. Four major hurricanes caused extensive damage and disruption to Florida communities in 2004.

4. In March 2004 southern Brazil suffered severe damage from a system that had hurricane characteristics, the first recorded cyclone of its type in the region.

5. Five fully developed cyclones passed through the Cook Islands in a five week period in February-March 2005.

6. The 2005 North Atlantic Hurricane Season broke several records including number of tropical cyclones, number of major hurricanes making landfall and number of category five hurricanes. In particular, the landfall of Hurricane Katrina at New Orleans and Mississippi caused unprecedented damage and more than 1300 deaths.

……………………………………………

*A hurricane is a cyclone in the Atlantic Basin and North Pacific east of the dateline. A typhoon is a cyclone in the Northwest Pacific west of the date line.

** This is what the report says on the issue of cyclone intensity:

“No single high impact tropical cyclone event of 2004 and 2005 can be directly attributed to global warming, though there may be an impact on the group as a whole;

– Emanuel (2005) has produced evidence for a substantial increase in the power of tropical cyclones (denoted by the integral of the cube of the maximum winds over time) during the last 50 years. This result is supported by the findings of Webster et al (2005) that there has been a substantial global increase (nearly 100%) in the proportion of the most severe tropical cyclones (category 4 and 5 on the Saffir-Simpson scale), from the period from 1970 to 1995, which has been accompanied by a similar decrease in weaker systems.

– The research community is deeply divided over whether the results of these studies are due, at least in part, to problems in the tropical cyclone data base. Precisely, the historical record of tropical cyclone tracks and intensities is a byproduct of real-time operations. Thus it’s accuracy and completeness changes continuously through the record as a result of the continuous changes and improvements in data density and quality, changes in satellite remote
sensing retrieval and dissemination, and changes in training. In particular a step-function change in methodologies for determination of satellite intensity occurred with the introduction of geosynchronous satellites in the mid to late 1970’s.

– The division in the community on the Webster et al and on the Emanuel papers is not as to whether Global Warming can cause a trend in tropical cyclone intensities. Rather it is on whether such a signal can be detected in the historical data base. Also it can be difficult to isolate the forced response of the climate system in the presence of substantial decadal and multi-decadal natural variability, such as the Atlantic Multi-decadal Oscillation.

– Whilst the existence of a large multi-decadal oscillation in Atlantic tropical cyclones is still generally accepted, some scientists believe that a trend towards more intense cyclones is emerging. This is a hotly debated area for which we can provide no definitive conclusion. It is agreed that there is no evidence for a decreasing trend in cyclone intensities.”

Update

Following comment and advice from readers of this blog I added the word ‘tropical’ to the title of this blog post.

Jennifer, 8pm. 22nd Feb.