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Subject: G3) How might global warming change hurricane intensity, frequency,
and rainfall ?
Contributed by Chris Landsea (NHC)
In November 2006 the global community of tropical cyclone researchers and
forecasters as met at the 6th International Workshop on Tropical
Cyclones of the World Meteorological Organization in San Jose, Costa Rica.
They released a statement on the links between anthropogenic (human-induced)
climate change and tropical cyclones, including hurricanes and typhoons.
The following is a summary of their report.
- There have been a number of recent high-impact tropical cyclone
events around the globe. These include 10 landfalling tropical cyclones in
Japan in 2004, five tropical cyclones affecting the Cook Islands in a
five-week period in 2005, Cyclone Gafilo in Madagascar in 2004, Cyclone Larry
in Australia in 2006, Typhoon Saomai in China in 2006, and the extremely
active 2004 and 2005 Atlantic tropical cyclone seasons - including the
catastrophic socio-economic impact of Hurricane Katrina.
- Some recent scientific articles have reported a large increase in
tropical cyclone energy, numbers, and wind-speeds in some regions during the
last few decades in association with warmer sea surface temperatures. Other
studies report that changes in observational techniques and instrumentation
are responsible for these increases.
Consensus Statements by International Workshop on Tropical Cyclones-VI
(IWTC-VI) Participants :
- 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.
- No individual tropical cyclone can be directly attributed to climate
change.
- The recent increase in societal impact from tropical cyclones has largely
been caused by rising concentrations of population and infrastructure in
coastal regions.
- Tropical cyclone wind-speed monitoring has changed dramatically over the
last few decades, leading to difficulties in determining accurate trends.
- 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.
- 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.
- There is an inconsistency between the small changes in wind-speed
projected by theory and modeling versus large changes reported by some
observational studies.
- Although recent climate model simulations project a decrease or no change
in global tropical cyclone numbers in a warmer climate, there is low confidence
in this projection. In addition, it is unknown how tropical cyclone tracks or
areas of impact will change in the future.
- Large regional variations exist in methods used to monitor tropical
cyclones. Also, most regions have no measurements by instrumented aircraft.
These significant limitations will continue to make detection of trends
difficult.
- If the projected rise in sea level due to global warming occurs, then the
vulnerability to tropical cyclone storm surge flooding would increase.
A PDF version of the official report is available
here.
Revised July 2, 2007
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