31 July 2007

More hurricanes, anyone?

One of the more contentious debates regarding climate change impacts is the projected increase in tropical cyclone activity. There has already been a stormy debate over claims that such an increase has already been detected. A new paper opens the debate again.

The article below summarizes the findings...

Climate change has triggered three major shifts in the number of tropical storms that rise up in the North Atlantic, according to a new analysis of 20th century records.

The first change came in 1905, starting a 25-year period with an average of 6.0 tropical storms or hurricanes per year. In 1931, the number jumped to 9.4 per year, and stayed at that level until 1994. The last big shift came in 1995, starting a period through to 2005 with an average of 14.8 storms per year.

However, as pointed out later in the article...

...NOAA's National Hurricane Center says that improvements in monitoring and technology over the last century mean that storms that were not picked up by meteorologists in the past are no longer overlooked (PDF).

The image, from the PDF link above, shows one source of errors in the long-term data set. This error is likely one of many. The errors can vary in many ways. I would think that another 'biggie' would be the different analysis techniques used throughout the dataset. Earlier, ship reports are likely less consistent (if not less accurate) than later satellite estimates. I don't know what, if any steps they have taken in the paper to address such inhomogeneities in the data.


...the improved data from the last half of the century cannot be solely responsible for the increase.

"We are led to the confident conclusion that the recent upsurge in the tropical cyclone frequency is due in part to greenhouse warming, and this is most likely the dominant effect," the authors wrote.

I think that both sides make a reasonable case, and there is probably a bit of all three going on. That is, we are looking at some (unknown) combination of data inhomogeneities, inter-decadal climate variability and anthropogenic climate change. The answers lies in discovering that proportion, a fairly difficult task. The errors are real, and probably more myriad in their ways than described in the linked paper. Nonetheless, one would think that we could get some handle on how large they really are. There really is inter-decadal variability, and it does modulate climate. Anthropogenic climate change is also a reality and the behavior of the weather and climate (including tropical cyclones) over the recent past (say 10 yrs +) is generally consistent with the modeled impacts on extreme weather.

Long time series of homogeneous data would be a boon! Potentially, marine sediment cores from coral reefs provide such a source of data.

The recent increase in the number of major Atlantic hurricanes may just be a return to the norm after a period of unusually low storm frequency, say researchers.

...marine sediment cores of coral samples from the northeast Caribbean ... build a proxy record of wind shear and sea-surface temperatures since 1730, and from this they estimated hurricane activity since that time.

High wind shear – the difference in speed and direction between low winds that blow close to the Earth's surface and winds that are higher up in the atmosphere – causes a decrease in rainfall, which generates denser coral. This is because less rain runs off the land into the sea, raising the water's salinity and affecting the way coral polyps build their skeletons.

To me, the connection to hurricanes seem a bit stretched...I think that a lot could happen to confound the presumed relationship at any given location. There are two at least semi-independent variables that the link depends on. How are the two potential effects separated? The general representativeness of the data also needs to be understood perhaps through a combination with other proxy data sets from around the region. Still, the dataset is a good start, but a considerable amount of work remains for the data to be taken at face value.

The implications of this research are important in that they tell us something of the relationship of the climate models to the observed climate. The models suggest that the changes observable to date should be fairly small to date, and difficult to observe. If changes this large are completely associated with climate change, then the climate models used in the IPCC reports are too conservative. The reality on the ground during the next few years will clarify this picture enormously. One can hope that our climate outlook isn't as grim as would appear from these last few years of observations.

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