Several studies have shown that hurricane activity is generally reduced during years when there is a thick aerosol haze over the subtropical Atlantic. The haze is comprised mainly of soil particles, stripped by wind erosion from the barren ground over the Sahara and Sahel. These particles are lifted into the atmosphere and carried by the Trade winds as far as the Caribbean and Amazon basin. Plumes of dust streaming off the African coast are easily recognized in satellite imagery, and were even described by Charles Darwin during his voyage on the Beagle.
Just as Typhoon Nargis has reminded us of the destructive power of tropical cyclones (with its horrible death toll in Burma–around 100,000 according to the UN), a new paper by Knutson et al in the latest issue of the journal Nature Geosciences purports to project a reduction in Atlantic hurricane activity (principally the ‘frequency’ but also integrated measures of powerfulness).
The close timing of the Knutson et al and Typhoon Nargis is of course coincidental. But the study has been accorded the unprecedented privilege (that is, for a climate change article published during the past 7 years) of a NOAA press conference. What’s the difference this time? Well, for one thing, the title of the paper: “Simulated reduction in Atlantic hurricane frequency under twenty-first-century warming conditions” (emphasis added).
When discussing the influence of anthropogenic global warming on hurricane or tropical cyclone (TC) frequency and intensity (see e.g. here, here, and here), it is important to examine observed past trends. As with all climate variables, the hurricane record becomes increasingly uncertain when we go back in time. However, the hurricane record has some peculiarities: hurricanes are highly confined structures, so you have to be at the right place at the right time to observe them. Secondly, hurricanes spend most of their life in the open oceans, i.e. in regions where there are very few people and no fixed observations. This means that the reliability of the long-term hurricane record is dependent on who was measuring them, and how, at any given time. The implementation of new observation methods, for example, might have altered the quality of the record considerably. But how much? This crucial question has been widely discussed in the recent scientific literature (e.g. Chang and Guo 2007, Holland and Webster 2007, Kossin et al. 2007, Landsea 2007, Mann et al. 2007). Where do we stand at the moment? This post will concentrate on the North Atlantic, which has the longest record.
If you are a RealClimate regular, you are undoubtedly aware of our ongoing interest in the developments in the scientific understanding of potential hurricane-climate change linkages. This is an area of the science where a substantial body of significant new research has emerged even since RealClimate’s inception in late 2004. The scientific research in this area, and the media frenzy and political theatrics that have inescapably followed it, are thoughtfully placed in a broader historical context in a fascinating new book by Chris Mooney entitled Storm World: Hurricanes, Politics, and the Battle over Global Warming. Anyone who is at all interested in the scientific history that has led to our current understanding of Hurricanes and their potential linkages with climate change, will find this book a page turner. The book is a nice complement to Kerry Emanuel’s recent book Divine Wind: The History and Science of Hurricanes (which too is so readable that it lies on our coffee table). Mooney in a sense picks up where Emanuel’s left off. Like Emanuel, he explores the history of the science. But he uses this historical context, and his studies of the personalities of key actors, to explore how the current scientific debate can be traced back to a rift that has emerged over many decades between distinct communities of atmospheric scientists. More »
A recent paper by Vecchi and Soden (preprint) published in the journal Geophysical Research Letters has been widely touted in the news (and some egregiously bad editorials), and the blogosphere as suggesting that increased vertical wind shear associated with tropical circulation changes may offset any tendencies for increased hurricane activity in the tropical Atlantic due to warming oceans. Some have even gone so far as to state that this study proves that recent trends in hurricane activity are part of a natural cycle. Most of this is just ‘spin’ (pun intended), but as usual, the real story is a little more nuanced. More »