The Summary for Policymakers in the 2007 United Nations Intergovernmental Panel on Climate Change (IPCC) states “There is observational evidence for an increase in intense tropical cyclone activity in the North Atlantic since about 1970, correlated with increases of tropical sea surface temperatures. There are also suggestions of increased intense tropical cyclone activity in some other regions where concerns over data quality are greater. Multi-decadal variability and the quality of the tropical cyclone records prior to routine satellite observations in about 1970 complicate the detection of long-term trends in tropical cyclone activity. There is no clear trend in the annual numbers of tropical cyclones.” Fair enough, but as we have written about repeatedly in World Climate Report, the scientific literature is full of articles disputing the claim that tropical cyclones are increasing in intensity in recent decades beyond where natural variations would have taken them.
We agree with the IPCC that the detection of trends in tropical cyclone activity is complicated by the lack of long-term records. Don’t look now, but an article has appeared in the prestigious journal Nature entitled “Intense hurricane activity over the past 5,000 years controlled by El Niño and the West African monsoon.” The title suggests that someone has a 5,000 year record of hurricane activity and that the activity is controlled by El Niño and weather in West Africa – there is no suggestion that hurricane activity is controlled by greenhouse gases, planetary temperature, or sea surface temperatures in the Atlantic.
Jeffrey Donnelly and Jonathan Woodruff of the Woods Hole Oceanographic Institution in Massachusetts begin their article noting that “At present there is significant debate about the cause of observed multi-decadal variability of hurricanes in the North Atlantic. To detect long-term patterns in tropical cyclone activity, reliable proxy reconstructions that extend back before the instrumental record are needed.” Finding proxy records of intense hurricane activity requires some imagination and some help from Mother Earth, and the pair found exactly what is needed on Puerto Rico’s island of Vieques.
Imagine a nice tropical beach backed by a vegetated barrier ridge about 10 feet tall. Behind the ridge is a back-barrier lagoon that over time becomes a playa that is only periodically under water. During large hurricane events, which are common in Puerto Rico, the ridge is breached and a large amount of material is deposited on the playa. Donnelly and Woodruff extracted cores from the playa, and they note that “Cores collected from the site contain several metres of organic-rich silt interbedded with coarse-grained event layers comprised of a mixture of siliciclastic sand and calcium carbonate shells and shell fragments. These layers are the result of marine flooding events overtopping or breaching the barrier and transporting these barrier and nearshore sediments into the lagoon.” Organic material can be dated, and just like magic, a long-term record of intense hurricane activity is produced.
The pair finds that “On the basis of our age model an interval of relatively frequent intense hurricane strikes at Vieques is evident between 5,400 and 3,600 calendar years before present (yr BP, where present is defined as 1950 AD by convention), with the exception of a short-lived quiescent interval between approximately 4,900 and 5,050 yr BP. Following this relatively active period is an interval of relatively few extreme coastal flooding events persisting from 3,600 until roughly 2,500 yr BP. Evidence of another relatively active interval of intense hurricane strikes is evident between 2,500 and approximately 1,000 yr BP. The interval from 1,000 to 250 yr BP was relatively quiescent with evidence of only one prominent event occurring around 500 yr BP. A relatively active regime has resumed since about 250 yr BP (1700 AD).”
With respect to the linkage between higher sea surface temperatures (SSTs) and hurricane activity, the pair notes that “Given the increase of intense hurricane landfalls during the later half of the Little Ice Age, tropical SSTs as warm as at present are apparently not a requisite condition for increased intense hurricane activity. In addition, the Caribbean experienced a relatively active interval of intense hurricanes for more than a millennium when local SSTs were on average cooler than modern.” They found that hurricane activity over the past 5,000 years has been modulated by the El Niño / La Niña cycle and the strength of the West African monsoon, not by the sea surface temperatures in the Atlantic and certainly not by global temperatures.
Should an unusually intense hurricane spin-up in the Atlantic during 2007, global warming advocates will be in front of every camera in sight claiming we are witnessing another manifestation of global warming. That is an interesting “spin,” but as we see in the recent Donnelly and Woodruff article, that Mother Nature may play a larger role in the process than she’ll be given credit for.
Donnelly, J.P., and J.D. Woodruff. 2007. Intense hurricane activity over the past 5,000 years controlled by El Niño and the West African monsoon. Nature, 447, 465-468.