A series of prominent papers has been recently published claiming a link between global warming and increasing power of Atlantic hurricanes. These papers became very prominent largely because of the large number of strong hurricanes that have hit the United States in recent years. But, is global warming really the cause?
The public is certainly divided on this one. On his blog, Roger Pielke, Jr. reports on a recent Gallup/CNN poll that queried Americans on their beliefs about the relationship between global warming and hurricanes. The question was posed “Thinking about the increase in the number and strength of hurricanes in recent years, do you think global warming has been a major cause, a minor cause, or not a cause of the increase in hurricanes?” 36 percent of the respondents answered “major cause,” 29 percent answered “minor cause,” and 30 percent thought that global warming played no role whatsoever in the upswing in recent hurricane activity (the remaining 5% must not have made up their minds yet).
Presumably, these opinions have been formed recently, because, until 2004’s four hits on Florida, hurricanes were not hot news. Since then, things have gotten much hotter. A year ago, several environmental groups corralled a few scientists to hold a press conference suggesting a between the burning of fossil fuels and stronger hurricanes. In response, leading hurricane-ologist Dr. Christopher Landsea pointed out that this opinion was not supported by the best available science. As one the of scientists publicly proclaiming the link was Dr. Kevin Trenberth, the lead-convening author of the currently in-the-works Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Landsea was especially miffed. The IPCC’s Third Assessment Report, released in 2001, specifically concluded that, “Changes globally in tropical and extra-tropical storm intensity and frequency are dominated by inter-decadal to multi-decadal variations, with no significant trends evident over the 20th century.” Without any new research, the IPCC’s Trenberth was touting an opinion that ran opposite to the findings of the IPCC itself. Landsea withdrew from any further participation in the preparation of the IPCC Fourth Assessment Report over Trenberth’s remarks.
More recently, this summer, three papers (Emanuel, 2005; Trenberth, 2005; Webster et al., 2005) were published in Nature and Science magazines that seemed to support Trenberth’s assertion, because of three observations: 1) measures of hurricane intensity have been increasing since about 1970, 2) sea surface temperatures (SST) have been increasing, and 3) greenhouse gases in the atmosphere have been increasing. The seemingly obvious logic is that increasing greenhouse gases are leading to rising sea surface temperatures which are leading to stronger hurricanes.
This chain of events is unproven; instead, it is merely asserted. In reality there is a somewhat cyclic behavior in the patterns of sea surface temperature in the Atlantic Ocean, called the Atlantic Multidecadal Oscillation (AMO), and is characterized by several decades-long periods when the SSTs in the tropical cyclone formation region in the Atlantic Ocean are either warmer or cooler than normal (Figure 1). The AMO has long been associated with hurricanes. From the mid-1920 to the late 1960s, the AMO was in a warm state, and hurricane activity in the Atlantic was high, from the late 1960s through 1994, the AMO was in a cool state and hurricane activity was correspondingly low. And in 1995, the AMO switched back to warm conditions and hurricane activity immediately increased. Thus, many hurricane specialists contend that this natural cycle is what is responsible for our current hurricane climate, rather than the effects of global warming. In fact, Chris Landsea began warning back in 1995, when the AMO switch was first observed, that hurricane activity in the Atlantic was going to pick up again, and remain high for possibly several decades. His warnings, which were in no way based upon global warming considerations, have proven correct.
Figure 1. The North Atlantic sea surface temperature anomalies related to the AMO index (source: Knight et al., 2005)
One problem with the natural-cycle theory is that we don’t have long-term observations that allow us to determine whether the AMO has been going on for some time (centuries or more), or whether, perhaps, it too is an effect of rising greenhouse gas levels—especially with regard to its recent behavior.
Until last week, that is. Geophysical Research Letters has just published a paper by a combination of climate modelers and paleoclimatologists testing this hypothesis. They found a realistic AMO signal in climate model run with constant level of external forcings (i.e. no changing greenhouse concentrations) for 1400 years. In fact, they found oscillations in SST patterns in the Atlantic that were similar to the AMO in the 20th century. They reported, “Our 1400 year model simulation exhibits multidecadal climate variability with a similar pattern and amplitude to that of the AMO in observations. Together with the similarity of the simulated 70-120 year period to the observed 65 year period, and the range of periods derived from paleodata (40-130 years), this suggests the model simulated a realistic AMO. Its presence over many centuries in the model supports the suggestion from observations and proxy data that the AMO is a genuine repeating mode of global-scale variability.”
The authors even go on the make a prediction that the AMO will stay in its current warm state for another couple of years and then slowly decrease over the next couple of decades. This should have the effect of lowering tropical North Atlantic SSTs and thus decreasing the frequency of intense hurricanes.
Between the early 1970s, when the AMO was most unfavorable for hurricanes, and now, when they are spinning up with reckless abandon, AMO-related temperatures in the Atlantic increased about 0.4°C. The total temperature rise in the Atlantic since the early 1970s is about 0.6°C, meaning that only a tiny 0.2°C may be because of global warming, instead of the AMO.
Let’s put that in perspective. Last year, Thomas Knutson and Robert Tuleya published a modeling study showing that a 2.0ºC increase in SST maximum hurricane wind speed of about 6 percent over eighty years. That’s ten times the non-AMO warming. That means global warming is likely to be responsible, right now, for, at best, an increase of about 0.6% in hurricane wind speeds—raising a decent hurricane of 120mph to 120.7mph, a change too small to measure.
Thus, there is simply no reliable scientific evidence that global warming is having a detectable influence on present day Atlantic hurricanes. So, the 30 percent of the Gallup/CNN poll respondents that answered “no role at all” are the ones who are likely closest to the truth.
Emanuel, K., 2005. Increasing destructiveness of tropical cyclones over the past 30 years. Nature, 436, 686-688.
Knight, J.R., et al., 2005. A signature of persistent natural thermohaline circulation cycles in observed climate. Geophysical Research Letters, doi:10.1029/2005GL024233.
Knutson, T., Tuleya, R., 2004. Impact of CO2-induced warming on simulated hurricane intensity and precipitation: Sensitivity t the choice of climate model and convective parameterization. Journal of Climate, 17, 3477-3495.
Trenberth, K., 2005. Uncertainty in hurricanes and global warming. Science, 308, 1753-1754.
Webster, P.J., et al., 2005. Changes in tropical cyclone number, duration, and intensity ion a warming environment. Science, 309, 1844-186.