Posted by Oliver Morton on behalf of Roger Pielke Jr.

Last week Kevin Trenberth offered a perspective on Nature Climate Feedback where he suggested that “clarity is actually emerging” in the scientific community on the relationship of greenhouse gas emissions and hurricane activity. My perspective on this debate differs from Kevin’s: Those in the tropical cyclone research community with different views on this subject remain as far apart as ever, and a community consensus beyond that presented in last year’s WMO report has yet to emerge.

Here are a few additional comments following Kevin’s piece:

1) The recent Holland (2007) paper in EOS dipped back into the same North Atlantic dataset that is the subject of intense debate to find that the area where storms have formed appears to have changed over time, observing that the dataset shows “the eastward spread of cyclone developments into the equatorial Atlantic.” Of course, those who argue that changes in observational systems are responsible for the increase in storm frequency found in the dataset can equally apply this logic to the location where storms are first observed. That is, they would argue that as observations have improved, more storms have been observed to have formed in the eastern Atlantic. Holland’s paper therefore provides ample support for both sides of the debate, which is not surprising since its analysis depends upon the exact same dataset that has been contested. Whether or not landfall rates are a constant proportion of basin activity remains a scientific frontier in hurricane climatology.

2) What has been neglected in the debate – by both sides – and is implicit in Holland (2007) is that whatever the cause of increased activity in the North Atlantic, the increase has taken place in the eastern Atlantic far from where landfalls occur (and this holds for storm counts and measures of intensity). On this point everyone seems to agree (see this exchange between ClimateAudit’s Steve McIntyre, who pointed out this trend on his blog and Georgia Tech’s Judy Curry, who is a collaborator with Holland). What this means is that if global warming has indeed led to an increase in North Atlantic hurricane activity, then it has necessarily (and ironically) had the effect of making the United States relatively safer from hurricanes as the proportion of total storms that makes landfall has decreased dramatically. (That is, landfalls show no trend while basin activity has increased due to global warming according to Mann et al. 2007 cited by Kevin). Imagine that headline – “Global Warming Protects U.S.from Increasing Landfalls”! Strange but true, but that is exactly the right message to take if Holland (2007) is taken at face value.

3) Kevin writes that none of the studies that he mentions cites Chang and Guo (2007), when in fact both studies that he references cite Chang and Guo. As I am sure that Kevin is aware, there are several other major studies of historical storm overcounts in the North Atlantic presently available in the community, which arrive at a range of conclusions which can be used to support either side of this debate. And there is also a major reanalysis project ongoing at NOAA as well. So it is perhaps a bit misleading to select one study as being “most definitive” at his point. On the historical undercounts there are various ways to assess whether they “matter” or not, and the Mann et al. (2007) paper cited by Kevin provides only one approach to this question. I have pointed out that if one accepts Chris Landsea’s estimates of historical storm undercounts, then one gets 529 total storms from 1905-1955 and 527 from 1956-2006, so it would seem to matter from that perspective. Obviously, the community’s lengthy arguments over this point would suggest at a very basic level that it does matter.

Debate over climate change and hurricanes exemplifies what Dan Sarewitz calls the “excess of objectivity” in science. Simply put, this means that in most any political issue involving science, there is an ample reservoir of knowledge from which partisans can selectively draw from to support a wide range of positions. Those skeptical of the human role in climate change are often criticized for cherrypicking science and selective presentation of science. And appropriately so. But these standards should be applied across the board, even to those who argue strongly for a human role in the climate system.

On the issue of climate change and hurricanes, clarity on the relationship of greenhouse gas emissions and hurricane behavior is not yet emerging, but the science is continuing and evolving in rapid fashion. The most recent study (what ever it is) will add to knowledge, but will not end debate. That will take some time.

Meantime, you can take a look at an exchange on this subject that Kevin and I were involved in last year, where we had similar views – Kevin’s that the debate was decided, and mine that it was still ongoing:

Pielke, Jr., R. A., C. Landsea, M. Mayfield, J. Laver and R. Pasch, 2005.

Hurricanes and global warming, Bulletin of the American Meteorological Society, 86:1571-1575. (PDF)

Comment by Anthes et al. 2006, Hurricanes and global warming: Potential linkage and consequences, BAMS, Vol. 87, pp. 623-628. (PDF)

Pielke, Jr., R. A., C.W. Landsea, M. Mayfield, J. Laver, R. Pasch, 2006.

Reply to Hurricanes and Global Warming Potential Linkages and Consequences, Bulletin of the American Meteorological Society, Vol. 87, pp. 628-631, May. (PDF)

— Roger Pielke Jr.