Katrina May Be A Harbinger Of Storms To Come, Science Author Says

Peter Webster, a top U.S. atmospheric scientist, discussed new research this week in an email interview with AAAS Senior Writer Edward W. Lempinen.

Let's start by jumping to the conclusion: What were the findings of your research team on the number and intensity of hurricanes globally? Did the North Atlantic-including the Caribbean and the Gulf of Mexico-differ in any way from what was seen elsewhere?

Earlier studies had indicated that there has been a steady increase in the number of hurricanes occurring each year in the North Atlantic Ocean from the 1960s onwards. Some attributed this increase to natural variability (oscillations in the climate system) and others, noting a steady increase in sea-surface temperature (SST), attributed the increase to global warming. We decided to extend the analysis to other parameters (the total number of hurricane days per year and the intensity) and to the other regions of the globe where hurricanes roughly occur.

In all of the other ocean basins the SST has increased steadily. But in the other ocean basins (the Northwest Pacific, where more hurricanes occur than elsewhere, the eastern North Pacific, the North and South Indian Oceans and the Southwest Pacific) there had not been a statistical increase in number of storms over the same period. However, the number of hurricane days sharply decreased in all basins (except the Atlantic) during the last 10-15 years. In the Atlantic, the number of hurricane days increased steadily over the period 1970-2004. Globally the number of hurricane days decreased from 600 to 400. In the Northwest Pacific Ocean, the decrease in hurricane days was from 300 to 180.

But the surprising result was that the intensity of hurricanes increased in all ocean basins, with categories 4 and 5 nearly doubling in occurrence between the 1970s and the 1990s. The largest increase in intensity has occurred in the Northwest Pacific Ocean. Although the smallest increase observed, the number of Category 4 and 5 hurricanes in the North Atlantic has increased by 54 percent between the '70s and the '90s.

On what evidence did the team base its findings?

We chose to limit our study to the period where there have been satellite observations (1970-2004). This has allowed a consistent data set of wind speeds estimated from satellite. We feel confident of the quality of the data because these satellite observations have been corroborated with aircraft hurricane penetrations in the North Pacific and the North Atlantic. Furthermore, the use of satellite data in all of the basins means that we have a globally consistent data set and can compare the statistics between one basin and another.

Do you have any insight into why the number of cyclones and cyclone days has decreased everywhere but the North Atlantic?

It is difficult to assess why the North Atlantic is an anomaly. Professor Bill Gray of Colorado State University believes that natural variability is responsible for all hurricane variability in the Atlantic. But his explanation doesn't explain the variability we note in the other ocean basins.

How strong is the evidence that increasing concentrations of CO₂ in the atmosphere, and resultant global warming, is contributing to the increasing intensity of North Atlantic hurricanes?

We have seen abundant evidence of warming in the atmosphere: retreats of glaciers and Artic summer ice, disappearing permafrost and, of particular relevance here, a steady increase of SST in all of the tropical ocean basins. Noting an increase in SST in the Atlantic, Kevin Trenberth of the National Center for Atmospheric Research speculated that there was a direct relationship the observed increases in frequency of hurricanes and, based on a theory that the hydrological cycle must speed up and increases in intensity should result. Our results are consistent with these speculations.

However, the length of the data record that we have available is insufficient to fully interpret the relationship that we observe between SST and hurricane intensity. Hence, we withhold the direct attribution to global warming because there are a number of issues that we do not understand about how hurricanes interact with the ocean or how hurricanes contribute to the heat balance of the planet. Specifically, we do not have enough data to be categorical about our understanding of the relationships between hurricane frequency and intensity, and hurricane frequency and SST.

There is some recent compelling evidence of causality from another source, though. Experiments with climate models at the NOAA Geophysical Fluid Dynamics Laboratory and in Tokyo using the Earth Simulator model have found that with a doubling of CO₂, there was an increase in intensity of hurricanes and, simultaneously, an overall decrease in frequency. These results are very much in accord with our findings.

Were these findings surprising? Or were they consistent with other research recently?

Before doing our global analysis, our expectation was that we would find that there would be similar changes in frequency and duration of hurricanes in the other ocean basins. One of the reasons that we had decided to do the study is that we were concerned that previous studies had looked only at the North Atlantic Ocean and had made statements pertaining to global warming. This "looking locally and inferring globally" concerned us because only 12 percent of hurricanes each year occur in the North Atlantic. So we were surprised to find that the hurricane trends in other basins were different from the North Atlantic. More surprising was the finding that in all other basins the number of hurricane days had decreased substantially while, in common with the Atlantic, there had been a decided shift to more intense hurricanes

Your paper clearly comes at an urgent time as scientists, policy-makers and the public try to understand Hurricane Katrina. I'm assuming that you were aware of the storm before it hit the Gulf Coast-maybe you watched it on television. If so, what were your thoughts about it at that time?

We followed the course of Katrina with great interest. First, the National Weather Service should be commended for issuing such a good forecast. Three days before landfall they predicted that an intense hurricane would hit New Orleans, including 20-30 ft storm surges. The director of the National Hurricane Center briefed federal, state and local officials of the forecast and of the potential damage to the area. Our thoughts were that the National Weather Service had provided emergency workers with all the information they needed to make reasoned risk analyses and to implement a risk management plan.

How would Katrina fit into the context of your findings? Can we look at it as the embodiment, or as a real-life manifestation, of your findings? Can we see it as a sort of proof-as a powerful piece of circumstantial evidence to back your findings?

It would be tempting to think that one hurricane was consistent with our statistical analysis. However, conclusions can't be drawn based upon a single storm. The significance of Katrina is that it has served as a wake-up call for the need to develop improved risk assessments and management plans to address the impacts of intense hurricanes.

Is there any way to gauge or predict how long this cycle of more intense hurricanes might last?

The variability of hurricane intensity is almost certainly a complex interaction of natural variability superimposed on a longer term warming trend. Natural cycles of variability (such as El Niño, North Atlantic Oscillation, Pacific Decadal Oscillation) individually have signals that are predominant in certain regions. However, the global signal that we are seeing is consistent with some component of the increase being associated with the long-term global warming trend. Given our current understanding of the warming trend and natural climate variability, it is very unlikely that we will return to the era of less intense hurricanes (seen in the 1960s and early 1970s) over the next two decades since there is no mechanism that we know of that would reduce the sea surface temperatures to 1970 levels short of massive volcanic eruptions.

Additional research and model simulations on understanding the role of the oceans in hurricane dynamics and the role of hurricanes in the global climate will provide further insights and will hopefully suggest some additional hypotheses that we can test with the observational data that we do have.

Given that increased intensity, what are the possible ramifications for policy-makers, generally? Would your research have any bearing, at least potentially, on decisions regarding the rebuilding of New Orleans and Gulf Coast?

The key inference from our study of relevance here is that storms like Katrina should not be regarded as a "once-in-a-lifetime" event in the coming decades, but may become more frequent. This suggests that risk assessment is needed for all coastal cities in the southern and southeastern U.S. for category 5 storms, and for the more northern cities (e.g. New York City) probably for category 3 storms (note the categorical risk for coastal cities at higher latitudes needs to be assessed using typhoon data from the Pacific). The southeastern U.S. needs to begin planning to manage the risk of category 5 hurricanes. There is an additional issue of communicating this increased risk to the public. Past strategies for "weathering the storm" will not work in the face of increased hurricane intensity.

More generally, what sort of hurricane activity should the Gulf Coast and the East Coast of the United States expect in the years ahead?

Besides the overall global trend of increasing hurricane intensity, the key issue of concern raised by our study is that the hurricane intensities in the North Atlantic for the last decade have been lower than elsewhere on the globe. It is likely that the differences among the different ocean basins is associated with natural variability. This implies that at some point within the next decade, there is the risk that the intensity of North Atlantic hurricanes could increase rapidly to the global average (with possibly a concurrent decrease in another ocean basin). The variation of Atlantic hurricanes relative to hurricanes in other basins in the context of known cycles of natural variability needs further investigation.

15 September 2005

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