Infamous hurricanes like Sandy and Katrina will be on the minds of many when the nation observes Hurricane Preparedness Week, and questions will continue to swirl around climate change and how it will, or already has, influenced hurricanes. Unfortunately, the scientific community is still searching for many of those answers. Scientists need a substantially long period of hurricane observations to detect any trends or changes in hurricane activity and there simply isn't enough reliable data to analyze - especially before 1966 when satellite observations began.
Image of Hurricane Floyd courtesy of NOAA.
In addition, the trends that scientists have been able to identify are relatively short-lived and dominated by high degrees of inter-annual and multi-decadal variability, both in terms of hurricane frequency and intensity. Therefore, scientists aren't sure if any of the past changes in tropical cyclone activity have exceeded the level they'd expect from natural forces. However, despite these limitations, scientists have still made progress by using novel techniques and research approaches to find answers to these questions. Some of their most robust findings about tropical cyclone behavior are listed below.
- Frequency: Even though several studies have claimed that Atlantic tropical cyclones have increased substantially over the past century, closer investigations have revealed that most of this jump can be explained by how scientists detect tropical cyclones. Before 1966, scientists used ships to find hurricanes and there weren't enough around to catch every storm. When satellites became available, less storms went undetected so the number of storms seemed to spike. Even in the satellite age, no significant changes in global tropical cyclone counts have been detected from 1970 to 2004. Scientists have improved the "hindcast" ability of sophisticated computer models, but replicating past climates with insufficient data only yields inferences that aren't as definitive as hard, observational data.
- Intensity: The global number of Category 4 and 5 hurricanes has nearly doubled from 1975 to 2004, but many scientists have cautioned that this time period is not sufficiently long to account for the considerable multi-decadal variability of the North Pacific. Therefore, it's especially difficult to ascribe this change to natural variability or higher greenhouse gas concentrations. Again, better data coverage would greatly enhance science's ability to detect trends and find their causes.
(Source: Thomas R. Knutson et al. 2010, "Review Article: Tropical cyclones and climate change," Nature Geoscience, doi: 10.1038/NGEO779.)