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Annual Public Lectures Series > Thomas Knutson

Scientists and Journalists:
Getting the Point Across
June 9-13, 2008

Tuesday, June 10, 3:30 p.m.
Hurricane Intensity: Warming Up the Debate
Have Humans Affected the Atlantic Hurricane Climate?

Tom Knutson, Research Meteorologist, Geophysical Fluid Dynamics Laboratory, National Oceanic and Atmospheric Administration

Summary

According to Tom Knutson, it is premature to conclude that humans have had a noticeable effect on Atlantic hurricane climate. This is the short answer to the big question raised in his lecture for Metcalf Institute at URI Graduate School of Oceanography.

The National Oceanic and Atmospheric Administration's Geophysical Fluid Dynamics Laboratory (GFDL), with Knutson at the lead, has created a computer model to better understand the frequency and intensity of past Atlantic hurricanes and possible future trends.

Knutson began his talk with the evidence put forth by the Intergovernmental Panel on Climate Change that the earth is warming significantly due to human influence. Knutson first presented model simulations of global temperature changes due to solar and volcanic activity alone and then with the additional influence of greenhouse gases. The results clearly showed that the earth's warming correlates with the warming simulated by models due to an increase in greenhouse gas emissions driven by human activities.

When sea surface temperatures are warmer, the most intense typhoons (or hurricanes) develop. If sea surface temperatures increase in the region where Atlantic hurricanes form and intensify, will hurricane intensity and frequency increase in the 21st Century, as the climate warms?

Knutson and his colleagues began looking at the Atlantic storm database to answer this question. They hypothesized that improvements in technology since 1965 may have led to an increase in storm recording - and by comparing modern-day storm tracks with historical ship tracks, they were able to estimate how many storms may have been missed before satellite technology. The long-term trends (since 1878) in the adjusted numbers of tropical storms and hurricanes in the Atlantic basin were "statistically no different from zero."

The high-resolution regional model that the GFDL created for predicting storm frequency and intensity during hurricane season accounts for sea surface temperature and general atmospheric conditions. In looking at past years, the model captured both the increase in hurricane activity since the 1980s and the year-by-year fluctuations in number of hurricanes. The control model reproduces a close relationship between sea surface temperatures and hurricane frequency for 1980 through 2006. However, this statistical relationship does not hold for future human-caused warming in the model.

According to Knutson, the model suggests that there is a climate-dependent threshold temperature for hurricane formation, which means that as the global sea surface temperatures increase, the threshold temperature at which hurricanes typically form also increases.

The question of model dependence is a factor when predicting future hurricane frequency and intensity. Out of the fourteen global climate models that the GFDL team analyzed with their regional model, the average global model projection calls for a slight decrease in Atlantic major hurricane counts by the late 21st Century. This number varies by model; in fact, one global model's projection implies a 70% increase in Atlantic major hurricane counts. This illustrates that further studies must narrow the uncertainty regarding details of future regional sea surface temperatures, wind shear, and other factors which may influence Atlantic hurricane activity.

Knutson noted that there is a significant computing expense for running these models. In fact, current limits on model resolution, due to limited computing resources, is one reason why the hurricanes simulated using even their high resolution regional model are generally weaker than observed and do not reach category 4 or 5 intensity. For these and other reasons, Knutson added, further studies are needed before we can confidently attribute past changes in hurricane activity to increased greenhouse gases or other human- caused factors.

At the end of the lecture, reflecting on Knutson's final conclusion, Metcalf director Sunshine Menezes pointed out that "It is important to realize what we don't know." Information about Knutson's studies are found on the web at: http://www.gfdl.noaa.gov/~tk.

Bio

Thomas Knutson has been a Research Meteorologist at the Geophysical Fluid Dynamics Laboratory (GFDL) in Princeton, New Jersey since 1990. GFDL, which is part of the National Oceanic and Atmospheric Administration, is one of the world's leading climate modeling centers. Mr. Knutson has authored several major studies in leading scientific journals on the potential impact of climate change on hurricanes. He now heads a project at GFDL aimed at simulating past and future Atlantic hurricane activity using regional high-resolution models. He currently serves as Co-Chair of the World Meteorological Organization (WMO) Expert Team on Climate Impacts on Tropical Cyclones, and was a key contributor to the December 2006 WMO "Statement on Tropical Cyclones and Climate Change." He is a lead author on the U.S. Climate Change Science Program (CCSP) assessment report on "Weather and Climate Extremes in a Changing Climate," a member of the American Meteorological Society's (AMS) Climate Variability and Change Committee, and an Associate Editor of the Journal of Climate. He has been an invited speaker on hurricanes and climate change at science meetings and workshops, universities, and in the private sector. Mr. Knutson has over 35 peer-reviewed science publications.

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August 7, 2008