September 21, 2007 12:00 Noon - 2:00 pm
Dirksen Senate Office Building, Room 106
Is there a scientific basis for anticipating that human-induced climate warming does and/or will affect hurricanes in some way, over and above natural climate variability? Do observations and model simulations support that expectation, or are there issues with data and observations that make the task of sorting out linkages more difficult? If the latter, what are the observational and data issues that continue to make this a challenging scientific problem? What do we know now that we did not know two years ago? What role do model simulations play in helping to sort out linkages, if any, between global warming and hurricanes, in the absence of data/observation or the presence of unreliable data/observations? How can we best develop a coordinated national effort to provide urgently required information for planning, community response and infrastructure development.
Dr. Anthony Socci, Senior Science Fellow, American Meteorological Society
Dr. Kerry Emanuel, Professor of Atmospheric Science, Massachusetts Institute of Technology, Cambridge, MA
Emanuel PowerPoint HTML Version
Dr. William K. M. Lau, Chief, Laboratory for Atmospheres, NASA/Goddard Space Flight Center, Greenbelt, MD
Lau PowerPoint HTML Version
Dr. Greg Holland, Director, Mesoscale and Microscale Meteorology Division, Earth and Sun Systems Laboratory, National Center for Atmospheric Research, Boulder, CO
Holland PowerPoint HTML Version
Dr. Gabriel Vecchi, Research Oceanographer, Climate diagnostics Group, Geophysical Fluid Dynamics Lab/NOAA, Princeton, NJ.
Vecchi PowerPoint HTML Version
Thomas R. Knutson, Research Meteorologist, Climate Dynamics and Prediction Group, Geophysical Fluid Dynamics Lab/NOAA, Princeton, NJ.
Overview of Hurricanes and Climate Change (a.k.a. global warming)
The understanding of climate change, including its effects on hurricanes, rests on three essential scientific techniques: theory, observation, and computational modeling. Each of these three approaches has unique strengths and limitations. In this talk, I will discuss the application of each of these to understanding the effect of climate change on hurricane activity and demonstrate that while each approach is compromised by uncertainties, taken together they present a persuasive picture of increasing hurricane risk as the climate warms.
Rainfall Extremes, Saharan Dust, Tropical Cyclones and Climate Change
Trends in tropical rainfall are more readily detectable in the form of changes in rainfall characteristics, rather than in rainfall total. From satellite data, we find that in the tropics there is a strong positive trend in extreme heavy and very light rains, coupled to a negative trend in moderate rain. Climatologically over tropical oceans, a large portion (over 60%) of most extreme heavy rainfall (top 5%) can be identified with those coming from tropical cyclones. Over the Atlantic, the contribution of tropical cyclones to heavy rain events has almost doubled in the last quarter century. Over the Pacific basin, the increase is lesser at about 10%. The differences in the basin may be related to the percentage change in the warm pool (SST> 28 ºC) areas in both oceans. Overall, tropical cyclones appear to be feeding more extreme rainfall events in the tropics in recent decades.
Saharan dust can affect tropical cyclones development, and may be a factor contributing to long-term hurricane statistics and possibly in seasonal hurricane forecasts. The Saharan Air Layer (SAL) can suppress tropical cyclogenesis through entrainment of hot, dry air into a developing cyclone, increasing stability and denying the developing system of its moisture supply. Saharan dust may also pre-condition the Atlantic, cooling the ocean surface through attenuation of solar radiation, during the early hurricane season. Additionally, differential radiative heating of the atmosphere by Saharan dust may induce changes in the large-scale circulation over the West Africa and Atlantic region. All these effects may provide a feedback on the coupled ocean-atmosphere system over the Atlantic, modulating the seasonal statistics of hurricanes. Analyses of satellite data and historical records show a more (less) active hurricane season is generally associated with less (more) Saharan dust over the Atlantic.
Global Warming and Hurricane Activity
The past century has seen North Atlantic hurricanes occurring in three periods of relatively stable frequency separated by sharp upward transitions. Each period has experienced 50% more hurricanes than the previous one and each was associated with a distinct change in eastern Atlantic sea surface temperatures (SSTs). After taking account of missing cyclones in earlier periods due to poor observing systems, we have experienced an 80-100% increase in hurricane frequency over since the early 1900s. Natural variability has contributed to some of the observed changes, but the compelling conclusion is that the overall increase has been substantially influenced by greenhouse warming. Superimposed on this increasing hurricane frequency is a completely independent oscillation in the proportions of major and minor hurricanes (compared to all storms). This oscillation has no distinguishable net trend and may arise largely from internal oscillations of the climate system. The period of enhanced major hurricane activity during 1945-1964 arose entirely from this oscillation. Unfortunately, the period since 1995 has experienced a double-whammy of a sharp increase in both numbers of hurricanes and the proportion of major hurricanes.
This heightened hurricane activity is unlikely to decrease in the future and we may see further increases. If so, current planning, building and coastal levee systems may prove to be inadequate, leading to more New Orleans-type disasters. The National Hurricane Research Initiative is designed to provide us with the tools to assess this future threat, to develop improved forecast and community response approaches, and to establish coastal planning approach to minimize the potential for future disasters. It is an initiative of critical national importance, which deserves strong and urgent support.
Long-term changes in Tropical Cyclone Activity: Looking Forward and Looking Back
To understand how human-induced climate change influences global and Atlantic tropical cyclone activity it is essential to have accurate records of past tropical cyclone variations and to model future climate conditions. The ways that tropical cyclones are measured have evolved over time, thereby influencing the homogeneity of the record. Statistical techniques can help, however, to estimate these deficiencies in the century-scale record. To project future conditions, global climate models (GCMs) – though not perfect – are our best tools. Although current computing power prevents GCMs from explicitly representing tropical cyclones, GCMs do indicate robust changes in many of the large-scale environmental conditions that are known to influence tropical cyclone activity, such as the thermodynamic structure of the atmosphere and vertical wind shear. Analyses of climate models and reconstructions of past tropical cyclone records indicate:
Observational evidence for century-scale changes in tropical cyclone activity is mixed, depending on the metric chosen, on the statistical correction applied to the data and on the time interval being examined.
Climate model projections of the Atlantic and East Pacific response to global warming exhibit mixed changes in cyclone-relevant parameters, with both an increase in thermodynamic potential intensity of tropical cyclones and an increase in vertical wind shear. More refined methods are needed to understand the detailed response of tropical cyclones to these environmental changes.
Outside of the Atlantic and East Pacific, projected changes to both the thermodynamic potential and the wind shear indicate conditions more favorable to tropical cyclone activity under global warming.
Although questions remain about the detailed response of tropical cyclone activity to human-induced climate change, we have relatively much greater confidence in the projected response of other large-scale climate conditions to increasing greenhouse gases (e.g., global warming, surface temperatures over land warm faster than over ocean, Arctic sea ice reduction, increase in ocean heat content, etc.).
Modeling the Response of Atlantic Hurricanes to Climate Variability and Change
A pressing question concerning ongoing global warming is whether human-caused warming of the planet has had any discernible impact on Atlantic hurricane activity. Confidence in any such a link is currently hampered by both data quality issues for the hurricane observational record and by limited work specifically targeting this question from a modeling perspective. Based on existing studies to date:
- Observed data, including consideration of data problems, give conflicting indications on whether there have been significant increases in Atlantic tropical storm and hurricane numbers. U.S. land-falling numbers have not increased. Models have not yet reproduced some reported long-term (~100 yr) increasing trends in basin-wide numbers.
- High resolution models consistently project increasing hurricane intensities and rainfall rates for the late 21st century, but whether there will be more or fewer hurricanes remains uncertain.
- A new modeling approach reproduces many important aspects of Atlantic hurricane activity observed since 1980, and thus shows promise as a tool for both understanding past variations and for making more reliable projections of future hurricane activity.
Dr. Kerry Emanuel is a professor of atmospheric science at the Massachusetts Institute of Technology, where he has been on the faculty since 1981, after spending three years as a faculty member at UCLA. Professor Emanuel's research interests focus on tropical meteorology and climate, with a specialty in hurricane physics. His interests also include cumulus convection, and advanced methods of sampling the atmosphere in aid of numerical weather prediction. He is the author or co-author of over 100 peer-reviewed scientific papers, and three books, including Divine Wind: The History and Science of Hurricanes, recently released by Oxford University Press and aimed at a general audience, and What We Know about Climate Change, published by the MIT Press.
Dr. William Lau is currently the Chief of the Laboratory for Atmospheres at NASA, Goddard Space Flight Center, and Adjunct Professor at Department of Meteorology U. of Maryland. His research work spans three decades and covers a wide range of topics including climate dynamics, tropical and monsoon meteorology, ocean-atmosphere interaction, and climate variability and change.
Dr. Lau has received numerous awards for his research and his scientific leadership, including among others, the AMS Meisinger Award in 1997; the John Lindsay Award,1998; the NASA Exceptional Science Achievement Award, 1991; the William Nordberg Award (GSFC highest award in Earth Sciences), 2002. He is a Goddard Senior Fellow, a fellow of the American Meteorological Society since 1988, and a fellow of the American Geophysical Union, 2007. Dr. Lau has published over 190 refereed papers, book Chapters in refereed journals. He is the principal author of a book “Intraseasonal Variability in the Tropical Ocean-Atmosphere System”, published in 2006. Dr. Lau received his B. Sc. in Physics and Mathematics from the University of Hong Kong, and his Ph.D. in Atmospheric Sciences from the University of Washington, Seattle.
Dr. Greg Holland is currently Director of the Mesoscale and Microscale Meteorology Division at the National Center for Atmospheric Research in Boulder, where he is involved scientifically with hurricane landfall, genesis and climate related work. He is a fellow of the American Meteorological Society as well as the Australian Meteorological and Oceanographic Society. Dr. Holland has several areas of research interests which have carried through to applications and include improved forecasting of tropical cyclone motion, scale interactions associated with cyclogenesis, establishment of field facilities, establishment of programs on coastal impacts of tropical cyclones and the development of Unmanned Aerial Vehicles (UAVs).
Dr. Holland has authored or co-authored more than 120 peer-reviewed scientific journal articles and book chapters, as well as dozens of planning documents, scientific prospectuses and workshop papers. He has given several hundred invited talks worldwide, as well as many contributed presentations at national and international conferences on hurricanes and related. He has also convened several national and international workshops, and served on several national and international committees and science-planning initiatives.
Dr. Gabriel Vecchi is a Research Oceanographer at the Geophysical Fluid Dynamics Laboratory (GFDL) in Princeton, New Jersey, where he has been working since 2003. GFDL, which is part of the National Oceanic and Atmospheric Administration (NOAA), is one of the world's leading climate modeling centers. Dr. Vecchi received a B.A. in Mathematics from Rutgers University, and an M.S. in Oceanography, an M.S. in Applied Mathematics and a Ph.D. in Oceanography from the University of Washington. His scientific research focuses on the interactions between the atmosphere and oceans on timescales from weeks to centuries. His recent research has focused on understanding long-term changes to tropical circulation and variability, including characterizing changes relevant to the possible impact of climate change on hurricanes.
Dr. Vecchi currently serves on the Climate Variability and Predictability (CLIVAR) Indian Ocean Panel, and is an Associate Editor of the Journal of the Atmospheric Sciences. His awards include the Presidential Early Career Award for Scientists and Engineers (PECASE), the American Geophysical Union’s Editor's Citation for Excellence in Refereeing for Geophysical Research Letters, and the Cook College, Rutgers University Marine Sciences Student of the Year. He has over 30 publications in peer-reviewed science journals or book chapters.
Thomas R. 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 modeling studies in major scientific journals on the potential impact of climate change on hurricanes. He now leads a project at GFDL aimed at simulating past and future Atlantic hurricane activity using regional high-resolution models.
He currently serves on the World Meteorological Organization (WMO) Expert Team on Climate Impacts on Tropical Cyclones, and was a major contributor to the December 2006 WMO "Statement on Tropical Cyclones and Climate Change". He is a member of a U.S. Climate Change Science Program (CCSP) committee developing an assessment report on "Weather and Climate Extremes in a Changing Climate," the AMS Climate Variability and Change Committee, and is an Associate Editor of the Journal of Climate. Mr. Knutson has over 30 publications in peer-reviewed science journals or book chapters.
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