Do theory and models suggest that in a warmer world, the intensity of hurricanes is expected to increase? Are observations/measurements of hurricane intensity over the past 50 years consistent with models and theory? Is it reasonable to assume that hurricanes and tropical storms would be insensitive to a climaticallyperturbed atmosphere? Are there sufficient data/observations of sufficient quality to help map and resolve the impact global warming might have on hurricane intensity? Among a host of dynamic processes and variables affecting hurricane formation and intensity, how important a role does sea surface temperature (SST) play? What are the other important variables affecting hurricane formation and intensity? Has SST in the regions that spawn hurricanes and tropical storms increased in recent decades? Is this warming attributable to natural variability, anthropogenic greenhouse gas emissions or both? What might we anticipate in the realm of hurricanes in an even warmer world?
Dr. Anthony Socci, Senior Science Fellow, American Meteorological Society
Dr. James P. Kossin, Atmospheric Research Scientist, Cooperative Institute for Meteorological Satellite Studies, University of Wisconsin-Madison, WI
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Dr. Tom M. L. Wigley, Senior Scientist and Director of the Consortium for the Application of Climate Impact Assessments (ACACIA), National Center for Atmospheric Research (NCAR), Boulder, CO
Dr. Greg Holland, Director of the Mesoscale and Microscale Meteorology Division, Earth-Sun Systems Laboratory, National Center for Atmospheric Research (NCAR), Boulder, CO
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Dr. Thomas L. Delworth, leader of the Climate Dynamics and Prediction Group at the NOAA Geophysical Fluid Dynamics Laboratory (GFDL) in Princeton, New Jersey
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Hurricanes and Global Warming:
Data Issues of Importance – Dr. Kossin
As the climate changes, it is reasonable to expect associated changes in hurricane behavior. This expectation can be tested in two ways: 1) through improving our understanding of how global climate change modulates hurricane activity, and 2) through analyzing historical records of hurricane activity to see if there are trends or other behaviors in the data that can be related to known changes in climate. The former is challenging and requires an extended scientific research effort, and the latter is simple but requires a long record of high quality data. Unfortunately, the historical records of global hurricane activity suffer from a number of inconsistencies that make them less-than-perfect for this task, and they are particularly ill-suited for identifying long-term trends.
The inconsistencies in the historical hurricane records arise from changes in the way that hurricanes are measured. For example, aircraft reconnaissance into hurricanes began after the 1940’s, and satellite measurements began after the 1970’s. Prior to the satellite era, hurricanes were measured infrequently and prior to the reconnaissance era, hurricanes could be missed entirely. This lack of consistency is problematic when we are looking for behaviors and trends that we expect to occur over multidecadal timescales. To address this issue, we used historical satellite data to construct a more consistent global data record for the 23-year period 1983–2005, and we compared the new data to the existing records that are being used to measure these behaviors and trends. We found that the upward trend in the Atlantic and downward trend in the East Pacific are well-supported by the new data, but the trends everywhere else (Western and Southern Pacific, Northern and Southern Indian Oceans) are significantly inflated by poor data quality. Since SST is increasing in all basins, our results challenge the paradigm that increasing SST alone will cause a concomitant increase in hurricane activity, and underscore the need for research toward better physical understanding of the relationship between climate and hurricane activity.
Implications of Warming in the Hurricane-Spawning
Regions – Dr. Wigley
Previous research has suggested links between changes in sea surface temperature (SST) and hurricane intensity. In particular, when SSTs are warmer in the areas where hurricanes develop (tropical “cyclogenesis” regions), the intensity of these storms is increased. SSTs have already warmed in both the Atlantic and Pacific tropical cyclogenesis regions: observed SST increases in these regions range from 0.32 to 0.67C over the 20th century. A leading question, therefore, is whether this observed warming can be attributed to human factors – i.e., whether the warming is due to external forcing or whether it can be explained entirely as a result of natural factors internal to the climate system. We can answer this question using climate model results.
In our latest research we used climate models to investigate the possible causes of SST changes in the Atlantic and Pacific tropical cyclogenesis regions. The 22 climate models examined here demonstrate that century-timescale SST changes of the observed magnitude cannot be explained solely by unforced (natural) variability of the climate system. Furthermore, model simulations of changes over the 20th century due to the combined effects of both natural (solar and volcanic) and anthropogenic (greenhouse gases, aerosols, etc.) forcing factors, are generally capable of replicating observed SST increases. In experiments where forcing factors are varied individually rather than jointly, human-caused changes in greenhouse gases are the main driver of the 20th century SST increases in both the Atlantic and Pacific tropical cyclogenesis regions. The ability of models to simulate past SST changes, and the expectation of substantially larger changes in the future with attendant effects on tropical storm intensity, provides yet another reason for concern about the future effects of human activity on the climate system.
Trends and Variability in Atlantic Hurricane Activity – Dr. Holland
Hurricane variations in the North Atlantic region contain a mix of short and long-term variability superimposed on a distinct trend. The short term variability arises from interannual changes associated with large-scale circulation changes, such as the El Nino. Long-period variations in named storms and hurricanes over the past century are distinguished by relatively stable regimes separated by sharp transitions in the 1930s and 1990s. Each regime has seen 50% more cyclones and hurricanes than the previous one and each is closely associated with a distinct increase in eastern Atlantic sea surface temperatures. These sharp changes cannot be attributed to errors in the hurricane data base. The end result has been a substantial, 100-year trend leading to related increases of over 0.7oC in SST and over 100% in tropical cyclone and hurricane numbers.
Superimposed on the evolving tropical cyclone and hurricane climatology is a completely independent variation in the proportions of major and minor hurricanes (compared to all named storms). This has no distinguishable net trend, is associated with concomitant variations in the proportion of tropical and subtropical hurricane developments, 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. However, while there is no trend in the proportion of major hurricanes, the increasing cyclone numbers has lead to a distinct trend in the number of major hurricanes. Other presentations will show that the change in ocean temperatures is largely due to greenhouse warming, which leads us the compelling conclusion that the overall trend in named storms, hurricanes and major hurricanes is also substantially influenced by greenhouse warming.
Changes in the Tropical Atlantic of Relevance for Hurricanes: Natural Variability and Anthropogenic Climate Change – Dr. Delworth
Variations in tropical storm and hurricane activity in the Atlantic have been linked with changes in sea surface temperature (SST). The observed record of SST in the Atlantic is characterized by both a long term warming trend and superimposed multidecadal fluctuations. There is a substantial (and growing) body of evidence that the long-term trend is driven by increasing greenhouse gases. A crucial issue is the degree to which the multidecadal fluctuations in Atlantic SST are a response to human activity (via increasing greenhouse gases and aerosols) or a manifestation of natural variability of the climate system. For example, it is highly likely that the Atlantic warming over the last 100 years is mainly a response to increasing greenhouse gases. However, what is far less clear is the relative contribution of natural variability versus anthropogenic forcing to the evolution of Atlantic SST from the 1950s to the present.
Making progress in resolving this issue is a key priority in order to improve our understanding of the physical causes of recent changes in Atlantic hurricane activity, and potential future changes. Some recent work suggests that part of this multidecadal variability may be a response to human activity, through a combination of increasing greenhouse gases (which warm the ocean) and changing aerosols (which cool the ocean). However, evidence at this point is insufficient to rule out a substantial contribution from natural variability. Resolution of this issue requires sustained detailed measurements of the properties of the Atlantic Ocean, the radiative forcing acting on the ocean, and the development of climate models that can adequately represent the main processes responsible for Atlantic SST changes on time scales of decades and longer, as well as their interactions with atmospheric circulation. Such models can then be used for more physically based predictions of the evolution of the Atlantic over the next decade, with associated implications for hurricane activity.
Dr. James Kossin is an atmospheric research scientist at the Cooperative Institute for Meteorological Satellite Studies (CIMSS) at the University of Wisconsin – Madison. His area of specialization is tropical meteorology with an emphasis on hurricane studies. Jim received his B.S. and M.S. degrees in Mathematics and Physics at Clarkson University in New York, and his Ph.D. in Atmospheric Sciences at Colorado State University. His scientific interest in hurricanes began in 1987 and he was onboard the historic aircraft flight into the eye of Hurricane Gilbert (1988) that measured the greatest intensity ever recorded in the Atlantic Ocean (this record was recently broken in 2005 by Hurricane Wilma).
Dr. Kossin’s research interests range from theoretical hurricane dynamics to hurricane variability on time scales from daily to decadal, and his earlier theoretical predictions of hurricane behavior were recently verified by observations in Hurricane Isabel (2003). His recent work on improving measurements of global hurricane trends has been presented to various risk prediction groups and he has been invited to speak at the US Climate Change Science Program’s (CCSP) first meeting on the Synthesis and Assessment Report "Weather and Climate Extremes in a Changing Climate". Dr. Kossin has authored and co-authored over 20 scientific articles since receiving his Ph.D. in 2000.
Dr. Tom M.L. Wigley was formerly Director of the Climatic Research Unit at the University of East Anglia (Norwich, U.K.) and is currently a Senior Scientist at the National Center for Atmospheric Research. He was born and educated in Australia where he trained as a meteorologist with the Commonwealth Bureau of Meteorology. His Ph.D. is in Theoretical Physics. He is a Fellow of the American Meteorological Society (AMS), the American Association for the Advancement of Science (AAAS) and the Royal Society for the Arts (RSA). His main current interests include projections of future climate and sea-level change, carbon-cycle modeling, the interpretation of past climate changes (including the detection of anthropogenic influences), climate model validation, and the economic and policy aspects of mitigation (i.e., reducing the emissions of greenhouse gases) and other approaches to solving the global warming problem. He has contributed as an author to all Intergovernmental Panel on Climate Change assessments, and developed the MAGICC coupled gas-cycle/climate model that has been used to produce the primary temperature and sea level projections given in these assessments. He has published widely in the field of climatology and related sciences. He is the author of more than 200 refereed journal articles and book chapters and is one of the most highly cited scientists in the field.
Dr. Greg Holland is Director of the Mesoscale and Microscale Meteorology Division in the Earth-Sun Systems Laboratory at NCAR. He is a Fellow of the American Meteorological Society and the Australian Meteorological and Oceanographic Society. A graduate of the Australian University of New South Wales in Math and Physics, Dr. Holland obtained his MS and PhD in Atmospheric Science from Colorado State University. He has several areas of research interests including hurricanes and tropical meteorology, and unmanned aerial vehicles (he led the development and commercialization of the Aerosonde robotic aircraft). His current active interests are on scale interactions, including: environmental influences on hurricane development, the impact of global change on hurricanes, the role of hurricanes in the global circulation and bridging the divide between climate and weather modeling. His interests in this area cover both the research side and carrying promising results through to applications. His publications have included major contributions to six textbooks and forecast manuals, together with over 100 research papers in atmospheric sciences and UAVs.
Dr. Thomas L. Delworth is leader of the Climate Dynamics and Prediction Group at the Geophysical Fluid Dynamics Laboratory (GFDL) in Princeton, New Jersey, where he has been a research scientist since 1984. GFDL is part of the National Oceanic and Atmospheric Administration, and is one of the world's leading climate modeling centers. Dr. Delworth's research focuses on using coupled ocean-atmosphere models to study climate variability and change on time scales of decades to centuries, with particular emphasis on decadal variability of the Atlantic ocean and its impact on climate and climate change. Dr. Delworth has served on national and international scientific research committees, has been a contributing author for the Intergovernmental Panel on Climate Change, and has authored over 50 papers for scientific journals and books.
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