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Global Warming and Hurricanes
1. New WMO/IWTC Statement on Tropical Cyclones and Climate Change
Given the high degree of interest in the possible relationship between climate change and tropical cyclones (including hurricanes and typhoons), a new summary statement on the topic has been developed by the global community of tropical cyclone researchers and forecasters as represented at the 6th International Workshop on Tropical Cyclones of the World Meteorological Organization (November 2006). A more comprehensive statement was also developed at the workshop.
The summary statement notes the following: "The surfaces of most tropical oceans have warmed by 0.25-0.5 degree Celsius during the past several decades. The Intergovernmental Panel on Climate Change (IPCC) considers that the likely primary cause of the rise in global mean surface temperature in the past 50 years is the increase in greenhouse gas concentrations....
...Some recent scientific articles have reported a large increase in tropical cyclone energy, numbers, and wind-speeds in some regions during the last few decades in association with warmer sea surface temperatures. Other studies report that changes in observational techniques and instrumentation are responsible for these increases."
Consensus statements by the workshop participants
"1. Though there is evidence both for and against the existence of a detectable anthropogenic signal in the tropical cyclone climate record to date, no firm conclusion can be made on this point.
2. No individual tropical cyclone can be directly attributed to climate change.
3. The recent increase in societal impact from tropical cyclones has been largely caused by rising concentrations of population and infrastructure in coastal regions.
4. Tropical cyclone wind-speed monitoring has changed dramatically over the last few decades leading to difficulties in determining accurate trends.
5. There is an observed multi-decadal variability of tropical cyclones in some regions whose causes, whether natural, anthropogenic or a combination, are currently being debated. This variability makes detecting any long-term trends in tropical cyclone activity difficult.
6. It is likely that some increase in tropical cyclone peak wind-speed and rainfall will occur if the climate continues to warm. Model studies and theory project a 3-5% increase in wind-speed per degree Celsius increase of tropical sea surface temperatures.
7. There is an inconsistency between the small changes in wind-speed projected by theory and modeling versus large changes reported by some observational studies.
8. Although recent climate model simulations project a decrease or no change in global tropical cyclone numbers in a warmer climate there is low confidence in this projection. In addition, it is unknown how tropical cyclone tracks or areas of impact will change in the future.
9. Large regional variations exist in methods used to monitor tropical cyclones. Also, most regions have no measurements by instrumented aircraft. These significant limitations will continue to make detection of trends difficult.
10. If the projected rise in sea level due to global warming occurs, then the vulnerability to tropical cyclone storm surge flooding would increase."
The full texts of the summary statement and comprehensive statement should be consulted for more details and context.
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2. NOAA State of the Science: Atlantic Hurricanes and Climate
This link is to a document representing the state of the science on Atlantic hurricanes and climate as developed by numerous NOAA researchers.
**** END OF MATERIAL RELATED TO THE NOAA DOCUMENT *****
Fig. 1. Comparison of simulated hurricane intensities. (more)
3. An Overview of GFDL Research on Global Warming and Hurricanes
The strongest hurricanes in the present climate may be upstaged by even more intense hurricanes over the next century as the earth's climate is warmed by increasing levels of greenhouse gases in the atmosphere. Although we cannot say at present whether more or fewer hurricanes will occur in the future with global warming, the hurricanes that do occur near the end of the 21st century are expected to be stronger and have significantly more intense rainfall than under present day climate conditions. This expectation (Figure 1) is based on an anticipated enhancement of energy available to the storms due to higher tropical sea surface temperatures.
The results shown in Figure 1 are based on a simulation study carried out by Thomas R. Knutson and Robert E. Tuleya at NOAA's Geophysical Fluid Dynamics Laboratory (GFDL). In this study hurricanes were simulated for a climate warming as projected to occur with a substantial build-up of atmospheric CO2. An increase of intensity of about one-half category on the Saffir-Simpson scale was simulated for an 80 year build-up of atmospheric CO2 at 1%/yr (compounded). For hurricane wind speeds, our model shows a sensitivity of about 4% per degree Celsius increase in tropical sea surface temperatures, with a larger percentage increase in near-storm rainfall.
4. An Overview of Recent Observational Studies and Debate
Recently, several studies have been published which argue that some measures of hurricane activity are already showing substantial increases over recent decades as the tropical oceans have warmed. Emanuel (Nature, Aug. 4, 2005) reports that a measure of the power dissipated by tropical cyclones (proportional to the cube of wind speeds accumulated over the North Atlantic and western North Pacific basins) has approximately doubled since about 1950, with most of the increase occurring over the past 30 years. According to Emanuel, increases in both intensity and duration of tropical cyclones have contributed to this apparent increase. Emanuel's power dissipation index (PDI) is strongly correlated with sea surface temperatures in these basins, which have increased markedly over the same period. For further discussion, see this discussion. A critique of Emanuel's Atlantic results by Chris Landsea appears in the 22 December 2005 issue of Nature. Landsea's reanalysis of the Atlantic PDI record indicates that there is no evidence for a trend in the Atlantic basin-wide PDI (1949-2004), in contrast to Emanuel's original Figure 1. In his reply in the same issue, Emanuel maintains that there is still a global warming-related signal of increasing hurricane intensity and duration emerging in the multi-basin indices. Mann and Emanuel (EOS, 2006) also report a long-term rise in Atlantic tropical cyclone counts, correlated with sea surface temperature increases in the basin.
Another research group, Webster et al. (Science, Sept. 16, 2005) report that the number of category 4 and 5 hurricanes has almost doubled globally over the past three decades. Although their analysis spans a shorter time period than Emanuel's, their results indicate that a substantial increase has occurred in all six tropical storm basins.
A key question for these studies is the quality and degree of homogeneity in the tropical cyclone data, a subject of debate in the hurricane research community. An example of some critical commentary on this issue is found in Landsea et al. (Science, 2006) who conclude that any trends in tropical cyclone intensity are "very likely to be much smaller (or even negligible) than those found in the recent studies."
It should be noted that the rate of increase of hurricane intensities implied in Emanuel's results (per degree of SST warming) is much greater than that simulated in our future projections. We are not yet able to reconcile these large differences in apparent sensitivity of the tropical cyclone intensities. We speculate that these discrepancies could arise from three sources: i) possible overestimation of the observed intensity trends; ii) possible underestimation by our model of the sensitivity of tropical cyclone intensities to SST changes; or iii) possible influence of related environmental variables such as trends in atmospheric temperatures (lapse rates) and moisture. Further investigation is ongoing.
Fig. 2. Tropical storm simulations. (more)
5. Further Background on Previous Studies at GFDL
An increase in the upper-limit intensity of hurricanes with global warming was suggested on theoretical grounds by M.I.T. Professor Kerry Emanuel in 1987. In the late 1990s, Knutson, Tuleya, and Kurihara at GFDL/NOAA began simulating samples of hurricanes from both the present-day climate and from a greenhouse-gas warmed climate. This was done by "telescoping-in" on coarsely resolved tropical storms in GFDL's global climate model using the high-resolution GFDL hurricane prediction model (Figure 2). A research report describing this work was published in the Feb 13, 1998 issue of Science, with a more detailed paper in Climate Dynamics (1999, vol. 15). All of these studies, as well as our more recent ones, include the moderating effect of atmospheric stabilization aloft under high CO2 conditions, rather than simply increasing the sea surface temperature alone.
In a follow-up study, which appeared in the Journal of Climate (June 2001), NOAA scientists Knutson and Tuleya teamed up with Isaac Ginis and Weixing Shen of the University of Rhode Island to explore the climate warming/ hurricane intensity issue using hurricane model coupled to a full ocean model. The coupled model was used to simulate the "cool SST wake" generated by the hurricanes as they moved over the simulated ocean (Figure 3). The model simulations including this additional feedback still showed a similar percentage increase of hurricane intensity under warm climate conditions as the original model without ocean coupling.
Fig. 3. Sea surface temperatures and sea level pressure. (more)
The most recent and comprehensive study by Knutson and Tuleya, published in Journal of Climate in September 2004 (download paper), confirms the general conclusions of previous studies but makes them more robust by using future climate projections from nine different global climate models and four different versions of the GFDL hurricane model. The GFDL hurricane model used for the study is an enhanced resolution version of the model used to predict hurricanes operationally at NOAA's National Centers for Environmental Prediction. According to this latest study, an 80 year build-up of atmospheric CO2 at 1%/yr (compounded) leads to roughly a one-half category increase in potential hurricane intensity on the Saffir-Simpson scale and an 18% increase in precipitation near the hurricane core. A 1%/yr CO2 increase is an idealized scenario of future climate forcing. As noted by the Intergovernmental Panel on Climate Change (IPCC), there is considerable uncertainty in projections of future radiative forcing of earth's climate. A criticism of our paper by Michaels et al. was recently published in the Journal of Climate. Our response is available here.
An implication of the GFDL studies is that if the frequency of tropical cyclones remains the same over the coming century, a greenhouse-gas induced warming may lead to an increasing risk in the occurrence of highly destructive category-5 storms.