Hurricanes in a changing climate

    In 2017, coastal residents in the U.S. have witnessed something extreme. Two Category 4 hurricanes made landfalls in the continental U.S. in the same year, first year on record. The old debate on hurricanes and climate change has been reignited. The relationship between hurricanes and climate change is very complicated, more so than many think. I am going to post part of a literature review report I wrote in graduate school (2009) here:

     "One possible manifestation or implication of climate change is the increasing hurricane activity in the past 35 years. Especially, an increase in the number and intensity of major hurricanes in Nothern Atlantic basin has been witnessed since 1995 (Goldenberg et al. 2001; Pielke et al. 2005; Webster et al. 2005). Since the limited instrumental record makes extensive analyses of hurricane activities globally impossible, the North Atlantic region with relatively reliable instruments which can be dated back to early 20th century has become the center of study on the relationship between climate change and hurricanes (Henderson-Sellers et al. 1998). Similar to the on-going discussion “human induction or nature?” in the broad realm of climate change, there is a debate on climate change and hurricanes. One side believes the upward trend of Atlantic hurricane activity is correlated with the increase of Sea Surface Temperature (SST) induced possibly by increasing green house gas forcing (Emanuel 2005; Webster et al. 2005; Elsner 2006; Emanuel 2007; Holland and Webster 2007). In comparison, the other side attributes this to natural variability. They believe the increased Atlantic hurricane activity in the recent years are just the reflection of the multidecadal-scale variability and it is premature to conclude that the increased activity is the result of green house warming (Goldenberg et al. 2001; Piekle et al. 2005; Landsea et al. 2006).

ANALYSIS

Definition
      A tropical cyclone (TC) is the generic term for a nonfrontal synoptic-scale low pressure system originating over tropical and subtropical waters with organized convection and definite cyclonic surface wind circulation (Henderson-Sellers et al. 1998). When the surface winds reach 33 m/s, a hurricane (the North Atlantic Ocean and the northeast Pacific Ocean) is formed (Henderson-Sellers et al. 1998).


Discussion

      Emanuel (2005 a.) applied a hurricane destruction index- power dissipation index (PDI), which is a combined measure of Atlantic hurricanes frequency, intensity and duration to his statistical analysis and found there was a significantly strong relationship between PDI and SST. Just after Emanuel’s article was published on Nature, Pielke and Landsea cast their doubts on Emanuel’s method. Pielke (2005) argued that the data used by Emanuel were not normalized and the effect of societal changes was another factor on the hurricane destruction. Landsea (2005) pointed out the flaws existing in Emanuel’s graphs in which the end-points remained unaltered in smoothing, bias-removal scheme used to alter the data for the Atlantic for 1949-69. In response, Emanuel (2005 b.) insisted on his conclusion about the trends in tropical-cyclone power dissipation since this trend was large and universal, though he gave credits to Piekle for taking societal changes into account and accepted the correction of dropping the end-points in smoothing while he also emphasized that “this error has comparatively little effect on the high correlation between PDI and SST.”(Emanuel 2005 b. E13)

     Webster et al. (2005) carefully examined the time series of SST by ocean basin, number of intense hurricanes and percentage of intense hurricanes and came to the conclusion that there is a 30-year trend toward more frequent and intense hurricanes based on the global data. On the other hand, Landsea et al. (2006) cast their doubt on the reliability of current global tropical cyclone databases to ascertain long-term trends in tropical cyclone intensity. They argued that constrained by the imperfect technique of monitoring and estimating the hurricane intensity, “the pre-1990 data for all basins are replete with large uncertainty, gaps, and biases.” (Landsea et al. 2006, 453) Holland & Webster (2007) did a more detailed study on the long-period variations in tropical cyclones and hurricane frequency in the North Atlantic Ocean and concluded the upward trend of the number of tropical cyclones concur with the increasing trend of SST.  To further complicate matters, Holland & Webster (2007) also found the contradiction of conclusions made by the other side. They pointed out that on one hand these papers (Landsea et al. 1999; Goldenberg et al. 2006; Owens & Landsea 2003; Landsea et al. 2006) “describe the data as being of high quality sufficient to determine natural variability in hurricane characteristics but, on the other hand, of insufficient quality to determine trends that are demonstrably of similar magnitude.” (Holland & Webster 2007, 2712)

    As one more piece of evidence to support the climate change effect on Atlantic hurricane hypothesis, Elsner (2006) applied Granger Causality Tests, which is used to determine whether one time series is useful in predicting another by comparing two sets of models involving lagged values of the predictor variable, and concluded that global temperature causes Atlantic SST and Atlantic SST causes the PDI in the Granger sense. Meanwhile, the projection model (Knutson & Tuleya 2004), lent  little evidence to the notion that green house warming leads to the upward trend of tropical storm  numbers, hurricane numbers despite the significant statistical correlation between SST and Atlantic hurricane activity in recent decades. However, they suggested the intensity of the hurricanes was projected to increase by a few percent (Knutson 2008).

      In the process of literature review, I have noticed one interesting thing. The authors of one paper (Henderson-Sellers et al. 1998) published in 1998 included the important representatives of the current two major schools. Emanuel, Holland, Webster of “climate change” and Gray, Landsea of “natural variability” contributed to the same paper. In the introduction section, these authors seem to all agree that “different definitions, techniques, and observational approaches may produce errors and biases in these datasets that could have implications for the study of the natural variation of tropical cyclone activist and the detection of possible historical trends.” (Henderson-Sellers et al. 1998, 20) This seems to imply that these scholars had realized that their clashing conclusions and views on the same issue were inevitable before they debated each other years later.

        The side of natural variability also provides a societal perspective to this connection of human-caused climate change to hurricane impacts. Pielke et al. (2005) put the emphasis on the hurricane damages to the society and stated that “the future damages to society of its projected changes in the behavior of hurricanes are dwarfed by the influence of its own projection of growing wealth and population.” (1574) In some sense, they should be given credit for their attempts to include the society vulnerability into this discussion. They stressed the importance of adaptation to hurricanes."

        In addition to the physical complexity of climate change and hurricane activities, let's not forget the basic definition of climate. Climate is the accumulation of weather over an extended period of time. If we continue to witness the overall pattern of increasing hurricane activities, we may be in a better position to draw a causal line between hurricanes and climate change. For this year's hurricanes alone, I am afraid we cannot directly connect them to climate change with substantial certainty.

Reference
Elsner, J. B., 2006. Evidence in support of the climate change-Atlantic hurricane hypothesis. Geophysical Res. Letters, Vol. 332, L16705
Emanuel, K. A. 2005 a. Increasing destructiveness of tropical cyclones over the past 30 years, Nature, 436, 686-688.
Emanuel, K. A. 2005 b. Emanuel replies, Nature, Brief Communications Arising 438 E13
Goldenberg, S.B., C.W. Landsea, A. M. Mestas-Nunez, and W. M. Gray, 2001: The recent increase in Atlantic hurricane activity: Causes and implications. Science, 293, 474-479.
Henderson-Sellers, A. et al. 1998. Tropical cyclones and global climate change: a post-IPCC assessment. Bull. Am. Meteor. Soc. 79, 19-38.
Holland, G. J., and P. J. Webster, 2007. Heightened tropical cyclone activity in the North Atlantic: natural variability or climate trend? Phil. Trans. R. Soc. A 2007 365, 2695-2716.
Knutson, T. R., and R. E. Tuleya, 2004. Impact of CO2-induced warming on simulated hurricane intensity and precipitation: Sensitivity to the choice of climate model and convective parameterization, J. Clim., 17, 3477-3495.
Knutson, T. R., J. J. Sirutis, S. T. Garner, G. A. Vecchi and I. M. Held, 2008 Simulated reduction in Atlantic hurricane frequency under twenty-first-century warming conditions. Nature Geoscience doi: 10.1038/ngeo202
Landsea, C. W. 2005. Hurricanes and global warming. Nature, Brief Communications Arising 438 E11-E12.
Landsea, C. W., Harper, B. A., Horau, K. and Knaff, J. A. 2006 Can we detect trends in extreme tropical cyclones? Science, Perspect. 313, 452-454.
Pielke, R. A., Jr., C. Landsea, M. Mayfield, J. Laver, and R. Pasch, 2005, Hurricanes and global warming, Bull. Am. Meteorol. Soc., 86, 1571-1575.
Pielke, R. A., Jr. 2005. Are there trends in hurricane destruction? Nature, Brief Communications Arising 438 E11
Trenberth, K. 2005. Uncertainty in hurricanes and global warming, Science, 308, 1753-1754.
Webster, P. J., G. J. Holland, J. A. Curry, and H.-R. Change, 2005. Changes in Tropical Cyclone Number, Duration, and Intensity in a Warming Environment, Science, 309, 1844-1846.