Prepared by Dr. Amanda Staudt, NWF's Climate Scientist. Dr. Staudt directed the National Academies of Science Climate Research Committee and helped author more than a dozen reports on topics including the U.S. strategy for supporting climate change research, radiative forcing of climate, past records of surface temperature, and practices for effective global change assessments. She holds a Ph.D. in atmospheric sciences (2001) and an A.B. in environmental engineering and sciences (1996) from Harvard University.

New evidence shows an increase in frequency of North Atlantic tropical cyclones associated with global warming.

Holland and Webster (2007) propose that there have been at least 3 distinct regimes for tropical storm activity in the North Atlantic since 1905. In each subsequent regime, there was a significant increase in hurricane activity and sea surface temperature in the area of the eastern North Atlantic Ocean where hurricanes form. They conclude that “the overall trend in sea surface temperatures, and tropical cyclone and hurricane numbers is substantially influenced by greenhouse warming.”

Other studies have suggested that natural oscillations explain the observed variation in tropical cyclone behavior (e.g., Vimont and Kossin, 2007). But, these natural oscillations themselves may have been impacted by global warming (Anthes et al., 2006).

Changes in observing system do not appear to fully explain the trend in storm frequency, especially for the past decade.

The shifts identified by Holland and Webster (2007) in the number of tropical storms recorded do not coincide with major changes to the observing system, which took place (1) in 1944-45 when routine aircraft reconnaissance began, (2) in the late 1960s when satellite observations began, (3) in 1970 when there was a change in analysis practice, and (4) in 2003 when some new satellites were launched. This suggests that the trend is independent of the way that tropical storms are recorded.

Some scientists have argued that the historical record of tropical storms in the North Atlantic probably does not include all the storms that took place each year, especially in the earlier part of the record. Landsea (2007) estimates that, without satellite surveillance, an average of 3.2 tropical storms were not counted from 1900-1965 because they did not make landfall and would not have been detected. Landsea (2007) also argues that about one tropical storm was missed each year for the period 1966-2002, before new satellite data were available to allow better identification of storms.

Many tropical storms that do not hit the coasts were included in the historical records because they were intercepted by ships. So, other scientists have argued that Landsea (2007) overestimated the number of tropical storms that were missed in the pre-satellite era. Without the benefit of satellite observations, Mann and Emanuel (2006) suggest that ships would have been more likely to intercept tropical storms. Using records of ship tracks, Chang and Guo (2007) concluded that, from 1946-1965, at most one North Atlantic tropical cyclones was missed each year because it did not make landfall.

Wave heights along the eastern United States during hurricane season have increased since the late 1970s, augmenting the overall storm-related hazards for coastal communities and habitats.

Komar and Allan (2007) studied how the heights of big waves—those greater than about 10 feet that are likely to be present during strong storms—have changed during the hurricane season of July-September. At an observing station off the North Carolina coast near Cape Hatteras, these large waves are about 2.4 feet higher today than they were 30 years ago, an increase of over 20 percent.

The highest waves measured during 1977-1990 were 23.6 feet; for the period of 1996-2005 they were 33.5 feet. The biggest waves were observed during Hurricanes Dennis (August 1999), Floyd (September 1999), and Isabel (September 2003).

The authors conclude that “the most likely explanation for increasing wave heights is that there has been a progressive intensification of hurricanes.” This is because wind speeds, which are the primary factor controlling the generation of waves, have increased within storms. Previous research has linked the increasing wind speeds to global warming (Emanuel, 2005). Komar and Allan (2007) further state that it is likely that wave heights will continue to increase in the future.

Scientific literature cited:
All of the following files are PDF files - PDF Help.

Anthes, R. A., R. W. Corell, G. Holland, J. W. Hurrell, M. C. MacCracken, and K. E. Trenberth. 2006: Hurricanes and Global Warming — Potential Linkages and Consequences. Bulletin of the American Meteorological Society, 87, 623–628. (328 Kb)

Chang, E. K. M. and Y. Guo. 2007: Is the number of North Atlantic tropical cyclones significantly underestimated prior to the availability of satellite observations? Geophysical Research Letters, 34, L14801. (128 Kb)

Emanuel, K. 2005: Increasing destructiveness of tropical cyclones over the past 30 years. Nature 436, 686–688. (159 Kb)

Holland, G. J. and P. J. Webster 2007: Heightened tropical cyclone activity in the North Atlantic: natural variability or climate trend? Philosophical Transactions of the Royal Society A (doi:10.1098/rsta.2007.2083) Published online. (569 Kb)

Komar, P. D. and J. C. Allan, 2007: Higher Waves Along U.S. East Coast Linked to Hurricanes. Eos 88, 301. (207 Kb)

Landsea, C. W. 2007: Counting Atlantic tropical cyclones back to 1900. Eos 88, 197–208. (458 Kb)

Mann, M. E. and Emanuel, K. 2006: Atlantic hurricane trends linked to climate change. Eos 87, 233–244.

Vimont, D. J. and J. P. Kossin. 2007: The Atlantic Meridional Mode and hurricane activity. Geophysical Research Letters. 34, L07709. (2.4 Mb)

Reproduced/modified by permission of American Geophysical Union

• Latest Science on Hurricanes and Global Warming as of August 2007 - summary prepared by Amanda Staudt Ph.D., NWF Climate Scientist