Two new studies by leading climate scientists quantify a connection between human-induced global warming and recent enhanced North Atlantic hurricane activity.  The studies conclude that natural variability plays only a minor role.  The research was supported by the National Science Foundation, a participating agency in the federal Climate Change Science Program. How will administration officials and federal scientists deal with these studies this year in their public statements about hurricanes and climate change? 

The Mann and Emanuel study seems to have received little media coverage. The Trenberth and Shea study received somewhat more coverage, though it tended to be overshadowed by the near-simultaneous release of a report by the National Research Council (National Academy of Sciences) on the long-term temperature record and current global warming.  These two powerful pieces of work by leading climate scientists, both of which conclude that increasing Atlantic hurricane activity is strongly influenced by long-term global warming and that cyclical natural variability plays at most a minor role, call for close attention.   

1.  “Atlantic hurricane trends linked to climate change,” by Michael Mann (Penn State University) and Kerry Emanuel (MIT), published in the June 13 issue of EOS, Transactions, American Geophysical Union

Mann, M. E., and K. A. Emanuel, 2006: Atlantic hurricane trends linked to climate change. EOS, 87, 233-244.  Copyright 2006,  American Geophysical Union. 

Intro and Abstract:

Increases in key measures of Atlantic hurricane activity over recent decades are believed to reflect, in large part, contemporaneous increases in tropical Atlantic warmth [e.g., Emanuel, 2005].  Some recent studies [e.g., Goldenberg et al., 2001] have attributed these increases to a natural climate cycle termed the Atlantic Multidecadal Oscillation (AMO), while other studies suggest that climate change may instead be playing the dominant role [Emanuel, 2005; Webster et al., 2005].

Using a formal statistical analysis to separate the estimated influences of anthropogenic climate change from possible natural cyclical influences, this article presents results indicating that anthropogenic factors are likely responsible for long-term trends in tropical Atlantic warmth and tropical cyclone activity. In addition, this analysis indicates that late twentieth century tropospheric aerosol cooling has offset a substantial fraction of anthropogenic warming in the region and has thus likely suppressed even greater potential increases in tropical cyclone activity.


Implications for Future Changes

There is a strong historical relationship between tropical Atlantic SST [sea surface temperature] and tropical cyclone activity extending back thgrough the late nineteenth century.  There is no apparent role of the Atlantic Multidecadal Oscillation…. These findings have implications for potential impacts of various alternative possible future anthropogenic forcing scenarios on Atlantic tropical cyclone trends.

2.  “Atlantic hurricanes and natural variability in 2005,” by Kevin Trenberth and Dennis Shea (National Center for Atmospheric Research), published June 27 in Geophysical Research Letters (by subscription) 
AGU, NCAR, NSF joint press release on the Trenberth and Shea study:

American Geophysical Union
National Center for Atmospheric Research
National Science Foundation
Joint Release

22 June 2006
AGU Release No. 06-21
For Immediate Release

Global Warming Beyond Natural Cycles Fueled 2005 Hurricane Season, New Study Concludes

WASHINGTON – Global warming accounted for around half of the extra hurricane-fueling warmth in the waters of the tropical North Atlantic in 2005, while natural cycles were only a minor factor, according to a new analysis by Kevin Trenberth and Dennis Shea of the National Center for Atmospheric Research (NCAR).

“The global warming influence provides a new background level that increases the risk of future enhancements in hurricane activity,” Trenberth says. The study appears 27 June in Geophysical Research Letters, published by the American Geophysical Union.

The study contradicts recent claims that natural cycles are responsible for the upturn in Atlantic hurricane activity since 1995.

It also adds support to the premise that hurricane seasons will become more active as global temperatures rise. Last year produced a record 28 tropical storms and hurricanes in the Atlantic. Hurricanes Katrina, Rita, and Wilma all reached Category 5 strength.

Trenberth and Shea’s research focuses on an increase in ocean temperatures. During much of last year’s hurricane season, sea-surface temperatures across the tropical Atlantic between 10 degrees north and 20 degrees north, which is where many Atlantic hurricanes originate, were a record 0.9 degrees Celsius [1.6 degrees Fahrenheit] above the 1901-1970 average. While researchers agree that the warming waters fueled hurricane intensity, they have been uncertain whether Atlantic waters have heated up because of a natural, decades-long cycle, or because of global warming.

By analyzing worldwide data on sea-surface temperatures (SSTs) since the early 20th century, Trenberth and Shea were able to calculate the causes of the increased temperatures in the tropical North Atlantic. Their calculations show that global warming explained about 0.45 degrees Celsius [0.81 degrees Fahrenheit] of this rise. Aftereffects from the 2004-2005 El Nino accounted for about 0.2 degrees Celsius [0.4 degrees Fahrenheit]. The Atlantic multidecadal oscillation (AMO), a 60-to-80-year natural cycle in sea surface temperature, explained less than 0.1 degrees Celsius [0.2 degrees Fahrenheit] of the rise, according to Trenberth. The remainder is due to year-to-year variability in temperatures.

Previous studies have attributed the warming and cooling patterns of North Atlantic ocean temperatures in the 20th century—and associated hurricane activity—to the Atlantic multidecadal oscillation. But Trenberth, suspecting that global warming was also playing a role, looked beyond the Atlantic to temperature patterns throughout Earth’s tropical and midlatitude waters. He subtracted the global trend from the irregular Atlantic temperatures—in effect, separating global warming from the Atlantic natural cycle. The results show that the AMO is actually much weaker now than it was in the 1950s, when Atlantic hurricanes were also quite active. However, the AMO did contribute to the lull in hurricane activity from about 1970 to 1990 in the Atlantic.

Global warming does not guarantee that each year will set new records for hurricanes, according to Trenberth. He notes that last year’s activity was related to very favorable upper-level winds as well as the extremely warm sea surface temperatures. Each year will bring ups and downs in tropical Atlantic SSTs, due to natural variations, such as the presence or absence of El Nino, he says. However, he adds, the long-term ocean warming should raise the baseline of hurricane activity.

The study was supported by the National Science Foundation.

Abstract of the Trenberth and Shea study:

GEOPHYSICAL RESEARCH LETTERS, VOL. 33, L12704, doi:10.1029/2006GL026894, 2006

Atlantic hurricanes and natural variability in 2005

Kevin E. Trenberth
National Center for Atmospheric Research, Boulder, Colorado, USA

Dennis J. Shea
National Center for Atmospheric Research, Boulder, Colorado, USA

The 2005 North Atlantic hurricane season (1 June to 30 November) was the most active on record by several measures, surpassing the very active season of 2004 and causing an unprecedented level of damage. Sea surface temperatures (SSTs) in the tropical North Atlantic (TNA) region critical for hurricanes (10 degrees to 20 degrees N) were at record high levels in the extended summer (June to October) of 2005 at 0.9 degree C above the 190170 normal and were a major reason for the record hurricane season. Changes in TNA SSTs are associated with a pattern of natural variation known as the Atlantic Multi-decadal Oscillation (AMO). However, previous AMO indices are conflated with linear trends and a revised AMO index accounts for between 0 and 0.1 degree C of the 2005 SST anomaly. About 0.45 degree C of the SST anomaly is common to global SST and is thus linked to global warming and, based on regression, about 0.2 degree C stemmed from after-effects of the 2004֖05 El Nio.

Coverage of the Trenberth and Shea study in ScienceNOW Daily News, National Geographic news on-line (archived), and USA Today (archived).