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Coral Decline on Australia's Great Barrier Reef
Volume 16, Number 14: 3 April 2013

In a paper published in the Proceedings of the National Academy of Sciences USA that focused on Australia's Great Barrier Reef, De'ath et al. (2012) write that "based on the world's most extensive time series data on reef condition (2,258 surveys of 214 reefs over 1985-2012), we show a major decline in coral cover from 28.0% to 13.8% (0.53% per year)," which they say equates to "a loss of 50.7% of initial cover." In addition, they report that "tropical cyclones, coral predation by crown-of-thorns starfish (COTS), and bleaching accounted for 48%, 42%, and 10% of the respective estimated losses, amounting to a 3.38% per year mortality rate."

On a more positive note, the four Australian researchers say "the estimated rate of increase in coral cover in the absence of cyclones, COTS, and bleaching was 2.85% per year, demonstrating substantial capacity for recovery of reefs." In fact, they indicate that in the absence of COTS alone, "coral cover would increase at 0.89% per year, despite ongoing losses due to cyclones and bleaching [italics added]." And confirming that such could indeed occur, they report that the relatively pristine northern region of the Great Barrier Reef "showed no overall decline" over the course of their study.

In light of these most interesting and impressive findings, De'ath et al. conclude that "reducing COTS populations by improving water quality and developing alternative control measures could prevent further coral decline and improve the outlook for the Great Barrier Reef." And so it could; and so, indeed, should the suggested improvements and control measures be rapidly implemented. However, they go on to state that such strategies can "only be successful if climatic conditions are stabilized." And why is that? Because, as they continue, they believe that "losses due to bleaching and cyclones will otherwise increase."

Real-world data, however, suggest something far different, especially in the case of tropical cyclones, which accounted for fully 48% of the yearly coral mortality rate in De'ath et al.'s study. Working in the North Indian Ocean, for example, Singh et al. (2000) found that tropical cyclone frequency experienced a slightly decreasing trend of -0.8/100 years between 1877 and 1998, over which time the earth warmed significantly as it recovered from the global chill of the Little Ice Age. Likewise, Harper et al. (2008) concluded that "there is no prima facie evidence of a potential climate-change induced trend in tropical cyclone intensity in northwestern Australia over the past 30 years." And focusing on the 39-year time period of 1970-2008 in the northwest Australian sub-basin of the southeastern Indian Ocean, Goebbert and Leslie (2010) say they could find "no significant linear trends in either mean annual tropical cyclone frequencies or tropical cyclone days," and they say there was also "no trend in the number of intense tropical cyclones.

Working at Curacoa Island, north Queensland, Australia, and taking a much longer look at the subject, Hayne and Chappell (2001) found that "cyclone frequency was statistically constant over the last 5,000 years," and they say they could also find "no indication that cyclones have changed in intensity," further noting that isotopic and trace element evidence from ancient corals indicates that sea surface temperatures were about 1°C warmer than they have been recently about 5,000 years ago.

In a considerably shorter but still long-term study, Nott et al. (2007) developed a 777-year-long annually-resolved record of land-falling tropical cyclones in northeast Australia based on analyses of isotope records of tropical cyclone rainfall in an annually-layered carbonate stalagmite from Chillagoe in northeast Queensland. This work revealed that the period between AD 1600 to 1800 - when the Little Ice Age held sway throughout the world - "had many more intense or hazardous cyclones impacting the site than the post AD 1800 period," when the planet gradually recovered from this cold interlude and began to warm at a rate that rose to ultimately become what climate alarmists typically characterize as unprecedented over the past millennium or more, and when temperatures rose to a level they claim was equally unprecedented.

Coming back to more recent times, Terry and Gienko (2010) analyzed various tropical cyclone characteristics based on four decades of cyclone season data (1969-2008) contained in the regional cyclone archive of the tropical South Pacific that is maintained by the Regional Specialized Meteorological Centre located at Nadi in the Fiji Islands, finding that "no linear trends were revealed in cyclogenesis origins, cyclone duration, track length or track azimuth over the four decades of records," which implies, in their words, that "there is as yet no evidence for climate-change forcing of these storm characteristics over recent historical times."

Even more revealing, in this regard, Callaghan and Power (2011) developed a new database of severe land-falling tropical cyclones for eastern Australia from numerous historical sources that took them over a decade to collect and analyze. And this database revealed that the number of severe tropical cyclones making land-fall over eastern Australia declined from about 0.45 per year in the early 1870s to about 0.17 per year in recent times, which amounts to a 62% decline.

As we contemplate the future in light of this further information on the impact of global warming on tropical cyclone activity in the area around Australia (as well as generally throughout the entirety of the tropics), we feel that the demonstrable fact that rising temperatures have either negligible impact on the yearly number and character of tropical cyclones, or that they actually tend to reduce such storm activity, is actually a really big "plus" for the Great Barrier Reef's corals, especially since cyclone activity was responsible for fully 48% of the coral mortality that was experienced on the Reef over the past 27 years.

Sherwood, Keith and Craig Idso

Callaghan, J. and Power, S.B. 2011. Variability and decline in the number of severe tropical cyclones making land-fall over eastern Australia since the late nineteenth century. Climate Dynamics 37: 647-662.

De'ath, G., Fabricius, K.E., Sweatman, H. and Puotinen, M. 2012. The 27-year decline of coral cover on the Great Barrier Reef and its causes. Proceedings of the National Academy of Sciences USA 109: 17,995-17,999.

Goebbert, K.H. and Leslie, L.M. 2010. Interannual variability of Northwest Australian tropical cyclones. Journal of Climate 23: 4538-4555.

Harper, B.A., Stroud, S.A., McCormack, M. and West, S. 2008. A review of historical tropical cyclone intensity in northwestern Australia and implications for climate change trend analysis. Australian Meteorological Magazine 57: 121-141.

Hayne, M. and Chappell, J. 2001. Cyclone frequency during the last 5000 years at Curacoa Island, north Queensland, Australia. Palaeogeography, Palaeoclimatology, Palaeoecology 168: 207-219.

Nott, J., Haig, J., Neil, H. and Gillieson, D. 2007. Greater frequency variability of landfalling tropical cyclones at centennial compared to seasonal and decadal scales. Earth and Planetary Science Letters 255: 367-372.

Singh, O.P., Ali Khan, T.M. and Rahman, Md.S. 2000. Changes in the frequency of tropical cyclones over the North Indian Ocean. Meteorology and Atmospheric Physics 75: 11-20.

Terry, J.P. and Gienko, G. 2010. Climatological aspects of South Pacific tropical cyclones, based on analysis of the RSMC-Nadi (Fiji) regional archive. Climate Research 42: 223-233.