How does rising atmospheric CO2 affect marine organisms?

Click to locate material archived on our website by topic

Coral Reefs (History - South Pacific Ocean) -- Summary
Are coral reefs really as susceptible to being destroyed by rising temperatures as climate alarmists claim they are? A look at the responses of some of them from the South Pacific Ocean to past periods of warmth provides a totally new perspective on their likely future.

Lough and Barnes (1997) determined annual density, linear extension and calcification rates of numerous massive Porites colonies based on data obtained from several coral cores extracted from 35 sites along Australia's Great Barrier Reef, stretching from 9 to 23S, which provided information about variability in coral growth there since the late 15th century. All three of the studied parameters showed a general decrease in values when progressing from the north (warmer) to the south (cooler). The data also indicated that the growth characteristics of Porites species are "highly variable," with extension and calcification rates varying "as much as 20-30% about the mean, both from year to year and over 10-30-year periods," while the temporal variability of coral density was much less at 10% about the mean, both inter-annually and over one- to three-decade periods.

Noting that their data showed frequent and extended periods of coral growth that were both above and below the long-term mean, the two researchers cautioned that "it would be unwise to rely on short-term values (say averages over less than 30 years) to assess mean conditions," adding that it would actually be "rash to compare one year's value with another and it would be reckless to compare individual years in different decades without analyzing the long-term trends." In addition, their analysis of calcification rates revealed a statistically significant correlation with sea surface temperature that suggested long-term variations in Porites calcification rates on the Great Barrier Reef are driven by variations in this parameter; and using their derived relationship, they calculated that an increase in sea surface temperature from 20 to 21C would tend to increase calcification rates by about 3.5%.

In viewing the researchers' long-term record, it is also evident there has been a recent decline in calcification rates in this region. However, their data show that "a decline in calcification equivalent to the recent decline occurred earlier this century and much greater declines occurred in the 18th and 19th centuries." Furthermore, analyses of annual density banding from the coral samples indicated that "the 20th century has witnessed the second highest period of above average calcification in the past 237 years." As a result, they concluded that "the observed decline in coral growth in recent decades may be, simply, a return to more 'normal' conditions."

Also studying the response of Porites corals on Australia's Great Barrier Reef to episodes of intense warming were Hendy et al. (2003), who report it has been suggested that since certain massive corals there, some as old as 700 years, "died as a result of the 1998 bleaching event, it must have been the most severe such event to hit the Great Barrier Reef over the last seven centuries (Hoegh-Guldberg, 1999)." Examining this contention in more detail, they considered "the likelihood of observing, in cores taken from Porites colonies, past mass coral mortality events equivalent in intensity and scale to the 1998 bleaching event," noting that "an historic record of past coral mortality events is needed to gain some perspective on current events and the impact of recent environmental change." This exercise included the careful examination of eight long Porites cores extracted from inshore and midshelf reefs in the central Great Barrier Reef; and it indicated the presence of two hiatuses in coral skeletal growth that were accurately dated to 1782-85 and 1817. Telltale "die-off scars" were observed in only one core for each event; and contemporary historical and proxy-climate records indicated that El Nio conditions occurred at the times of both growth discontinuities, with those of 1782-83 being termed "exceptional" by Whetton and Rutherfurd (1994). Other data indicated that low salinity from river runoff was a contributor to bleaching during the 1817 event; and the three researchers note that similar environmental conditions were associated with the 1998 bleaching of the Great Barrier Reef. Together, these findings demonstrated that Porites colonies can readily recover and continue growing for centuries after a partial mortality event such as that experienced on the Great Barrier Reef in 1998.

Based on the work of Marshall and Baird (2000), who studied the bleaching responses of different coral taxa to the environmental conditions that produced the 1998 event in the Great Barrier Reef, Hendy et al. additionally calculated the probability of sampling an event of equivalent severity that may have occurred in the more distant past. The results of this effort suggested that the chance of seeing an event across all eight of the cores they examined is exceedingly unlikely, "even for one as dramatic as the 1998 bleaching event." In fact, they calculated that "a growth discontinuity is most likely to be observed in only one of the cores in any sample population size smaller than 17 cores," which finding suggests that such coral bleaching events may well have occurred periodically in past centuries, but that they may be difficult to detect without massive multiple-coring studies. Until such work is conducted, therefore, we will not be able to accurately assess the uniqueness of the 1998 bleaching event, although there is now solid evidence that indicates it may not have been as unusual as climate alarmists have claimed it was.

Also working in Australian waters, Webster and Davies (2003) analyzed variations in lithology and coral assemblages of long drill cores made in the northern Great Barrier Reef by the International Consortium for Great Barrier Reef Drilling (Alexander et al., 2001). One of the cores came from an inner-shelf reef (Boulder Reef) and one from an outer-shelf reef (Ribbon Reef 5) located 5 and 49 km east of Cooktown on the northeast coast of Australia, respectively, which they used to characterize the nature of the Great Barrier Reef throughout the Pleistocene. This work revealed "the repeated occurrence of similar coral assemblages in both drill cores," which demonstrates, in the words of the two researchers, that "the Great Barrier Reef has been able to re-establish itself and produce reefs of similar composition again and again over hundreds of thousands of years, despite major environmental fluctuations (i.e. sea-level and temperature changes)."

Last of all, Linsley et al. (2000) retrieved a 3.5-meter core of continuous coral from a massive colony of Porites lutea far to the east of Australia on the southwest side of Rarotonga in the Cook Islands, from which they obtained Sr/Ca ratios on 1-mm-interval sections spanning the entire core (representing 271 years of growth), as well as δ18O values at the same resolution from 1726 to 1770 and from 1950 to 1997, the latter of which they used for calibration purposes together with sea surface temperature (SST) data, ultimately constructing a long-term temperature history of the area. Interestingly, their analyses revealed the existence of a quarter-century period centered on about the year 1745 when SSTs in the vicinity of Rarotonga were at least 1.5C warmer than they were at the end of the 20th century. This finding clearly demonstrates that corals in this region have survived sustained temperatures of a magnitude supposedly great enough to destroy them -- according to typical climate-alarmist thinking -- which obviously did not happen and demonstrates the incorrectness of their thinking.

In conclusion, substantial evidence indicates recent major bleaching events experienced by coral reefs of the South Pacific Ocean are not as unusual as recent observations might seem to imply, as much longer records reveal instances of similar bleachings in prior centuries. In addition, there is substantial evidence for much earlier greater-than-present sea surface temperatures that did not elicit dramatic bleaching; and when such bleaching has occurred, it typically has not spelled the end of the reefs involved, for many of them have regularly reconstructed themselves in their own prior image.

Alexander, I., Andres, M.S., Braithwaite, C.J.R., Braga, J.C., Davies, P.J., Elderfield, H., Gilmour, M.A., Kay, R.L., Kroon, D., McKenzie, J.A., Montaggioni, L.F., Skinner, A., Thompson, R., Vasconcelos, C., Webster, J.M. and Wilson, P.A. 2001. New constraints on the origin of the Australian Great Barrier Reef: results from an international project of deep coring. Geology 29: 483-486.

Hendy, E.J., Lough, J.M. and Gagan, M.K. 2003. Historical mortality in massive Porites from the central Great Barrier Reef, Australia: evidence for past environmental stress? Coral Reefs 22: 207-215.

Hoegh-Guldberg, O. 1999. Climate change, coral bleaching and the future of the world's coral reefs. Marine and Freshwater Research 50: 839-866.

Linsley, B.K., Wellington, G.M. and Schrag, D.P. 2000. Decadal sea surface temperature variability in the subtropical South Pacific from 1726 to 1997 A.D. Science 290: 1145-1148.

Lough, J.M. and Barnes, D.J. 1997. Several centuries of variation in skeletal extension, density and calcification in massive Porites colonies from the Great Barrier Reef: A proxy for seawater temperature and a background of variability against which to identify unnatural change. Journal of Experimental and Marine Biology and Ecology 211: 29-67.

Marshall, P.A. and Baird, A.H. 2000. Bleaching of corals in the Great Barrier Reef: differential susceptibilities among taxa. Coral Reefs 19: 155-163.

Webster, J.M. and Davies, P.J. 2003. Coral variation in two deep drill cores: significance for the Pleistocene development of the Great Barrier Reef. Sedimentary Geology 159: 61-80.

Whetton, P. and Rutherfurd, I. 1994. Historical ENSO teleconnections in the eastern hemisphere. Climatic Change 28: 221-253.

Last updated 13 February 2008