“RESEARCHERS in Australia say the growth of coral on the country’s iconic Great Barrier Reef has fallen since 1990 to its lowest rate in 400 years”, variations of this message have been repeated around the world with global warming, and the associated acidification of oceans, claimed to be the cause. 
That was at the beginning of this year, in January, and the media frenzy was based on a paper by Glenn De’ath, Janice Lough and Katrina Fabricius published in the peer-reviewed journal ‘Science’. 
Canadian statistician Steve McIntyre has recently had an opportunity to download all the data used to construct the key illustrations that suggested catastrophe.
Mr McIntyre concludes that looking at just yearly averages (above graph) there has been a general trend of increasing coral calcification rates at the Great Barrier Reef since the late 1500s.
In recent years there have been fewer measurements of coral at the Great Barrier Reef and the apparent downward spike in 2004-05 may be an artifact of the data – lack of data. 
Mr McIntyre writes:
“To see the impact of unsmoothed data, I did a simple plot of the average calcification by year over the data set. I understand that the coral data spans a considerable length and that various sorts of adjustments might be justified, but it’s never a bad idea to plot an average. Here are two plots, showing a simple average, first from 1572-2005 and then in the 20th century. Based only on the first plot, one could NOT say that even the 2004-2005 results were “unprecedented in at least 400 years” – values in 1852 were lower. So I can confirm that the values before adjustment are unprecedented since at least 1852.
Visually, this graph looks to me like calcification has been increasing over time, with a downspike in 2004-5…”
Mr McIntyre goes on to comment that there is a very high correlation (0.48) between calcification and the number of measurements available in a year. The unsmoothed data gives a very different impression … Unsmoothed, years up to 2003 were not particularly low; it’s only two years – 2004 and 2005 – that have anomalously low values.
Glenn De’ath et. al. commented on ocean acidification in the same paper in ‘Science’ but provide no data. Wei et al have recently published on the pH history of Arlington Reef, which is part of the Great Barrier Reef, and they conclude that there was a ten-year pH minimum centered at about 1935 and a shorter more variable minimum very recently. Apart from these two non-CO2-related exceptions, the majority of the data fall within a band that exhibits no long-term trend in acidification – according to Sherwood, Keith and Craig Idso writing for C02 Science. [4,5]
Notes and Links
1. Global Warming Unlikely Reason for Slow Coral Growth, by Jennifer Marohasy, January 4th, 2009
2. G. De’ath, J.M. Lough and K. Fabricius (2009) Declining Coral Calcification on the Great Barrier Reef. Science. Volumne 323, pages 116-119
3. “Unprecedented” in the past 153 Years, by Steve McIntyre on June 3rd, 2009
4. The Ocean Acidification Fiction, Volume 12, Number 22: 3 June 2009
5. Wei, G., McCulloch, M.T., Mortimer, G., Deng, W. and Xie, L. 2009. Evidence for ocean acidification in the Great Barrier Reef of Australia. Geochimica et Cosmochimica Acta 73: 2332-2346.
Click on the above graph for a better/larger view of calcification rates for the Great Barrier Reef shown simply as yearly averages.