GBR Coral Growth Study from AIMS

This newly released study from the Australian Institute of Marine Science in Townsville is getting a lot of press. An interview with the author Glen De’ath by the ABC claims a tipping point for coral growth has already been reached in 1990. Mongabay.com claims the growth of coral in Australia’s Great Barrier Reef has slowed its lowest rate in at least 400 years as a result of warming waters and ocean acidification.

The claims are made on the basis of data apparently showing the rate of calcification from 328 colonies of massive Porites corals from 69 reefs of the Great Barrier Reef (GBR) has declined by 14.2% since 1990. These data would seem to be worthwhile checking by independent sources. Abstract from Science below, a News of the Week article by Elizabeth Pennisi. As the claims are based essentially on an upside-down hockey stick, there seems to be a contradiction in the abstract.

Declining Coral Calcification on the Great Barrier Reef
Glenn De’ath,* Janice M. Lough, Katharina E. Fabricius

Reef-building corals are under increasing physiological stress from a changing climate and ocean absorption of increasing atmospheric carbon dioxide. We investigated 328 colonies of massive Porites corals from 69 reefs of the Great Barrier Reef (GBR) in Australia. Their skeletal records show that throughout the GBR, calcification has declined by 14.2% since 1990, predominantly because extension (linear growth) has declined by 13.3%. The data suggest that such a severe and sudden decline in calcification is unprecedented in at least the past 400 years. Calcification increases linearly with increasing large-scale sea surface temperature but responds nonlinearly to annual temperature anomalies. The causes of the decline remain unknown; however, this study suggests that increasing temperature stress and a declining saturation state of seawater aragonite may be diminishing the ability of GBR corals to deposit calcium carbonate.

Australian Institute of Marine Science, Townsville, Queensland 4810, Australia.

How can calcification simultaneously increase linearly with increasing sea surface temperature, and at the same time, have remained relatively constant over the last 400 years? Either sea surface temperatures were constant, or calcification rate did not respond to temperature.

Update: jae pointed to some contradictory studies.
Crabbe, M.J.C., Wilson, M.E.J. and Smith, D.J. 2006. :

Crabbe et al. report, first of all, that the Quaternary corals they studied appear to have grown “in a comparable environment to modern reefs at Kaledupa and Hoga,” except, of course, for the air’s CO2 concentration, which is currently higher than it has been at any other time throughout the entire Quaternary, i.e., the past 1.8 million years. Second, the results of their measurements indicate that the radial growth rates of the modern corals are 31% greater than those of their more ancient Quaternary cousins, in the case of Porites species, and 34% greater in the case of Favites species.

Review of long-term coral data sets:

So what did Bessat and Buigues find? First of all, they found that a 1°C increase in water temperature increased coral calcification rate at the site they studied by fully 4.5%. Then they found that “instead of a 6-14% decline in calcification over the past 100 years [as] computed by the Kleypas group, the calcification has increased, in accordance with [what] Australian scientists Lough and Barnes [found].” They also observed patterns of “jumps or stages” in the record, which were characterized by an increase in the annual rate of calcification, particularly at the beginning of the past century “and in a more marked way around 1940, 1960 and 1976,” stating once again that their results “do not confirm those predicted by the Kleypas et al. (1999) model.”

There seems a variation from other results in the field.