What was done
The authors used digital photography, image analysis and measurements in the field to determine the original growth rates of long-dead Quaternary corals found in exposed onshore limestone deposits near the margins of Hoga and Kaledupa Islands in the Wakatobi Marine National Park of Indonesia, after which they compared them to the growth rates of present-day corals of the same genera (Porites and Favites) living in the same area.

What was learned
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.

What it means
Clearly, the impact of the historical increase in the atmosphere's CO2 concentration on the corals in question has not been as catastrophically negative as the rantings and ravings of the world's climate alarmists suggest it should have been. In fact, the increase in the CO2 content of the modern atmosphere appears to have not been negative at all. In fact, it appears to have been positive, which should only have been expected in light of what we know about the beneficial influence of atmospheric CO2 enrichment on the photosynthetic rates of marine microalgae, such as those that comprise the food-producing symbiotic zooxanthellae of corals (see, in this regard, Aquatic Plants (Marine: Microalgae) plus many of the items archived under Coral Reefs (Calcification) in our Subject Index).