Two research expeditions to a unique location in Papua New Guinea have given scientists rare insights into what tropical coral reefs could look like if human-induced atmospheric CO2 concentrations continue to rise unabated.
At present rates of increase, the Intergovernmental Panel on Climate Change (IPCC) forecasts a doubling of atmospheric CO2 levels by 2100. About a third of the increased load will be absorbed by the oceans. As a consequence, pH levels will drop from 8.1 to 7.8 and the reduced alkalinity (also known as the ocean acidification problem) should make it more difficult for calcifying organisms to build shells and skeletons.
In 2010-11, AIMS scientist Dr Katharina Fabricius led researchers from six countries to study coral reefs occurring near three natural CO2 seeps in Milne Bay, Papua New Guinea. This location is the only site presently known where cool CO2 gas bubbles up in tropical waters.
A scientific paper on the first results of the study was published in the prestigious, international scientific journal, Nature Climate Change. It is the first scientific paper to present data on tropical coral reef ecosystems that are naturally adapted and acclimatised to elevated CO2.
In the past, scientists have relied on short-term laboratory experiments to tell us what happens to marine organisms exposed to ocean acidification. While these experiments are important, the natural gasseeps in Milne Bay provide a convincing demonstration of what happens to coral reef communities chronically exposed to CO2 gradients from present day to future conditions.
The research showed there will be some winners but many more losers when tropical coral reefs are exposed to widespread ocean acidification. The researchers found that as pH decreases, the number and types of corals making up coral reefs are much reduced. Diversity of corals drops by 40 per cent and the reef becomes dominated by the massive Porites corals referred to in the previous story.
The cover of the more delicate branching corals was reduced three-fold near the CO2 seeps. Similarly, the abundance of soft corals and sponges were also significantly reduced. Most importantly, the researchers found that reef development ceased below pH level 7.7.
The study showed that significant acidification of the oceans will lead to profound changes in coral reef ecosystems. The demise of the structurally complex corals means that future reefs will be much simpler than current ones with the risk of losing much of the rich biodiversity that we associate with modern coral reefs (see CReefs Highlight).