# Coral growth keeping up with sea level rise?

On 11 sep 2016, the Australian Herald Sun placed an opinion piece by Andrew Bolt titled Drowning in cash: Pacific islands love this scare* about the $300 million** financial aid pledged by the Australian Prime Minister, Malcolm Turnbull, at a Pacific leaders’ forum in Micronesia last week (I guess that's this meeting), to help the Pacific “manage climate change and improve disaster resilience". Bolt's critique is that these islands do not actually face much danger: The scare that these islands risk drowning as man-made warming melts our ice caps is wildly exaggerated. Professor Paul Kench, an Auckland University coastal geomorphologist, along with Australian scientists, has studied more than 600 coral reef islands. His findings: about 40 per cent have actually grown in size. Another 40 per cent stayed stable, and just 20 per cent have shrunk. Much of that is because coral islands, essentially living things, grow with rising seas, as well as with sand washed up by the waves. What’s more, populated islands can reclaim land. Paul Kench's page at the University of Auckland lists several of his publications, and maybe the blog post refers to this one: Destruction or persistence of coral atoll islands in the face of 20th and 21st century sea-level rise? From the abstract: Here, we analyze the physical changes in over 200 islands on 12 atolls in the central and western Pacific in the past few decades when sea level in the region increased at rates three to four times the global average. Results show little evidence of heightened erosion or reduction in island size. Instead island shores have adjusted their position and morphology in response to human impacts such as seawall construction and to variations in climate–ocean processes. The article is behind a paywall, so I cannot look at the actual study - the abstract does not mention anything about coral growth. As far as I know, sea level rise is going to be much faster than coral can grow, plus we have the complication factor of reduced grow (or actual coral death) because of rising sea water acidity and temperature. But it's still an intriguing question: Could there be a hint of truth in this, that coral growth could keep up with sea level rise? Maybe with a little help from us? Has this been investigated? * You may have trouble following this link directly. In that case go to the start page, Andrew Bolt's blog, and look up the article there. ** Australian dollars, I assume. That would be US$ 227 million

## 1 Answer

I have worked as a climate-change specialist in many countries including those with coral atolls, such as the Maldives, Solomon Islands and Samoa. I have analysed, at first hand, both the the tide-gauge and satellite-based rates of sea level rise. Notwithstanding the non-scientific writings of that well-known journalist, Andrew Bolt, the demonstrable truth is that ocean acidification is real and unstopable for the foreseeable future. The sea surface is warming faster than expected a decade ago, and the rate of sea level rise is accelerating. In the southwest Pacific, which is particularly prone to El Nino intensification, these affects are even more alarming.

I point out that Bolt has zero scientific credentials, has never done any climate research, and is only able to make a living as a climate-skeptic writer because many Australians are, by nature, predisposed to enjoy 'contrary' arguments, irrespective of the science.

Much research has gone into coral growth over the last decade, particularly in peri-Australian waters. Current findings are that, under optimum conditions, the fastest growing corals can indeed grow fast enough to keep up with the current rates of sea level rise. However, this cannot continue indefinitely. The reef is a total ecosystem, of fast and slow growing corals, bryozoa, etc., which has an aggregate rate of growth of less than the rise of sea level. Fast growing pioneering corals eventually collapse if left to continue growing ahead of the rest of the biome. Note also the phrase "under optimum conditions". As Jan Doggen rightly points out, sea-surface warming beyond the optimum thermo-tolerance, acidification, and water pollution are all serious stress factors. Between them one can say that, apart from deep-water corals, there is hardly a coral reef on the planet that isn't suffering from sub-optimal growth conditions.

There is another somewhat unexpected threat to corals: ordinary sun-cream from holidaymakers swimming near reefs. The common sun-cream component of oxy-benzone (present in most sun-creams) is toxic to corals at only about 60 parts per trillion. Globally, an estimated 10,000 tonnes of sun-screen ends up in the oceans, near corals, every year. This has a noticeable negative impact on the reef.

Taken altogether, this quadruple whammy bodes ill for the world's coral reefs. When the reefs die, and sea level rises there will be nothing to stop sub-tropical waves eroding the very land that some Pacific Islanders live on. Even islands with higher elevations still have fringing reefs. They are all in trouble, because sea level rise and higher erosion rates will force a retreat from the coast (hopefully planned), with massive social, environmental and economic disruption. So yes, they are absolutely justified in getting agitated about reef degradation!