Coral reef islands under a warming climate

This post was written by PLOS Climate editorial board member Liqiang Xu

Oceans extend over vast areas of water, but typically their surface is locally dotted by islands. In warmer climates many of these islands have developed on coral reefs, and many are well known for their attractive landscape and amazing natural scenery that attracts many visitors to tropical resorts such as those on the Great Barrier Reef, the Maldives, the Marshall Islands, and around Fiji. The importance of these coral reef islands is highlighted by the fact that they are support over 500 million people worldwide. But now, those islands and the people that inhabit and rely on them are facing unprecedented challenges.

Reef-building corals are sensitive, requiring strict environmental conditions, including temperature, salinity, and seawater transparency. Coral reef ecosystems are often referred to as underwater rainforests, as they are believed by many to support the highest level of biodiversity in the oceans. Coral reef ecosystems usually produce a large underwater platform, on which coral reef islands are developed. Reef islands are then built of coral skeletons and numerous other organisms that rely on corals. The islands are generally small in area and low-lying, mostly only a few meters above sea level. These reef islands have vital ecological functions, as well as aesthetic significance.

The year 2021 has been especially challenging, as coral island communities have suffered great losses in their tourist industries due to the Covid-19 pandemic. However, the biggest challenge for those islands is not from the pandemic, but from climate change. Reef-building corals and reef ecosystems are extremely vulnerable, and climate change imposes a variety of negative, and in some cases deadly, impacts on them. The corals and reef islands are critically dependent on the tropical oceans, which have experienced some of the highest rates of sea level rise, due to global warming. The stability of reef islands and their response to global warming is a matter of global concern.

In fact, the degree of sea level rise is quite small, but its consequences are huge. The most straightforward effect of sea level rise is inundation of low-lying reef islands. Additionally, sea level rise changes wave regimes, increases wave energy and causes extensive coastal erosion. Climate change impacts not only the islands themselves, but also their surrounding marine ecosystems. The main cause of climate change is increased CO₂ in the atmosphere, which shifts the equilibrium between gaseous CO₂ and carbonic acid concentrations in the ocean, resulting in ocean acidification. Ocean acidification can lead to coral bleaching, which can in turn kill corals and destroy reef ecosystems. This ultimately cuts off the source materials required to build and maintain reef islands, resulting in decline and erosion of the islands.

According to the Fifth Assessment Report of Intergovernmental Panel on Climate Change (IPCC), the Earth surface temperature increased by 0.85 °C from 1880 to 2012. It is almost certain that we will get a higher number in the forthcoming Sixth Assessment Report by Working Group I of the IPCC on August 9th, 2021, which implies higher risk. To cope with challenges from climate change and to better preserve reef islands, extensive and deep cooperation and multidisciplinary study are indispensable. A global commitment to reduce Greenhouse Gas (GHG) emissions is also required. Many countries have taken action. For example, the Biden Administration in the USA re-joined the Paris Climate Agreement. China, as the second largest global economy, set up its first roadmap this March to achieve net zero emissions by 2060. In terms of science, our new journal PLOS Climate provides a communication platform for diverse science communities to observe what is happening to the Earth system and address causes and effects of climate change.

Liqiang Xu
Hefei University of Technology, China
Editorial Board Member of PLOS Climate, Science writer