Coral reefs in jeopardy

Devastating coral bleaching will be more common, start earlier and last longer unless we cut emissions

Sarah_lewis/Shutterstock

Camille Mellin, University of Adelaide and Damien Fordham, University of Adelaide

Coral bleaching is becoming much more common as a result of increasingly severe and frequent marine heatwaves. Four global mass bleaching events have happened since 1998. Two of these were in the past decade.

Unless greenhouse gas emissions are cut to slow global warming, our new research shows that, by 2080, coral bleaching will start in spring, rather than late summer. Some events will last into autumn. The Great Barrier Reef’s maximum annual heat stress will double by 2050 if emissions do not slow.

Marine heatwaves stress corals, which then expel the symbiotic algae living in their tissue. These corals are left white and weakened. While not all bleached corals die immediately, prolonged heat stress harms their health and reproduction.

Our research used daily data on sea surface temperatures (instead of monthly data that models typically use) and supercomputing to produce high-resolution projections of marine heatwaves. We showed the risk of coral bleaching will be greatest along the equator. That’s also where the most biodiverse coral reefs are found.

Coral reefs cover only 1% of our oceans, but host at least 25% of all marine species. More than half a billion people worldwide depend on coral reefs for food.

So coral reefs are vital for the health of the ocean and people. They are also among the ecosystems most at risk from climate change.

Longer bleaching season will hit spawning

The US National Oceanic and Atmospheric Administration monitors marine heatwaves globally. Seasonal coral bleaching alerts are based on this data. Predicting coral bleaching risk over entire decades has proved much more challenging.

Recent improvements in climate modelling now allow marine heatwaves and coral bleaching risks to be predicted with high accuracy. Using daily projections of heat stress from many global climate models, we show the severity and duration of coral bleaching will soon reach uncharted territory.

By mid-century coral bleaching is expected to start in spring for most of Earth’s reefs, rather than late summer as is typical today. In equatorial regions, corals will be at high risk of bleaching all year round by the end of the century.

In many regions, corals spawn only once a year. These spectacular mass spawning events happen in a single week following a full moon in spring.

By 2040, this spawning event could coincide with severe bleaching risk. This would greatly reduce their reproductive success, causing large-scale coral loss.

Acropora coral spawning on Magnetic Island in Queensland, Australia. Coral Brunner/Shutterstock

Equatorial regions most at risk

We show the future risk of severe coral bleaching is uneven globally.

The greatest risk is along the equator. Equatorial regions are home to the most biodiverse coral reefs, including conservation hotspots such as the Coral Triangle. To make matters worse, marine life in these regions is particularly vulnerable to accelerated climate change.

Many equatorial species are already living at temperatures near their upper tolerance. They also generally have low abilities to move to track shifting climates. This leaves them at high risk of extinction.

Future risk of coral bleaching under a high-emission scenario (top) and benefit from climate mitigation (bottom). Adapted from Mellin et al. Science Advances 2024

Our research shows equatorial regions are set to benefit least from efforts to curb emissions. We expect significant emission cuts will reduce the annual duration of severe bleaching conditions in all areas except these regions.

The projected highest climate impacts coincide with highest social reliance on coral reefs. This will challenge human populations that rely heavily on their local reefs for their livelihoods and nutrition.

Improving coral reef management

Our research identifies Earth’s reef regions that are at lowest risk of increased bleaching. This will help conservation managers and policymakers prioritise efforts to limit loss of coral reef biodiversity.

We predict much less risk of coral bleaching in regions such as the northern coasts of Venezuela and Colombia, Socotra Island (opposite the Gulf of Aden) and Alor Kecil in Indonesia. Seasonal upwellings occur here, bringing cooler water to the surface that’s likely to limit the severity of heatwaves.

Identifying these future havens for coral reefs will help maximise the success of coral conservation strategies such as assisted evolution, coral restoration or transplantation.

These strategies can help maintain healthy coral populations at local scales, particularly if used on reefs where future climate impacts will be lower. By pinpointing these havens, our research will strengthen coral conservation.

Our research includes a user-friendly web-based tool for mapping future coral bleaching. It will help pinpoint locations for effective management interventions.

Curbing greenhouse gas emissions is the main solution to reduce future climate impacts on corals. However, other strategies are also vital to maximise coral reefs’ adaptation to climate change.

Camille Mellin, Senior Lecturer and ARC Future Fellow, School of Biological Sciences, University of Adelaide and Damien Fordham, Associate Professor of Global Change Ecology, University of Adelaide

This article is republished from The Conversation under a Creative Commons license. Read the original article.

The risk of ocean change and climate impact

Atlantic Ocean is headed for a tipping point − once melting glaciers shut down the Gulf Stream, we would see extreme climate change within decades, study shows

Too much fresh water from Greenland’s ice sheet can slow the Atlantic Ocean’s circulation. Paul Souders/Stone via Getty Images

René van Westen, Utrecht University; Henk A. Dijkstra, Utrecht University, and Michael Kliphuis, Utrecht University

Superstorms, abrupt climate shifts and New York City frozen in ice. That’s how the blockbuster Hollywood movie “The Day After Tomorrow” depicted an abrupt shutdown of the Atlantic Ocean’s circulation and the catastrophic consequences.

While Hollywood’s vision was over the top, the 2004 movie raised a serious question: If global warming shuts down the Atlantic Meridional Overturning Circulation, which is crucial for carrying heat from the tropics to the northern latitudes, how abrupt and severe would the climate changes be?

Twenty years after the movie’s release, we know a lot more about the Atlantic Ocean’s circulation. Instruments deployed in the ocean starting in 2004 show that the Atlantic Ocean circulation has observably slowed over the past two decades, possibly to its weakest state in almost a millennium. Studies also suggest that the circulation has reached a dangerous tipping point in the past that sent it into a precipitous, unstoppable decline, and that it could hit that tipping point again as the planet warms and glaciers and ice sheets melt.

In a new study using the latest generation of Earth’s climate models, we simulated the flow of fresh water until the ocean circulation reached that tipping point.

The results showed that the circulation could fully shut down within a century of hitting the tipping point, and that it’s headed in that direction. If that happened, average temperatures would drop by several degrees in North America, parts of Asia and Europe, and people would see severe and cascading consequences around the world.

We also discovered a physics-based early warning signal that can alert the world when the Atlantic Ocean circulation is nearing its tipping point.

The ocean’s conveyor belt

Ocean currents are driven by winds, tides and water density differences.

In the Atlantic Ocean circulation, the relatively warm and salty surface water near the equator flows toward Greenland. During its journey it crosses the Caribbean Sea, loops up into the Gulf of Mexico, and then flows along the U.S. East Coast before crossing the Atlantic.

How the Atlantic Ocean circulation changes as it slows. IPCC 6th Assessment Report

This current, also known as the Gulf Stream, brings heat to Europe. As it flows northward and cools, the water mass becomes heavier. By the time it reaches Greenland, it starts to sink and flow southward. The sinking of water near Greenland pulls water from elsewhere in the Atlantic Ocean and the cycle repeats, like a conveyor belt.

Too much fresh water from melting glaciers and the Greenland ice sheet can dilute the saltiness of the water, preventing it from sinking, and weaken this ocean conveyor belt. A weaker conveyor belt transports less heat northward and also enables less heavy water to reach Greenland, which further weakens the conveyor belt’s strength. Once it reaches the tipping point, it shuts down quickly.

What happens to the climate at the tipping point?

The existence of a tipping point was first noticed in an overly simplified model of the Atlantic Ocean circulation in the early 1960s. Today’s more detailed climate models indicate a continued slowing of the conveyor belt’s strength under climate change. However, an abrupt shutdown of the Atlantic Ocean circulation appeared to be absent in these climate models.

How the ocean conveyor belt works.

This is where our study comes in. We performed an experiment with a detailed climate model to find the tipping point for an abrupt shutdown by slowly increasing the input of fresh water.

We found that once it reaches the tipping point, the conveyor belt shuts down within 100 years. The heat transport toward the north is strongly reduced, leading to abrupt climate shifts.

The result: Dangerous cold in the North

Regions that are influenced by the Gulf Stream receive substantially less heat when the circulation stops. This cools the North American and European continents by a few degrees.

The European climate is much more influenced by the Gulf Stream than other regions. In our experiment, that meant parts of the continent changed at more than 5 degrees Fahrenheit (3 degrees Celsius) per decade – far faster than today’s global warming of about 0.36 F (0.2 C) per decade. We found that parts of Norway would experience temperature drops of more than 36 F (20 C). On the other hand, regions in the Southern Hemisphere would warm by a few degrees.

The annual mean temperature changes after the conveyor belt stops reflect an extreme temperature drop in northern Europe in particular. René M. van Westen

These temperature changes develop over about 100 years. That might seem like a long time, but on typical climate time scales, it is abrupt.

The conveyor belt shutting down would also affect sea level and precipitation patterns, which can push other ecosystems closer to their tipping points. For example, the Amazon rainforest is vulnerable to declining precipitation. If its forest ecosystem turned to grassland, the transition would release carbon to the atmosphere and result in the loss of a valuable carbon sink, further accelerating climate change.

The Atlantic circulation has slowed significantly in the distant past. During glacial periods when ice sheets that covered large parts of the planet were melting, the influx of fresh water slowed the Atlantic circulation, triggering huge climate fluctuations.

So, when will we see this tipping point?

The big question – when will the Atlantic circulation reach a tipping point – remains unanswered. Observations don’t go back far enough to provide a clear result. While a recent study suggested that the conveyor belt is rapidly approaching its tipping point, possibly within a few years, these statistical analyses made several assumptions that give rise to uncertainty.

Instead, we were able to develop a physics-based and observable early warning signal involving the salinity transport at the southern boundary of the Atlantic Ocean. Once a threshold is reached, the tipping point is likely to follow in one to four decades.

A climate model experiment shows how quickly the AMOC slows once it reaches a tipping point with a threshold of fresh water entering the ocean. How soon that will happen remains an open question. René M. van Westen

The climate impacts from our study underline the severity of such an abrupt conveyor belt collapse. The temperature, sea level and precipitation changes will severely affect society, and the climate shifts are unstoppable on human time scales.

It might seem counterintuitive to worry about extreme cold as the planet warms, but if the main Atlantic Ocean circulation shuts down from too much meltwater pouring in, that’s the risk ahead.

This article was updated on Feb. 11, 2024, to fix a typo: The experiment found temperatures in parts of Europe changed by more than 5 F per decade.

René van Westen, Postdoctoral Researcher in Climate Physics, Utrecht University; Henk A. Dijkstra, Professor of Physics, Utrecht University, and Michael Kliphuis, Climate Model Specialist, Utrecht University

This article is republished from The Conversation under a Creative Commons license. Read the original article.

The changing composition of the world's oceans - acidification

The 12th meeting of the Parties to the Convention on Biological Diversity is currently underway at Pyeongchang in South Korea (6-17 October, 2014). One of the research papers presented at the meeting has addressed the issue of ocean acidification. Acidification of the ocean, caused by increased level of absorption of carbon emissions, has often been cited and identified as major threat to the environment and marine life in particular. Increasingly the evidence of this serious threat to marine life has needed quantification which at last appears to have occured. The sea's acidity level has increased by over 26% during the past 200 years with impacts on corals, shellfish and other calcium-making organisms. The effect of acidification is most strongly felt by tropical coral reefs such as the Great Barrier Reef which is already exposed to warmer water. The impact extends beyond marine organisms and ecosystems but also to over 400 million people who depend on the ocean habitats for survival.

The link is below:

12th Meeting of the Convention on Biological Diversity

Climate Change Insight: Impact of cooler oceans is only temporary

Researchers at UCLA in San Diego in the United States have just released research that shows that the cooling of eastern Pacific Ocean waters has been counteracting the warming effect of greenhouse gases. The impact from this natural variability in ocean cycles is responsible for the pause or “hiatus” in global warming over the last ten years. This is not a permanent effect and will end leading to a resumption in global warming as before.

The UCLA  study examines the tropical Pacific Decadal Oscillation, a climate cycle that occurs over the course of several decades. Within this large pattern are the El Niño and La Niña  cycles that cause shifts in the distribution of warm water in the equatorial Pacific Ocean. While El Niño and La Niña last only a few years, the Pacific Decadal Oscillation lasts several decades. The Oscillation has been in a cooling phase since 1998.

When the climate cycle that governs that ocean cooling reverses and begins warming again, the planet-wide direction toward higher temperatures will resume.

As the researchers have noted, before 2000, global temperatures had risen at a rate of 0.13C per decade since 1950. The hiatus in warming has happened while levels of carbon dioxide, the main greenhouse gas, continue rising steadily. In May 2013, carbon dioxide reached 400 parts per million in the atmosphere for the first time in human history. This study does not refute climate change models, but only reinforces the understanding of the various dynamic forces at work in the environment.

UNESCO's concern over the future of Australia's Great Barrier Reef

Bleached coral - Great Barrier Reef, Australia

The latest report from the World Heritage Committee of the United Nations Educational, Scientific and Cultural Organisation (UNESCO) tabled at the thirty-seventh session in June 2013, raises the uncomfortable proposal to list the Great Barrier Reef on the list of World Heritage sites in danger. UNESCO routinely monitors World Heritage sites and comments on efforts by states parties to maintain these listed valuable sites within their geographical borders. The UN body has raised concerns about coastal development, particularly the Port of Gladstone, and water quality with the Australian and Queensland Governments in previous years but found that key issues were not being fully addressed. In addition there was a lack of transparency in information being provided by the Queensland Government.

In relation to the Great Barrier Reef and Australia, the UN report specifically states:

The World Heritage Centre and IUCN recommend that the World Heritage Committee reiterate its request to the State Party (Australia) to undertake the following actions:

a) make a clear financial commitment to maintain the Reef Rescue programme and ensure water quality continues to improve,
 
b) halt the approval of coastal development projects that could individually or cumulatively impact on the property’s Oustanding Universal Value (OUV) and compromise the ongoing Strategic Assessment, and
 
c) ensure that the legislation protecting the property remains strong and adequate to maintain and enhance its OUV.
 
They further recommend that the Committee consider the Great Barrier Reef for inscription on the List of World Heritage in Danger at its 38th session in 2014 in the absence of a firm and demonstrable commitment on these priority issues by the State Party.

The Great Barrier Reef is one of the wonders of the world and the largest coral reef on the planet. The international recognition of serious threats to its' survival should galvanise further action domestically as warnings from local marine scientists appear to have been largely unsuccessful.

The Great Barrier Reef and climate change

Great Barrier Reef, Queensland Australia

New Australian research casts doubt on the forecasts that the Great Barrier Reef will be destroyed within a generation by climate change, while finding that corals are capable of better adaptation than previously believed.  The study, reported in the international journal Science, accepts that reefs are threatened by global warming and are already deteriorating but not at a rate which predictions given for the Great Barrier Reef.

Coral reefs are, by nature, highly diverse and resilient, and could cope with climate change in various ways. In the conclusion, the research report  states that “New knowledge confirms that coral reefs, at least as presently structured, are indeed threatened by climate change, but that current projections of global-scale collapse of reefs within the next few decades probably overestimate the rapidity and spatial homogeneity of the decline"

Prominent reef scientist Ove Hoegh-Guldberg said in April 2011 the reef would die unless carbon emissions were dramatically cut within the next decade.

The review of the science on global reef health by Professor Connolly and three other leading researchers, including Dr John Pandolfi of Queensland University, found that recent mathematical modelling of coral thermal tolerances suggested a wide range of outcomes, from a complete collapse of reefs by mid-century to maintenance of existing coral cover to 2100 and beyond.

The study stated that abundant evidence of coral sensitivity to ocean warming and acidification had played a key role in many predictions that the disappearance of coral reefs on a global scale will be irreversibly under way within a matter of decades.  However, this may not "adequately take account" of the capacity of corals to cope with and adapt to environmental stress. On the negative side, the Great Barrier Reef was more vulnerable to acidification hitting calcium carbonate levels, which are the building blocks of coral growth.

Science 22 July 2011:
Vol. 333 no. 6041 pp. 418-422
DOI: 10.1126/science.1204794