Destructive Marine Heatwaves Are Wreaking Havoc On Australia’s Great Barrier Reef

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It’s a World Heritage site that humans have seen from outer space. It’s the planet’s biggest single structure made by living organisms. Home to 30 species of cetaceans, it is Australia’s Great Barrier Reef, and hundreds of miles of its most pristine northern sector are dead. Overheated seawater is the culprit, and we humans are responsible.

Climate change is the greatest threat to the Great Barrier Reef and coral reefs worldwide. Current rates of greenhouse gas emissions will spur global average temperature rise, and a special report by the Intergovernmental Panel on Climate Change projects that 1.5°C – 2°C temperature rises will cause the health and viability of the Reef to diminish exponentially, with dire consequential effects on communities that depend on the Reef.

A position paper by the Australian government says that the worst is upon us. Only the strongest and fastest possible actions to decrease global greenhouse gas emissions, they argue, will reduce the risks and limit the impacts of climate change on the Reef.

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In March 2017, the journal Nature published a paper showing that huge sections of an 800-kilometer (500 mile) stretch in the northern part of the reef had died the previous year due to high water temperatures, an event that the authors ascribed to the effects of global climate change.

Global analyses show climate change has contributed to a 5x increase in the frequency of severe coral bleaching events over the past 40 years. Multiple severe tropical cyclones and floods have had cumulative impacts on the Reef’s ecosystem, including seagrass meadows, coral reefs, and dugong and turtle populations. Pressures on the Reef and its ecosystem include runoff, dumping of dredging sludge, and cyclic population outbreaks of the crown-of-thorns starfish.

Other impacts such as rising sea level are projected to result in increased coastal erosion and inundation of critical nesting habitats, while changes in the patterns of ocean circulation will likely lead to shifts in the distribution and abundance of species in the Great Coral Reef.

From Emissions to Marine Heatwaves

Global emissions of greenhouse gases such as carbon dioxide from the burning of fossil fuels, agriculture, and land clearing are causing climate change. It is estimated these human activities have driven an approximate 1°C increase in global average temperature above pre-industrial levels.

Continuing and rapid increase in global temperature increases the likelihood of marine heatwaves. Coral reef ecosystems depend on the health of reef-building coral species, which have limited capacity to endure heat stress. Increased sea temperature can directly cause mass bleaching and mortality. Associated impacts from altered weather patterns — such as more intense storms, tropical cyclones and flood events — ocean acidification, and rising sea level also damage coral reef ecosystems.

Strong global action to curb climate change is needed urgently to give the Great Barrier Reef the best its only chance of survival.

Local Impacts of Climate Change on the Great Barrier Reef

The widespread impacts of climate change on the Reef are already evident.

• Bleaching events were observed in the 1980s at temperatures of 0.5°C above pre-industrial conditions.
• In 2016, an average of 30% of shallow-water corals (at depths between two and 10 meters) were lost across the whole Reef, with the majority of mortality occurring in the northern third. Bleaching and mortality generally declined with depth: however, severe bleaching and some mortality of corals were also observed on northern reefs along the outer shelf at 40 meters depth.
• In 2017, the spatial extent of severe bleaching was estimated by aerial surveys only. Given the severity of bleaching observed, it is certain that this event caused a further decline in coral cover across the northern two-thirds of the Marine Park.
• Together with the impacts of 11 severe tropical cyclones since 2005, these events have caused an  unprecedented decline in the health of the Reef.
• Climate change is causing the rapid feminization of green turtles, as the sex of turtle hatchlings is determined by temperature. In a recent study, only about 1% of juvenile turtles from warmer nesting beaches in the northern Great Barrier Reef hatched male, compared to 31-34% from cooler southern nesting beaches, raising concerns about viability of future populations.

Why Should We Care about the Great Barrier Reef?

Coral reefs are important for many different reasons, including containing the most diverse ecosystems on the planet. According to the Queensland government, they:

• protect coastlines from the damaging effects of wave action and tropical storms
• provide habitats and shelter for many marine organisms
• are the source of nitrogen and other essential nutrients for marine food chains
• assist in carbon and nitrogen fixing
• help with nutrient recycling

Other reasons why they are so important include:

• The fishing industry depends on coral reefs because many fish spawn there, and juvenile fish spend time there before making their way to the open sea.
• The Great Barrier Reef generates more than $1.5 billion  every year for the Australian economy, from fishing and tourism.
• The study of coral reefs is important for providing a clear, scientifically-testable record of climatic events over the past million years or so. This includes records of recent major storms and human impacts that are recorded by the changes in coral growth patterns.

Reducing biodiversity through the extinction of species inevitably leads to the breakdown in ecosystem health and function. Healthy ecosystems are essential to provide us with:

• natural resources, such as foods and drugs
• services we depend upon, such as recycling and purification of water and air, the creation of soil, and the break-down of pollutants
• social, cultural and recreational activities, such as those found in our many unique National Parks, World Heritage Areas and the other special places we like to visit with high species diversity

Climate Action to Mitigate Further Reef Decline

Only the strongest and fastest possible action on climate change will reduce the risks and limit the impacts of climate change on the Reef. The required reduction in global greenhouse gas emissions demands an international and national response to secure a better future for the Reef.

The Great Barrier Reef Marine Park Authority outlines the following actions:

• partnerships, plans, and actions that reduce cumulative impacts on the Reef
• actions that build Reef resilience and protect key species for reef recovery
• actions that enable adaptation and restoration of reef habitats

Even relatively small increases in acidity reduce the capability of corals and other calcifying organisms to build skeletons and shells, which, in turn, leads to a reduction in habitat available to support reef biodiversity. The weakening of skeletons of coral and other reef-builders further affects their capacity to resist and recover from physical damage caused by tropical cyclones.

These effects are likely to include loss of properties and infrastructure, loss of cultural and regional identity, and, unless urgent action is taken, subsequent declines in regional economies. Impacts on communities globally are likely to be significant, as more than 500 million people depend on coral reefs for their livelihoods and food security.

Final Thoughts

For the Reef and coral reefs worldwide, there is growing recognition that limiting the increase in global average temperature to 1.5°C — and ideally less — is critical to minimize significant environmental and societal costs from the loss of reef habitats. Coral reefs are projected to decline by a further 70-90% at a 1.5°C increase in temperature, with greater losses at a 2°C increase.

Due to the greenhouse gases already in the atmosphere, delays in taking strong action to reduce global carbon emissions decrease the likelihood of limiting the temperature increase to below 1.5°C. Of particular concern are projections that the Reef could be affected by bleaching events twice per decade by about 2035 and annually by about 2044 if greenhouse gas emissions continue to increase at the current rate.

If bleaching becomes more frequent and more intense, there will not be enough time for reefs to recover and persist as coral-dominated systems in their current form.