Is There Evidence That Climate Change Is Impacting the Great Barrier Reef?

Impact of climate change on the Great Barrier Reef

Check out this answer from Consensus:

The evidence strongly indicates that climate change is having a profound impact on the Great Barrier Reef, primarily through increased coral bleaching, mortality, and changes in coral assemblages. The combined effects of climate change and local stressors are exacerbating the decline of the reef. However, effective local management and strong global mitigation efforts can improve the resilience of the reef and delay its decline. Immediate and sustained actions are crucial to safeguard the future of the Great Barrier Reef.

The Great Barrier Reef (GBR) is facing significant threats from climate change, which is exacerbating existing stressors and leading to severe ecological consequences. This synthesis examines the evidence from multiple research papers on how climate change is impacting the GBR.

Key Insights

• Coral Bleaching and Mortality:
  • Elevated ocean temperatures are causing increased coral bleaching and mortality, with severe events noted during marine heatwaves, such as the one in 20162 3 4.
  • The loss of thermal tolerance mechanisms in corals due to rising temperatures is leading to higher rates of bleaching and death2.
• Changes in Coral Assemblages:
  • Heat stress from global warming is transforming coral reef assemblages, with fast-growing corals suffering catastrophic die-offs, altering the ecological structure and function of the reefs3.
• Cumulative Stressors:
  • The combined effects of climate change and local stressors, such as nutrient runoff, cyclones, and crown-of-thorns starfish predation, are significantly reducing coral cover and reef performance1 4 7 8.
  • Increased cyclone intensity due to climate change is accelerating coral reef degradation, causing extensive ecological damage and slow recovery10.
• Management and Mitigation:
  • Effective local management, such as improving water quality and controlling crown-of-thorns starfish populations, can enhance coral resilience and delay reef decline1 7 8 9.
  • Scenarios with strong carbon mitigation and improved local management predict better outcomes for coral recovery and sustained reef health1 8.
• Future Projections:
  • Without significant interventions, all climate scenarios predict a steep decline in coral cover over the next 50 years7.
  • Scenarios with strong mitigation efforts show potential for significant coral recovery and reduced vulnerability to climate change1 8.

Is there evidence that climate change is impacting the Great Barrier Reef?

Roger Beeden has answered Near Certain

An expert from Great Barrier Reef Marine Park Authority in Environmental Science

Monitoring evidence from the past 30 years clearly demonstrates that the Great Barrier Reef is being increasingly impacted by disturbances; several of which are already being exacerbated by climate change driven increases in temperature, particularly the very severe mass coral bleaching events of 2016 and 2017. Those two back to back events were caused by extended periods of above average sea temperatures during the first half of 2016 and then 2017 that is attributable to climate change.

Corals required tropical temperatures for the symbiosis between themselves and the tiny unicellular algae inside them to work. The algae provide most of the coral’s energy through photosynthesis, and the proper functioning of the symbiosis has been so successful that it has enabled the relatively simple invertebrate coral animals to build the world’s largest living structure the Great Barrier Reef. However, when sea temperatures exceed the tolerance level of corals then the symbiosis breaks down, the corals expel the algae in an effort to remain alive, the transparent coral tissues that house the algae become see through revealing the white calcium carbonate coral skeleton below. This process is widely known as coral bleaching and while it can occur as result of several different stressors (changes in salinity, pollution and even cold water) when it occurs over large areas it is exclusively a result of extended periods of elevated summer temperatures and excess light. If temperatures reduce to normal levels relatively quickly then corals can reabsorb their symbiotic algae and recover. However, if temperatures remain abnormally high then the corals will die primarily because they are starved of their source of energy.

There is no doubt that average sea surface temperatures have been increasing as these have been reliably measured over long periods on the Great Barrier Reef through partnerships between AIMS, BoM and CSIRO. There is a clear relationship between elevated temperatures in the Great Barrier Reef region and the mass bleaching events of 1998, 2002, 2016 and 2017 (and a localised event in 2006). The 2016 and 2017 were the most severe coral bleaching events on record causing extensive mortality to corals across the top two thirds of the Great Barrier Reef. Importantly, the bottom third of the Great Barrier Reef Marine Park was much less affected as temperatures were not as consistently elevated in that region. Hence media reports from places such as the Keppel Islands suggesting that bleaching wasnt occuring on the Great Barrier Reef based on observations in areas away from the main impact region are inconsistent with extensive coral bleaching aerial survey and in water field survey evidence from the top two thirds of the Reef.

Beyond the Great Barrier Reef region, extended periods of elevated sea surface temperatures resulted in a worldwide coral bleaching that resulted in unprecedented reef impacts.

Temperature impacts are not the only risks that climate change and global emissions driven ocean acidification poses to coral reef ecosystems worldwide. While it is too early to definitively link individual cyclones to climate change it is anticipated that severe cyclones are likely to become more frequent even if the overall number of cyclones does not increase. The more severe a cyclone is the more likely it is to be able to cause substantial structural damage to reefs. The recent seriece of very severe cyclones crossing the Great Barrier Reef over the past decade are combined with COTS and coral bleaching impacts the major causes of decline in coral cover and condition that has been observed through the AIMS long term monitoring program.

An Assessment of the Vulnerability of the Great Barrier Reef to Climate Change was completed in 2007. The assessment employed more than 70 leading experts in the evaluation of how climate change is currently and is most likely to affect the wide range of habitats. species groups and species found in the Great Barrier Reef. The findings were a core source of information for the 2009 Great Barrier Reef Outlook Report which highlighted climate change as the most significant threat to the Great Barrier Reef. This assessment was updated in 2014 summarising further evidence of the existing impacts and future threats posed by climate change.

In summary, there is abundant evidence that climate change is already severely affecting the health and resilience of the Great Barrier Reef and those impacts are forecast to worsen as the climate continues to change. This is the primary reason for increasing investment in additional measures to support the resilience of the system and a reminder of the urgent need for decisive global action to reduce greenhouse gas emissions.

Is there evidence that climate change is impacting the Great Barrier Reef?

Tom Bridge has answered Near Certain

An expert from James Cook University in Marine Biology

There is now incontrovertible evidence that climate change is affecting coral reefs worldwide, including Australia’s Great Barrier Reef. Coral reef ecosystems are particularly sensitive to temperature because corals rely on a partnership with symbiotic algae, called zooxanthellae, in order to survive. Abnormally high water temperatures, which are increasingly common due to increased atmospheric carbon dioxide, can kill corals both directly through heat stress and indirectly through multiple factors, such a breakdown of the relationship between corals and zooxanthellae and increased disease incidence following heat stress. The stress caused by high temperatures causes corals to expel their zooxanthellae in a process known as coral bleaching, so called because the corals turn white.

We also know that thermal stress and associated coral bleaching events are becoming more frequent and severe. Bleaching of corals over hundreds to thousands of kilometres, known as mass bleaching events, were unknown until the 1980s. The first major global-scale bleaching event was in 1997-98, with subsequent events in 2010 and 2015-16. There have been numerous other regional-scale bleaching events in other years; for example, the second mass bleaching event on the Great Barrier Reef occurred in 2002.

Throughout their lifetimes, corals produce a skeleton made of a substance called aragonite, a form of calcium carbonate. The chemical composition of this skeleton can reveal information about the environment at the time, including the temperature and salinity of the seawater. Some corals can survive for hundreds of years, thereby providing scientists a record of changes in seawater conditions over several centuries, just like ice cores collected from polar regions. Since the growth rates of corals change seasonally, their skeletons contain growth rings like those on trees. By collecting large numbers of coral cores across large geographic scales, scientists can accurately reconstruct environmental conditions in each year going back several centuries. Coral bleaching, in particular, leaves distinctive layers in the coral skeleton. Using these coral archives, we can confidently say that mass bleaching events did not occur on the Great Barrier Reef prior to the 1980s, even though there were no divers out there conducting surveys. Mass bleaching is clearly a recent phenomenon, not something that has always occurred on the Great Barrier Reef.

Bleaching events tend to be caused by ‘marine heatwaves’, periods of weeks to months where sea temperatures are elevated well above normal, rather than ‘background’ warming. Marine heatwaves are increasing in frequency in many parts of the world, including Australia, and will continue to become more frequent as temperatures rise. In addition, background warming continues to increase long-term average sea temperatures, which means marine heatwaves that may have had relatively little impact 30 years ago can now cause widespread damage. El Nino events, such as those that occurred in 1997-98 and 2016-17, are often associated with mass bleaching. However, warmer average sea temperatures mean that any local conditions that increase summer temperatures can now cause bleaching; for example, the 2002 bleaching event on the Great Barrier Reef was not due to El Nino, just a period of hot summer weather. While El Nino conditions do tend to result in higher than average global temperatures, we no longer need an El Nino to generate bleaching conditions. Similarly, we also know from our coral cores that prior to the 1980s, even warmer temperatures associated with El Nino were not sufficient to cause mass coral bleaching. Clearly, coral bleaching is a recent phenomenon that is directly attributable to rising temperatures caused by increased atmospheric carbon dioxide concentrations.

In addition to temperature extremes, other aspects of climate change can negatively impact upon the Great Barrier Reef. For example, the Great Barrier Reef has recently seen a spate of particularly severe tropical cyclones. While cyclones have always occurred on the Great Barrier Reef, severe tropical cyclones cause disproportionately more damage than weaker storms. In the last ten years there has been an unusually high number of severe tropical cyclones that have cumulatively impacted almost the entire Great Barrier Reef. While this time frame is too short to say definitively whether these observations represent a long term trend, climate models generally indicate an increase in storm intensity due to climate change. Importantly, research has shown that increased cyclone frequency alone could dramatically decrease coral cover across the entire Great Barrier Reef, even if there were no effects from other climate-related stresses such as mass bleaching.

There is now irrefutable evidence that climate change is negatively impacting the Great Barrier Reef, and every effort to reduce atmospheric carbon dioxide must be made to ensure its persistence.

Is there evidence that climate change is impacting the Great Barrier Reef?

Sjannie Lefevre has answered Near Certain

An expert from University of Oslo in Ecophysiology

As the sea surface temperature has increased and continue to rise as a result of climate change, it also increases the likelyhood of reaching temperatures above the tolerance levels of the algae living inside corals – the core organism of the reefs. Given the relationship between high temperature and coral bleaching, the most likely explanation for the increasing number of bleaching events is global warming (sources: https://www.nature.com/articles/s41586-018-0041-2, https://www.coralcoe.org.au/for-managers/coral-bleaching-and-the-great-barrier-reef).

Is there evidence that climate change is impacting the Great Barrier Reef?

Timothy A C Gordon has answered Near Certain

An expert from University of Exeter in Marine Biology

Climate change is causing sea temperatures to rise, which causes significant damage to corals. Corals are damaged by bleaching, where in high temperatures they lose their symbiotic zooxanthellae and are no longer able to photosynthesise. The death of many corals as a result of this process causes the whole ecosystem to collapse. In some areas of the Great Barrier Reef, around 80% of the corals died in the space of several weeks in the 2016-2017 bleaching event. The increasing frequency of bleaching events due to climate change, and the damage that they have on the Great Barrier Reef and other reefs worldwide, are well documented in rigorous peer-reviewed research such as Hughes et al 2018 (“Spatial and temporal patterns of mass bleaching of corals in the Anthropocene”, Science), Hughes et al 2017 (“Global warming and recurrent mass bleaching of corals”, Nature), and Ainsworth et al 2016 (“Climate change disables coral bleaching protection on the Great Barrier Reef”, Science).