Half of Earth’s coral reefs will be permanently damaged by 2035 if climate change continues unabated, a new study has warned.
Researchers from the University of Hawaii predicted the effects to Earth’s coral reefs based on changes to sea surface temperature, ocean acidification, tropical storms, land use and human population.
Their findings suggest that, under a worst-case scenario, half of all coral reef ecosystems worldwide will be damaged in just a dozen years.
‘While the negative impacts of climate change on coral reefs are well known, this research shows that they are actually worse than anticipated due to a broad combination of climate change-induced stressors,’ said lead author Renee Setter.
Half of Earth’s coral reefs will be permanently damaged by 2035 if climate change continues unabated, a new study has warned
Previous studies have shown how coral reefs are extremely sensitive to changes in their environment.
For example, research has shown how marine heatwaves can cause huge swathes of reefs to bleach.
But in the new study, the researchers set out to understand the timeline for these effects.
The researchers used climate models to look at global projections of five environmental stressors – sea surface temperature, ocean acidification, tropical storm, land use and human population projections.
Their results revealed that, when looking at a single stressor under current conditions, half of the world’s coral reefs will be permanently damaged by 2050.
However, when multiple stressors are considered, this date falls to 2035.
‘It was enlightening to find that coral would face multiple stressors – posing an even greater hurdle and challenge that would need to be overcome to increase the possibility of survival,’ Ms Setter said.
The researchers used climate models to look at global projections of five environmental stressors – sea surface temperature, ocean acidification, tropical storm, land use and human population projections
In addition, the researchers found that by 2055, 99 per cent of the world’s coral reefs will face unsuitable conditions based on at least one of the five stressors.
And by 2100, 93 per cent of reefs will be under threat by two or more of the stressors, according to the study.
‘We know that corals are vulnerable to increasing sea surface temperatures and marine heatwaves due to climate change,’ said Erik Franklin, co-author of the study.
‘But it is important to include the complete anthropogenic impact and numerous stressors that coral reefs are exposed to in order to get a better sense of the overall risks to these ecosystems.
‘This has great implications for our local Hawaiian reefs that are key to local biodiversity, culture, fisheries and tourism.’
The team now plans to carry out further studies to assess how climate change will affect individual coral species.
WHAT ARE MARINE HEATWAVES AND WHAT DO WE KNOW ABOUT THEM?
On land, heatwaves can be deadly for humans and wildlife and can devastate crops and forests.
Unusually warm periods can also occur in the ocean. These can last for weeks or months, killing off kelp forests and corals, and producing other significant impacts on marine ecosystems, fishing and aquaculture industries.
Yet until recently, the formation, distribution and frequency of marine heatwaves had received little research attention.
Long-term change
Climate change is warming ocean waters and causing shifts in the distribution and abundance of seaweeds, corals, fish and other marine species. For example, tropical fish species are now commonly found in Sydney Harbour.
But these changes in ocean temperatures are not steady or even, and scientists have lacked the tools to define, synthesize and understand the global patterns of marine heatwaves and their biological impacts.
At a meeting in early 2015, we convened a group of scientists with expertise in atmospheric climatology, oceanography and ecology to form a marine heatwaves working group to develop a definition for the phenomenon: A prolonged period of unusually warm water at a particular location for that time of the year. Importantly, marine heatwaves can occur at any time of the year, summer or winter.
Unusually warm periods can last for weeks or months, killing off kelp forests and corals, and producing other significant impacts on marine ecosystems, fishing and aquaculture industries worldwide (pictured)
With the definition in hand, we were finally able to analyse historical data to determine patterns in their occurrence.
Analysis of marine heatwave trends
Over the past century, marine heatwaves have become longer and more frequent around the world. The number of marine heatwave days increased by 54 per cent from 1925 to 2016, with an accelerating trend since 1982.
We collated more than 100 years of sea surface temperature data around the world from ship-based measurements, shore station records and satellite observations, and looked for changes in how often marine heatwaves occurred and how long they lasted.
This graph shows a yearly count of marine heatwave days from 1900 to 2016, as a global average.
We found that from 1925 to 1954 and 1987 to 2016, the frequency of heatwaves increased 34 per cent and their duration grew by 17 per cent.
These long-term trends can be explained by ongoing increases in ocean temperatures. Given the likelihood of continued ocean surface warming throughout the 21st century, we can expect to see more marine heatwaves globally in the future, with implications for marine biodiversity.
‘The Blob’ effect
Numbers and statistics are informative, but here’s what that means underwater.
A marine ecosystem that had 30 days of extreme heat in the early 20th century might now experience 45 days of extreme heat. That extra exposure can have detrimental effects on the health of the ecosystem and the economic benefits, such as fisheries and aquaculture, derived from it.
A number of recent marine heatwaves have done just that.
In 2011, a marine heatwave off western Australia killed off a kelp forest and replaced it with turf seaweed. The ecosystem shift remained even after water temperatures returned to normal, signalling a long-lasting or maybe even permanent change.
That same event led to widespread loss of seagrass meadows from the iconic Shark Bay area, with consequences for biodiversity including increased bacterial blooms, declines in blue crabs, scallops and the health of green turtles, and reductions in the long-term carbon storage of these important habitats.
Examples of marine heatwave impacts on ecosystems and species. Coral bleaching and seagrass die-back (top left and right). Mass mortality and changes in patterns of commercially important species s (bottom left and right)
Similarly, a marine heatwave in the Gulf of Maine disrupted the lucrative lobster fishery in 2012. The warm water in late spring allowed lobsters to move inshore earlier in the year than usual, which led to early landings, and an unexpected and significant price drop.
More recently, a persistent area of warm water in the North Pacific, nicknamed ‘The Blob’, stayed put for years (2014-2016), and caused fishery closures, mass strandings of marine mammals and harmful algal bloom outbreaks along the coast. It even changed large-scale weather patterns in the Pacific Northwest.
As global ocean temperatures continue to rise and marine heatwaves become more widespread, the marine ecosystems many rely upon for food, livelihoods and recreation will become increasingly less stable and predictable.
The climate change link
Anthropogenic, that is human-caused, climate change is linked to some of these recent marine heatwaves.
For example, human emissions of greenhouse gases made the 2016 marine heatwave in tropical Australia, which led to massive bleaching of the Great Barrier Reef, 53 times more likely to occur.
Even more dramatically, the 2015-16 marine heatwave in the Tasman Sea that persisted for more than eight months and disrupted Tasmanian fisheries and aquaculture industries was over 300 times more likely, thanks to anthropogenic climate change.
For scientists, the next step is to quantify future changes under different warming scenarios. How much more often will they occur? How much warmer will they be? And how much longer will they last?
Ultimately, scientists should develop forecasts for policy makers, managers and industry that could predict the future impacts of marine heatwaves for weeks or months ahead. Having that information would help fishery managers know when to open or close a fishery, aquaculture businesses to plan harvest dates and conservation managers to implement additional monitoring efforts.
Forecasts can help manage the risks, but in the end, we still need urgent action to curb greenhouse gas emissions and limit global warming. If not, marine ecosystems are set for an ever-increasing hammering from extreme ocean heat.
Source: Eric Oliver, Assistant Professor, Dalhousie University; Alistair Hobday, Senior Principal Research Scientist – Oceans and Atmosphere, CSIRO; Dan Smale, Research Fellow in Marine Ecology, Marine Biological Association; Neil Holbrook, Professor, University of Tasmania; Thomas Wernberg, ARC Future Fellow in Marine Ecology, University of Western Australia in a piece for The Conversation.
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