Our research, published today in Nature, found far fewer baby corals are being produced than are needed to replace the large number of adult corals that have died. The rate at which baby corals are settling on the Great Barrier Reef has fallen by nearly 90% since 2016.
While coral does not always die after bleaching, repeated bleaching has killed large numbers of coral. This new research has negative implications for the Reef’s capacity to recover from high ocean temperatures.
How coral recovers
Most corals reproduce by “spawning”: releasing thousands of tight, buoyant bundles with remarkable synchronisation. The bundles burst when they hit the ocean surface, releasing eggs and/or sperm. Fertilised eggs develop into larvae as they are moved about by ocean currents. The larvae settle in new places, forming entirely new coral colonies. This coral “recruitment” is essential to reef recovery.
The research team, led by my colleague Terry Hughes from the ARC Centre of Excellence for Coral Reef Studies, measured rates of coral recruitment by attaching small clay tiles to the reef just before the predicted mass spawning each year. These settlement panels represent a standardised habitat that allows for improved detection of the coral recruits, which are just 1-2mm in size.
Almost 1,000 tiles were deployed across 17 widely separated reefs after the recent mass bleaching, in late 2016 and 2017. After eight weeks they were collected and carefully inspected under a microscope to count the number of newly settled coral recruits. Resulting estimates of coral recruitment were compared to recruitment rates recorded over two decades prior to the recent bleaching.
Rates of coral recruitment recorded in the aftermath of the recent coral bleaching were just 11% of levels recorded during the preceding decades. Whereas there were more than 40 coral recruits per tile before the bleaching, there was an average of just five coral recruits per tile in the past couple of years.
The Great Barrier Reef (GBR) is the world’s largest reef system. The large overall size and high number of distinct reefs provides a buffer against most major disturbances. Even if large tracts of the GBR are disturbed, there is a good chance at least some areas will have healthy stocks of adult corals, representing a source of new larvae to enable replenishment and recovery.
Larvae produced by spawning corals on one reef may settle on other nearby reefs to effectively replace corals lost to localised disturbances.
It is reassuring there is at least some new coral recruitment in the aftermath of severe bleaching and mass mortality of adult corals on the GBR. However, the substantial and widespread reduction of regrowth indicates the magnitude of the disturbance caused by recent heatwaves.
Declines in rates of coral recruitment were greatest in the northern parts of the GBR. This is where bleaching was most pronounced in 2016 and 2017, and there was the greatest loss of adult corals. There were much more moderate declines in coral recruitment in the southern GBR, reflecting generally higher abundance of adults corals in these areas. However, prevailing southerly currents (and the large distances involved) make it very unlikely coral larvae from southern parts of the Reef will drift naturally to the hardest-hit northern areas.
It is hard to say how long it will take for coral assemblages to recover from the recent mass bleaching. What is certain is low levels of coral recruitment will constrain coral recovery and greatly increase the recovery time. Any further large-scale developments with also greatly reduce coral cover and impede recovery.
This study further highlights the vulnerability of coral reefs to sustained and ongoing global warming. Not only do adult corals bleach and die when exposed to elevated temperatures, this prevents new coral recruitment and undermines ecosystem resilience.
The only way to effectively redress global warming is to immediately and substantially reduce global carbon emissions. This requires that all countries, including Australia, renew and strengthen their commitments to the Paris Agreement on climate change.
While further management is required to minimise more direct human pressure on coral reefs – such as sediment run-off and pollution – all these efforts will be futile if we do not address global climate change.
Environmental news is rarely good. But even by those low standards, 2018 was especially bad. That is the main conclusion from Australia’s Environment in 2018, the latest in an annual series of environmental condition reports, released today.
Every year, we analyse vast amounts of measurements from satellites and on-ground stations using algorithms and prediction models on a supercomputer. These volumes of data are turned into regional summary accounts that can be explored on our Australian Environment Explorer website. We interpret these data, along with other information from national and international reports, to assess how our environment is tracking.
Temperatures went up again, rainfall declined further, and the destruction of vegetation and ecosystems by drought, fire and land clearing continued. Soil moisture, rivers and wetlands all declined, and vegetation growth was poor.
The combined pressures from habitat destruction, climate change, and invasive pests and diseases are taking their toll on our unique plants and animals. Another 54 species were added to the official list of threatened species, which now stands at 1,775. That is 47% more than 18 years ago and puts Australia among the world’s worst performers in biodiversity protection. On the upside, the number of predator-proof islands or fenced-off reserves in Australia reached 188 in 2018, covering close to 2,500 square kilometres. They offer good prospects of saving at least 13 mammal species from extinction.
Globally, the increase of greenhouse gases in the atmosphere accelerated again after slowing down in 2017. Global air and ocean temperatures remained high, sea levels increased further, and even the ozone hole grew again, after shrinking during the previous two years.
Sea surface temperatures around Australia did not increase in 2018, but they nevertheless were well above long-term averages. Surveys of the Great Barrier Reef showed further declining health across the entire reef. An exceptional heatwave in late 2018 in Far North Queensland raised fears for yet another bout of coral bleaching, but this was averted when sudden massive downpours cooled surface waters.
The hot conditions did cause much damage to wildlife and vegetation, however, with spectacled flying foxes dropping dead from trees and fire ravaging what was once a tropical rainforest.
While previous environmental scorecards showed a mixed bag of regional impacts, 2018 was a poor year in all states and territories. Particularly badly hit was New South Wales, where after a second year of very poor rainfall, ecosystems and communities reached crisis point. Least affected was southern Western Australia, which enjoyed relatively cool and wet conditions.
It was a poor year for nature and farmers alike, with growing conditions in grazing, irrigated agriculture and dryland cropping each declining by 17-20% at a national scale. The only upside was improved cropping conditions in WA, which mitigated the 34% decline elsewhere.
It would be comforting to believe that our environment merely waxes and wanes with rainfall, and is resilient to yearly variations. To some extent, this is true. The current year may still turn wet and improve conditions, although a developing El Niño makes this less likely.
However, while we are good at acknowledging rapid changes, we are terrible at recognising slow, long-term ones. Underlying the yearly variations in weather is an unmistakable pattern of environmental decline that threatens our future.
What can we do about it?
Global warming is already with us, and strong action is required to avoid an even more dire future of rolling heatwaves and year-round bushfires. But while global climate change requires global action, there is a lot we can and have to do ourselves.
Australia is one of the world’s most wasteful societies, and there are many opportunities to clean up our act. Achieving progress is not hard, and despite shrill protests from vested interests and the ideologically blind, taking action will not take away our prosperity. Home solar systems and more efficient transport can in fact save money. Our country has huge opportunities for renewable energy, which can potentially create thousands of jobs. Together, we can indeed reduce emissions “in a canter” – all it takes is some clear national leadership.
The ongoing destruction of natural vegetation is as damaging as it is unnecessary, and stopping it will bring a raft of benefits. Our rivers and wetlands are more than just a source of cheap irrigation for big businesses. With more effort, we can save many species from extinction. Our farmers play a vital role in caring for our country, and we need to support them better in doing so.
A catastrophic event occurred on Earth 66 million years ago. A huge meteorite struck our planet in what is now Mexico, triggering mass extinctions of the dinosaurs and most other living creatures.
A new paper shows the first recorded victims of this impact were fish and other marine animals, stranded by a wave that left them high and dry in an ancient river in North Dakota, at a site called Tanis.
For scientists unpacking the evidence around the event, a full picture of the cataclysm has involved looking into the details of planetary surface physics during giant impacts.
If you’ve never heard of a form of wave called a seiche, this is your chance to catch up.
Waves of damage
The Chicxulub meteorite crater in coastal Mexico is strongly associated with the mass extinction of the dinosaurs (and 75% of all species), 66 million years ago.
The first victims were right at the site. Any marine creatures close to the point of impact would have been instantly vaporised (sadly leaving no fossil record), along with much of the surrounding rock.
Around the periphery, the energy of the impact melted and ejected tonnes of molten rock, which together with condensing rock vapour, formed little glass beads (“impact spherules”) that can be found in a layer around the world at this time.
The shock wave itself pulverised the adjacent rock enough to metamorphise it, forming features like “shocked quartz” – fractured quartz indicative of enormous pressures. It carried the energy equivalent of a magnitude 11 earthquake – 1,000 times more energy than the 2004 Boxing Day quake which killed almost 230,000 people.
Vast inland sea now gone
North Dakota is more than 3,000km away from the Chicxulub crater, and was a similar distance at the time of the meteorite impact event.
Separating them back then, however, was a vast inland sea that covered much of midwest USA, from Texas up to the Dakotas. Feeding into that inland sea was a river system upon which the Tanis site in North Dakota was formed. This site has preserved the earliest recorded deaths of the Chicxulub impact.
The site itself is unusual. The deposition of sediments can tell us about the flow of water in the river.
Most ripples (or flame structures) indicate a southerly flow of the river before and after the Tanis deposit. However, these flow indicators point the wrong way during the time the Tanis unit formed. Water was flowing upstream, fast.
At the site are also found the fossilised remains of species, like sharks and rays, that occupied brackish water, rather than the freshwater of the stream. These had to be brought inland from the sea by something, and left to die, smothered in sediment, on a riverbank.
Stranded in Dakota
The obvious candidate is an impact tsunami. Perhaps the impact of the meteorite hitting the ocean generated a huge wave that carried fish from the inland sea, and against the flow of fresh water, to leave the creatures stranded in Dakota?
But there are problems with this hypothesis. The tiny impact spherules that formed in Chicxulub can be found throughout the deposit (many clogging the gills of fish), and pockmarks in the sedimentary layers means rocks were still raining down. This means the surge of water occurred within around 15 minutes to two hours of the impact itself.
For a tsunami to travel the 3,000km from the point of impact, to the Tanis site across the inland sea, would have taken almost 18 hours. Something else killed these creatures.
The seismic waves from the impact would have travelled through the Earth much faster than a tsunami travelled across water – and arrived near Tanis between 6-13 minutes later. The authors of the Tanis study suggest these seismic waves may have triggered an unusual type of wave in the inland sea, called a seiche.
Seiches are standing waves in bodies of water, and are often found in large lake systems during strong winds. The winds themselves cause waves and water displacement, which can have a harmonic effect, causing the water to slosh side to side like an overfull bathtub.
However, earthquakes are also known to cause seiches. Particularly dramatic seiches are often seen in swimming pools during large quakes. The interaction of the seismic wave’s period (the time between two waves) with the timescale of waves sloshing in a pool can amplify their effect.
But seiches can affect larger bodies of water too.
During the 2011 Tohuku earthquake in Japan, seiches over 1m high were observed in Norwegian fjords more than 8,000km away. With an energy more than 1,000 times greater, the Chicxulub event could quite conceivably have generated bigger than 10 metre swells in the North American inland sea – the scale implied by the deposition of the Tanis site.
Given a seiche can be driven by seismic waves, it’s conceivable that one drove the surge that stranded marine creatures at Tanis, resulting in the short time between the impact debris and the surge deposit.
Still lots of questions
But a lot remains unclear regarding exactly what did happen 66 million years ago.
Could the fish stranding have been driven by the first seismic activity to appear at Tanis (the P and S waves in science parlance, which travel through the interior of the Earth, arriving at Tanis 6 and 10 minutes after impact, respectively), or the more destructive but slower surface waves at the top of the Earth’s crust, which arrived 13 minutes after impact?
Would the period of sloshing of a seiche be consistent with the scale of the inland sea? (The inland sea was much larger than most lakes seiches are traditionally observed in – and may or may not have been open to the ocean). Given so little is really known about the dimensions of the inland sea, this is hard to constrain.
The Tanis site has given us an incredible window into the first few hours of a mass-extinction. But it has also highlighted how little we have probed into the fatal surface physics of these extreme events.