An alarming claim has been made about the future of our Earth in recent months: that, at best, we're facing the collapse of civilisation or, at worst, a mass extinction.
The claims have been highlighted by environmental protest movement Extinction Rebellion and from climate crisis activist Greta Thunberg.
Ms Thunberg has warned of the "end of our civilisation as we know it". A rebellion spokeswoman told The Age: "We are risking the collapse of human civilisation and the deaths of billions of people."
What do they mean? And are they right?
What's behind the alarm?
The protesters warn of two separate but related threats: the holocene extinction, and global heating.
Holocene is the name of the geological epoch we live in. It's simply a category in science. The holocene stretches back 11,650 years to the end of the last ice age, and coincides with the rise of human civilisation.
The holocene extinction, also known as the sixth mass extinction, refers to fears that human activity is pushing a huge number of plants and animals to the brink of extinction.
There have been five confirmed mass extinction events in the Earth's history, such as the one that wiped out the dinosaurs. These were cataclysmic events triggered by phenomena such as volcanoes or meteorites. Scientists are still debating whether the current situation should be classified as a "mass extinction" of the kind previously seen or whether we are simply sending lots of species extinct very quickly. This would be the first mass extinction caused by human activity such as polluting and land clearing.
The holocene extinction is related to, but distinct from, the threat posed by global heating.
Scientists agree that if the world's emissions keep growing at their current rate, we are on track for warming of between 3 and 5 degrees by the end of the century.
Global governments pledged to keep warming to 2 degrees but pledges alone have not stopped greenhouse gas emissions increasing at unprecedented levels, driven largely by the industrialisation of China and India.
Global heating has its own worrying effects too.
So is a huge extinction in store?
For plants and animals, yes.
About 25 per cent of land species and 40 per cent of amphibians are now threatened with extinction, a 2019 UN evidence synthesis estimated. Some scientists have estimated that between 24 and 150 species become extinct every single day.
One in every 10 remaining species is already "committed to extinction", meaning without action to restore their habitats they will die out, many within decades. Most of the damage has been done by land clearing, logging, hunting, fishing and other human activities. Human actions have driven at least 680 vertebrate species extinct since 1500.
This is what Ms Thunberg refers to when she warns of the sixth mass extinction.
So far only one animal – as it happens, an Australian creature called the Bramble Cay melomys – has been declared extinct as a direct result of global heating.
One in every 10 remaining species will die out, many within decades.
This small rodent lived only on a low-lying island off the coast of Queensland; scientists believe rising sea levels and storm surges caused by climate change either deprived it of its food or literally swept it away to its doom.
Unlike humans, animals and plants cannot adapt to a quickly changing environment; scientists expect global heating to put more and more pressure on threatened species, adding to existing pressures from human activity and pushing them further toward extinction.
What about us? Will humans become extinct?
When Ms Thunberg and Extinction Rebellion talk about the collapse of civilisation, they are talking about the risks posed by global heating.
Will global heating mean the end of the human race?
“No. People will find refuge,” says Professor Steven Sherwood, deputy director of the University of NSW’s Climate Change Research Centre.
“But, will it bring about the downfall of civilisation? That’s tricky.”
There is good evidence that the predicted 3 to 5 degrees of global heating will result in the deaths of many millions of people living in poverty.
They will run out of water, starve or die in more frequent cyclones, floods, wildfires and storm surges.
But humans won’t die out. With enough money, we are extremely adaptable. Air-conditioning and desalination plants will allow those in more affluent societies to live in extremely hot, dry places. The rich can build sea walls, move to colder places, and pay more for food – which will become much more expensive.
But, even for them, life will be different.
“If you find you have 60 days a year where it’s above 40 degrees, it will change many aspects of our lives,” says Professor Brendan Mackey, director of Griffith University’s Climate Change Response Program.
The poor will not have the luxury of adapting.
Crop yields are projected to fall by 60 per cent in the Middle East and North Africa, exposing hundreds of millions to starvation, according to the World Bank (this may be counterbalanced with increases in farming productivity). What crops do grow would become less nutritious. The number of fish available in key fisheries could fall by up to 50 per cent.
Large parts of Bangladesh, home to 164 million people, will be inundated by sea levels that will rise by almost one metre. It is possible, although not certain, the Antarctic and Greenland ice sheets will melt (the Arctic will definitely melt), raising sea levels further.
Global heating will increase the number of droughts the world will see, and even the chance that conflicts will break out over remaining water supplies.
Available drinking water in already parched North Africa and the Middle East could fall by more than 45 per cent, the World Bank predicts.
Global heating is already causing more deadly heat waves, hotter days and nights, more floods, more storm surges, more cyclones and more wildfires. There will be more pandemics too, in so far as hotter weather increases the spread of fleas, ticks and mosquitoes all of which are disease carriers.
The frequency of these events will continue to increase as the world warms.
What if the world gets even hotter than 5 degrees?
We don’t know. Our existing models are less accurate for that time span.
Scientists who warn of catastrophic impacts say the things we should really fear are "feedback loops", vicious cycles of accelerated climate change.
“The effects of climate change are not linear. They compound. They cascade,” says Professor Mackey.
We know such loops exist, but scientists do not yet know how hot the Earth needs to get before they activate.
The effects of climate change are not linear. They compound. They cascade.
One example is the thawing of the permafrost in the Arctic, which holds millions of tonnes of frozen greenhouse gases. The more permafrost you melt, the more greenhouse gases you might release, increasing global heating and melting more permafrost. This process has already begun to happen; scientists aren't sure how hot we would need to get to thaw large amounts of it and kick off a feedback loop.
“If we release that methane, the quantities suggest that could lead to runaway climate change,” says Dr Steven Crimp from the Australian National University's Climate Change Institute.
There are other known feedback loops, and there could be unknown ones too.
Some scientists fear these feedback loops might lead us to "hothouse Earth", an apocalyptic scenario when our planet becomes too warm to support any life, although there is not a lot of evidence to support that fear.
So when Extinction Rebellion and Ms Thunberg warn of catastrophe, they are not wrong. Global heating might not threaten the human race with extinction, but it may well be the end of the world as we know it.