What started as quiet concern is growing much louder among climate scientists: without a very radical change of course within the next few years, we're not going to stop the world from passing 1.5 degrees Celsius of warming.
At the rate we're going, we've got around 10 years until we hit 1.5C.
In terms of the numbers: the Intergovernmental Panel on Climate Change (IPCC) finds that limiting warming to 1.5C requires global emissions to be slashed by 45 per cent by 2030, compared to 2010 levels.
Research published in Nature forecasts 2030 CO2 emissions to be between 6 -10 per cent higher than 2010 levels.
"It's exceedingly unlikely that we will manage to limit warming to below 1.5C without overshoot," according to climate scientist Zeke Hausfather from Berkeley Earth and the Breakthrough Institute, who co-authored a commentary on the research in Nature, but wasn't associated with the study.
The Nature research shows that even if — and it's a big if — every signatory country meets their 2030 emissions reduction commitments and net-zero commitments, which were agreed to at or since last year's COP26 climate summit in Glasgow, warming could flatten out at between 1.9C and 2C by the end of the century.
Previous analysis has put us on track to hit at least 2.1C, and as much as 3.9C, by the end of the century.
However, there have been some changes to commitments since then.
But what's the big deal about 1.5C anyway? Why is that the number that scientists focus on, and how much does it matter if we overshoot 1.5C and instead limit warming to around 2C?
These are some of the differences that half a degree will make.
'1.5 to stay alive'
"1.5 to stay alive" is a movement spearheaded by many low-lying island nations, including our Pacific neighbours, who see any breach of 1.5C as literally that bleak.
A report by Carbon Brief, which analysed 70 research papers comparing 1.5C and 2C, found that globally we can expect an average of about 56 centimetres of sea level rise this century at 2C — but up to 96cm in the worst-case scenario.
That's about 10cm more, on average, when compared to 1.5C.
That extra 10cm, according to the IPCC, is expected to impact an extra 10.4 million people, either directly through inundation of coastal homes or through crippling of food-growing lands and livelihoods.
Many of those 10.4 million, including residents of some of the low-lying islands of our Pacific region, are expected to become climate refugees.
Sea-level rise doesn't happen equally around the globe. One variable is what is called gravitational self-attraction.
Things with mass have gravity, and things with huge mass like the Greenland and Antarctic ice sheets have enough gravity that they pull the oceans towards them.
As their mass is reduced, the seas will settle out towards the lower latitudes, causing more substantial rises around the tropics and sub-tropics, which includes the Australian coastline and our Pacific neighbours.
One research paper published in Nature Climate Change in 2020 forecast that at 70cm of sea-level rise, around 12,000 kilometres of Australia's coastline — around 40 per cent of our beaches — will recede by more than 100 metres.
At 1.5C, the Northern Hemisphere can also expect an ice-free Arctic summer around every once in 33 years, rising to around once every seven years at 2C (and nearly two in every three years at 3C).
An extra 1.5 to 2.5 million square kilometres of permafrost will also melt at 2C by 2300.
Floods, droughts and heatwaves
Though weather varies regionally, on average we can expect longer, more intense droughts, longer, hotter heatwaves and fewer but more extreme rainfall events at both 1.5C and 2C.
According to the Carbon Brief report, globally we can expect a 16 per cent increase in hot days each year at 1.5C, and a 25 per cent increase or 35 more hot days each year at 2C.
Just over a seventh of the world's population will face a severe heat wave at least once in five years at 1.5C, but that jumps to more than a third at 2C, and the frequency of warm extremes jumps by 343 per cent at 2C.
An extra 62 million people will be exposed to drought each year if we reach 2C, compared to 1.5C.
Tropical cyclones are expected to become less frequent, but extreme cyclones are predicted to be more intense when they do hit.
At 2C, an extra 12 million people are expected to be flooded in coastal areas each year compared to at 1.5C.
In Australia, the frequency of warm extremes is forecast to jump more than 400 per cent in the north at 2C, and 234 per cent in southern regions.
Cold extremes will become less frequent nationwide, but extreme rainfall events will increase.
The length of droughts is also expected to marginally increase, while as many as 1 million people in northern Australia could face water scarcity at 2C.
Every point-of-a-degree has health impacts
It's worth pointing out here that if we breach 1.5C, the next target doesn't automatically default to 2C.
Every nth-of-a-degree is crucial, according to Kathryn Bowen, deputy director of Melbourne Climate Futures at the University of Melbourne.
"Everything we know about the impacts of climate change and health is going to be made worse if we miss 1.5 degrees," Professor Bowen said.
In many developing countries, the biggest health threats posed by climate change involve nutrition and water.
"Globally, the biggest threat is coming from food and water insecurity and human health," Professor Bowen said.
In northern Africa, for instance, the average drought is predicted to be 7 months longer at 1.5C versus 20 months longer at 2C (at 3C, it's 60 months longer).
In East Africa, 6 million more people are expected to suffer water scarcity at 1.5C. At 2C, that rises to 22 million more people.
But Australia faces different challenges.
"In Australia it's mainly around heat, and extreme weather events — fire, droughts, floods."
At 2C of warming, the IPCC predicts extreme temperatures will reach 50C or more in Melbourne and Sydney.
And annual heat-related deaths in Melbourne, Sydney and Brisbane could average 600 per year between 2031 and 2081, up from the current average 142.
But almost every climate impact is intertwined with health — both directly and indirectly — and the closer we get to 2C, the greater the risks become.
The obvious ones are deaths during heatwaves or floods, bushfires or cyclones. But others are less visible.
Longer droughts or intense floods threaten food availability and nutrition; extreme weather events can damage medical infrastructure and cut access to medicine and treatment in the aftermath; lingering flood waters can carry disease; lost income means families may be less able to afford medical care.
And then there's the compounding trauma of suffering extreme weather events in succession, such as we've seen recently with flooding in northern New South Wales.
Although there is no research yet to quantify the potential jump in negative mental health outcomes between 1.5C and 2C, extreme weather events can be triggers for disorders like PTSD, anxiety and depression.
"There's a lot more research pointing to the negative impacts of health and climate," Professor Bowen said.
"We could expect that the mental health impacts, like other health issues, would be amplified."
Eco-anxiety and "solastalgia" — the distress of seeing natural environments that are important to us being harmed — are already on the rise.
"The eco-anxiety stuff is particularly linked with [climate change] and it's particularly coming out in young people," Professor Bowen said.
Australian ecosystems already collapsing
We don't have to look far to see the early impacts of climate change on animals and ecosystems.
"The Great Barrier Reef is the canary in the coal mine," according to Malte Meinshausen, University of Melbourne climate scientist and co-author of the Nature paper.
He said we could have saved most of the reef if we'd acted decisively 20 years ago, and that today we're faced with a similar choice.
"What we have in our hands is whether we put other ecosystems though the same fate as the Great Barrier Reef or we limit warming to as close to 1.5C as possible."
Between 70 and 90 per cent of tropical corals will disappear at 1.5C, rising to more than 99 per cent at 2C, according to the IPCC.
In the Daintree rainforest in tropical far north Queensland, ecologists say they are already recording the decline of a number of bird and mammal species.
Some are being pushed further up mountains as the lowlands warm, and others are suffering in longer heatwaves.
"The lowland species are moving into the midlands, the midlands species are declining," Steven Williams from James Cook University told the ABC earlier this year.
It's a trend that is going to get worse in many ecosystems across the planet.
At 1.5C, 4 per cent of mammals will lose more than half their natural range. At 2C, that doubles to 8 per cent.
For insects, those numbers are 6 per cent at 1.5 degrees and 18 per cent at 2C (at 4.5C it jumps to 67 per cent).
Many species already living in reduced habitat and facing other pressures, like deforestation, are likely to go extinct.
But predicting exactly which species are going to be driven to extinction is difficult, according to Lesley Hughes from the Climate Council and a biologist at Macquarie University.
"It's often those relatively rare, but getting less rare, extreme events that absolutely have a transformational impact in a very short time span," Professor Hughes said.
We've already seen bushfires pushing into Gondwanan rainforest in Tasmania and sub-tropical rainforest in Queensland — habitat that should not burn and which will need thousands of years to recover.
And there are plenty more ecosystems in Australia already on the decline.
"We know that there are at least 19 significant ecosystems in Australia that are collapsing in their range right now," Professor Hughes said.
"This has happened in some cases quite quickly and in others it's happened over decades.
"It doesn't take an ecologist to say that if this is happening at 1.3 degrees, well 1.4C in Australia ... let's imagine Black Summers occurring twice as often, floods occurring twice as often, bleaching occurring twice as often, then it's pretty easy to say its going to be worse."
Tipping points and the good news
Tipping points are the great unknowns of climate change.
A tipping point is a threshold which, when crossed, sets off an unstoppable cascade toward a complete change in the state of a system.
The Amazon rainforest is believed to be approaching a tipping point which could see it transform to savanna and grassland over decades.
There are also warning signs of the collapse of the Gulf Stream — a warm Atlantic current which helps moderate the climate of the eastern United States and western Europe.
And then there are the tipping points for the Greenland and Antarctic ice sheets.
"We don't know where the tipping point for the Greenland ice sheet to melt irreversibly is — it may be at 1.5 degrees, it may be at 3C — we don't know, but we keep our chances best if we limit warming to 1.5 degrees."
So what can possibly be the good news in all of this?
The good news is, we have the ability to stop it — to limit warming to as close to 1.5C as possible and avoid the worst impacts of climate change.
The technology exists and the know-how exists.
If political leaders act with an immediate urgency to drive a global overhaul of energy, transport, agriculture and industry, the very rapid replacement of coal, oil and gas with renewables, and to plug the emissions gaps with scientifically validated offsets and drawdown — it can still happen.
At this point what is missing is the political will, according to Dr Meinshausen.
"To be as close as possible to 1.5 degrees, there needs to be accelerated action to 2030," he said.
"Most countries updated their 2030 [emissions reduction] targets. Australia is one of the few who didn't.
"The world still has a chance to avoid catastrophic climate change."
