Some of Australia's iconic and unique natural ecosystems may disappear for good if we keep emitting carbon at current rates, climate experts warn.
According to the latest Intergovernmental Panel on Climate Change (IPCC) report on the effects of and adaptation to climate change, Australia can expect more hotter days, fewer cold ones, more extreme fire weather, and heatwaves on land and in oceans under 1.5 to 2C of global warming above pre-industrial levels.
The world is currently at an average of about 1C of warming, and, exacerbated by other human actions, this is already modifying Australian ecosystems, says Griffith University climate and environmental scientist Brendan Mackey.
"We know how the climate has changed in the last 100 years in Australia, and we have some evidence now for how ecosystems have been impacted by those changes."
Professor Mackey was a coordinating lead author for the Australasian chapter of the IPCC report, which outlined nine key climate risks for the region.
Four of those risks focused on ecosystems at risk of severe damage — or even collapse — under climate pressures.
So let's take a tour of the continent and have a look at what's at stake — and they're not necessarily the ecosystems you might expect to see.
Up high in alpine regions
When we think of alpine regions, often the images that spring to mind might be the snow-covered peaks of Europe or the Himalayas.
Though much lower, Australia's alpine regions host some of the most unique biodiversity on the planet — mountain pygmy possums, corroboree frogs, spiny crayfish, snow gums, and myriad plant species.
But the short stature of our mountains is their Achilles heel.
Many species have adapted to living in the cooler climes found on our mountainsides above 1,000 metres or so.
As the climate warms, those animal and plant species are forced to migrate further and further up the mountains to find a suitable, cooler habitat.
But there's not very much further for them to go, according to Professor Mackey.
"If you're in New Zealand, as you go up in elevation, they have real mountains with pointy tops.
"In New Zealand, the alpine vegetation can potentially grow uphill a bit, and any plants or animals that are dependent upon that habitat can go with it.
The IPCC singled out Australian alpine regions for special mention in the latest report.
There's predicted to be a sharp increase in extinctions in these regions as warming goes beyond 1.5C toward 2C.
On the latest predictions, we're forecast to hit 1.5C around 2030.
In Australia's tropical north, species have already been observed pushing further up mountains, according to Steven Williams from James Cook University.
"The lowland species are moving into the midlands, the midlands species are declining," Professor Williams said.
"A lot of the endemic [bird] species … most of them have already declined by up to 30 per cent, some to 60 per cent."
For mammal species like possums, he said it seemed to be the increase in extremely hot days that had been doing the most damage.
"The ring-tailed possums endemic to the area have already declined by 50 per cent," he said.
"Two other species are declining badly.
"The paper that we’re about to bring out on the possum shows that most of the possum species will be in really serious trouble by 2050.4
But so far Australia's southern alpine regions are yet to feel the real brunt of climate change, according to Ary Hoffman from the University of Melbourne.
Mt Buller's mountain pygmy possum population appears to be healthy, for instance.
Its key threat today, according to Professor Mackey, is logging.
In the southern alpine regions more generally, Professor Hoffman said the biggest threat right now is from weeds and ferals like the wild brumbies.
But the changes will come, he said.
"There is no doubt about it. There are plants you only find in snow patches, and they will disappear," Professor Hoffman said.
"Animals associated with the wetlands and the bogs will start disappearing; frog species like the corroboree frog and Baw Baw frog, they’ll be in trouble."
But he said it's the unexpected interactions between species that create the most uncertainty.
Scientists suspect warmer temperatures and longer drought in the Australian alps are responsible for a boom in longicorn beetles, for instance.
The beetles in turn are causing an extensive and sudden dieback in our snow gums — trees a few hundred years old have been dying en masse as they're essentially ringbarked by the beetles.
"That's the problem with climate change — you get these unexpected things happening," Professor Hoffman said.
Some forests on land …
Speaking of snow gums, forests of southern and south-west Australia were also singled out as Australian ecosystems at risk of the one-two punch of climate change and human mismanagement.
But not the eucalypt forests of Victoria and NSW that were decimated in the 2019-20 bushfires … at least not immediately.
That's mainly because those trees are known as "sprouters", and store tiny buds under trunk and branch bark which are ready to grow out as green shoots should a fire go through the area, Professor Mackey said.
The forests at risk are of towering jarrah and mountain ash, found in the continent's south-west and south-east corners respectively, as well as gnarled snow gum woodland and Tasmania's pencil pines.
And each is affected slightly differently by a warming world.
In southern Western Australia, winter rainfall would normally replenish underground reserves of water, which the deep roots of jarrah trees tap into to keep going over summer.
But regional winter rainfall has steadily dropped over the past 30 years, and their subterranean reservoirs haven't been topped up as much.
On the other side of the country, mountain ash trees are "seeders". Older trees can cope with intense bushfires, but seedlings must grow for around 20 years before they can reproduce.
"So if you start getting catastrophic wildfires more than two to three times every 10 years, that's faster than it takes the ash forest to grow," Professor Mackey said.
"And if you have a commercial logging regime that's keeping ash forest at a young age — so rather than letting the trees grow to be 400 years old, you're harvesting them between 48 years old and 46 years old — they're younger trees, they're more vulnerable, and they're more likely to be killed by fire.
More fires, more often, affect snow gums in a similar way. These trees resprout from the base, where they keep a stock of starches and sugars for this purpose.
"But if you increase the frequency of catastrophic fire events ... and snow gums get hit too often, they don't have time to replenish the energy stock they use to regenerate," Professor Mackey said.
And Tasmania's ancient pencil pines haven't had to deal with catastrophic fires — until recently.
Bushfires in 2016 razed swathes of pencil pine forest, and even six years on, large areas are not recovering well, Professor Mackey said.
So what would happen if these forests disappeared?
They wouldn't leave a barren plain. Rather, a different type of forest would grow in its place.
For instance, if snow gums disappeared, they might be replaced by woody shrubs that can regenerate after fire quickly — something that's already happening in parts of the country.
And this, Professor Mackey says, results in an overall loss of biodiversity.
"You lose something that was particular to a local environmental condition, and it's replaced by something which is more common elsewhere."
… and forests beneath the waves
When Gretta Pecl started diving in the kelp forests off the east coast of Tasmania in the mid-1990s, towering stands of giant kelp stretched from seabed to surface.
"By around 2005, I started noticing changes," said Professor Pecl, now a marine ecologist at the University of Tasmania and a lead author on the Australasian chapter of the IPCC report.
Different and diminutive kelp species started moving in while the long ropey strands of giant kelp dwindled.
"And when you consider the kinds of [kelp] habitats and systems that we had in the late 90s and early 2000s to what we've got now, they actually look like two different ecosystems."
Indeed, the IPCC report states "less than 10 per cent of giant kelp in Tasmania was remaining by 2011 due to ocean warming".
The loss of kelp forests in southern Australia and south-east New Zealand — highlighted as a key risk — is already severe, Professor Pecl said.
"But it could be reduced substantially by rapid, large-scale and effective mitigation and adaptation strategies", such as transplanting more heat-tolerant types.
So what happened to the once-dominant kelp forests?
Giant kelp is sensitive to temperature and thrives in the cold, nutrient-rich water that laps at the Tasmanian coastline.
But on the east coast, this has been replaced by water that's warmer and comparatively devoid of nutrients.
This warm water is shuttled down by the East Australian Current, which has moved 350 kilometres south since the middle of last century, Professor Pecl said.
"That extension of the East Australian Current, as shown in the working group 1 IPCC report, is largely driven by warming over the Pacific."
This underlying ocean warming is separate to marine heatwaves — days-long bursts of particularly warm seawater.
Kelp must also contend with overgrazing by nibbling fish and sea urchins that are moving southward with the East Australian Current.
"We know that at the global level, around half of plants and animals, including marine species, are moving poleward … and changing where they live now as a function of climate.
"Effectively everything is moving at the same time, but at different rates.
"Connections are being broken apart and new connections are being formed … and the kelp forests around Australia are a victim of that."
But worst affected will be coral reefs
By now we've all heard how climate change is going to kill a lot of coral. We've already seen several mass bleaching events on the Great Barrier Reef since 2016.
According to the latest report, we're expecting to see bleaching conditions on the reef at least once every five years by 2035, even if we aggressively cut emissions from today.
That's likely to increase to every year by around 2050.
Without long periods in between bleaching events for the coral to recover, mass mortality is a near certainty.
At 1.5C of warming, the IPCC predicts we'll lose between 70 to 90 per cent of coral diversity. At 2C, that rises to more than 99 per cent.
But the problem is far bigger than losing pretty corals. The corals are merely one part of a massive food web that provides nutrients for marine species well beyond the reef itself, according to Scott Heron from James Cook University.
"Coral reefs cover less than one-tenth of 1 per cent of the ocean floor," Dr Heron said.
"But they support more than 25 per cent of oceanic fish species.
Losing 90 per cent or more of coral species is going to have a profound effect on fish abundance, including on many of the commercial fish species that we rely on.
Then there is the physical buffering that reefs provide our coastlines.
Reefs help dissipate wave action, sheltering our coastal communities from the worst impacts of storms and cyclones.
"That's all the more important in an era where we're seeing storm intensity increase," Dr Heron said.
"We're not necessarily seeing more tropical cyclones, but we're seeing more severe tropical cyclones."
Despite the dire predictions for our reefs, Dr Heron says he thinks they're still worth fighting for, and that we need to make an equitable and rapid transition from greenhouse-gas-emitting technologies.
"The number of options and ideas that we have available to us are rapidly diminishing," he said.
