Ancient ice cores, drilled from Antarctic glaciers more than 6,000 kilometres from Western Australia, are the unlikely gatekeepers of insights into the drying climate in the state's South-West.
Perth has ended June with less than half of the average monthly rainfall in what has been a dry start to winter in the South-West corner of the state.
It is a scenario familiar to the region, which is in the midst of a 50-year drying trend, considered one of the worst in the world, due to climate change.
But as scientists try to untangle human-caused impacts from natural variability, one of the biggest challenges has been the short 140-year period of climate records kept by the Bureau of Meteorology.
An ancient ice core and chance "teleconnection" between Antarctica and the South-West has now given scientists insight into 2,000 years of drought history in the region.
Three mega droughts in last two millennia
According to Antarctic climate scientist Tas van Ommen from the Australian Antarctic Division, the analysis showed the South-West had experienced a comparable period of drought just twice in the last two millennia.
Dr van Ommen said this occurred around about 400 AD, and again in 750 AD.
He said they were able to come to this conclusion thanks to a "teleconnection" between Antarctica and the South-West recognised in 2010 using a 750-year-old ice core.
He said they found when snowfall increased in Antarctica, rainfall decreased in the region.
“As we looked at the atmospheric patterns that actually brought that moisture and snowfall to Antarctica, we could start to see that in this pattern, there was also a signature of bringing cold dry air to Western Australia,” he said.
The latest research, led by PhD student Yaowen Zheng, extended on Dr van Ommen’s previous work by reconstructing the snowfall in Antarctica using a longer 2037-year-old ice core from the same area.
Through that, they were able to draw conclusions about the past climate of South-West WA for the same period.
The ice core research is the longest reconstruction of past rainfall variability in the region to date, other than long-range modelling.
The research was accepted through peer review, but some caution was noted because the evidence assumes the link between snow at Law Dome and rainfall in South-West WA remains strong through time, which may not have been the case in past centuries.
Drought 'unusual' in long term
Dr van Ommen said the ice-core research showed the time between the previous drying events meant the current drought was very unusual.
But he said just because it had happened before, it did not mean human influence had not played a part.
Dr van Ommen said their records showed the current snowfall increase in Antarctica was “slightly more extreme” than the past two events.
He said they were also able to use the ice-core as a “check” of their climate models.
He said the models matched what the ice-core indicated were periods of snowfall increase and rainfall decrease in the past quite well, but could not produce the current drought until carbon dioxide increase was added.
"And so everything matches against everything else and we get more confidence in what the model is telling us," he said.
Cautious conclusions about the past
Other methods have been used to study past climate in the region, including a 2021 study of tree rings, which looked into the climate of the last 668 years in the Wheatbelt region of WA.
Similar to the ice core research, the tree-ring research found it was likely both natural climate variability and human-caused climate change contributed to the recent decline in rainfall.
But in contrast to the ice-core research, the tree-ring study found the Wheatbelt had suffered five periods of drought since 1350.
Dr van Ommen said the differing result was likely to do with the geographical areas of the South-West that each piece of research focused on.
"The ice core record is most applicable for the Perth water catchment and the higher rainfall western wheatbelt, while the Callitris (tree) is more applicable for the more arid eastern inland area," he said.
Nevertheless, he said their ice-core evidence remained open to ongoing scrutiny.
Climate history key to robust research
Dr van Ommen said understanding past climate was not just a look back in history, but a way to make climate models more robust.
"One part of doing that is to test the models with longer-term records from past climate," he said.
“So that's really the key is to predict better what is going to happen.
"There's another strand too, and that is that we need to understand what might happen if our present understanding is inadequate – that is where past evidence of how the climate has changed is critical."
The oldest ice core is 800,000 years old, but Dr van Ommen said their sights were set on understanding even more from longer records.
