As the climate changes, it is likely that southern Australia will continue to get drier than average, especially in the southwest.
This year, however has been wet. Yesterday, the Bureau of Meteorology announced we are now in another La Niña event. This large-scale, common climate pattern can last for months and will increase the likelihood of further wetter weather across the country.
We are now in a more wet period than usual, despite the long-term trend towards dry conditions. What’s the deal?
Explainer: El Niño and La Niña
Current climate change assessments
In southern Australia, rainfall has been decreasing in the last decade (see below). The Intergovernmental Panel on Climate Change’s Sixth Assessment report – which draws on the latest climate research – found the recent drying trend in southern Australia is Most likelyTo continue, especially during the cool seasons.
Cool-season rainfall has been decreasing in southwest Western Australia. Drying in this area is also a concern. Very likely. There’s strong evidence this trend contains a “signal” of climate change, otherwise known as a climate change “fingerprint”.
We know that the changes in weather patterns have driven the drying. Future projections of drying are strong in the southwest.
The situation in the southeast of Australia is not as clear, but there’s strong evidence for an ongoing decrease in rainfall here, too.
Yet, we find it has been wet in many southwest and southeast regions this year – including in the cool season. In fact, some areas received “very much above average” rainfall in the ten months to the end of October, seemingly defying the drying trend.
Variability is a virtue in Australia
Australia has always been a land of flooding rains and droughts. It’s normal to be beset by a multi-year drought, before experiencing major floods. Year-to-year variability here is large compared with most of the world’s other land regions.
In 2021, we’ve seen a wet swing of the climate needle due to several global climate drivers. Alongside the Pacific now being in the La Niña phase, the Indian Ocean Dipole is in a negative phase.
We know that the deck is set for a wetter season when we see these drivers lined up. This is what we’ve always found, and we can count on Australia’s climate variability to remain, through this century and beyond.
Does a wet-year cast doubt on climate change assessment? No.
2020 State of the ClimateReport released by the CSIRO/Bureau of Meteorology. We noted that recent rainfall trends are within projections.
We actually showed rainfall trends from the baseline of 1986-2005 to 2020. These are following in the dry end projections for the southwest, Victoria. This suggests that the projections are probably conservative.
There are wet year, like this one and Victoria in 2010. The longer-term trend has been down. This is consistent with our expectations for climate change, based on multiple lines of evidence and climate modeling.
2021 is not an extraordinary year in the long-term. Even if the whole of 2021 ends up being very wet, it won’t shift the long-term trend by much. Only a long-term, persistently wet trend for decades or more would raise questions about climate change and current projections.
What does a ‘drier’ climate look like?
As the global climate warms, global average rainfall is increasing – and we expect most regions of the world will actually become wetter.
However, there are certain regions that will become dryer, such as southern Australia, the Mediterranean, and southern Africa. So how will this change look?
There will be many ups and downs as a trend towards a dryer climate develops. For example, between 1900 and 2020 southwest Western Australia’s annual rainfall varied between about 393mm and 1035mm (-40% and +55% from the average). This is a large range.
Climate projections suggestA persistent reduction of rainfall of up to 15% in the Southwest of Australia is possible over the first half century. Climate change will be a key factor in shaping the future of the century.
We can expect to see a drop in rainfall if we limit it to as high as 20%. If we don’t, it will decrease by up to 35% – with larger reductions very plausible in the cool season. These projected average changes are still lower than the year to-year variability in Australia over many years.
But this doesn’t mean the trends and projections aren’t important. A persistent and ongoing shifting towards drier conditions, with drier (and hotter) dry spells is a huge challenge for Australia’s water managers and primary producers. With a lower average rainfall, the chances for unprecedented dry periods is higher.
Wet extremes expected
Not every year will be dry. There will be wet days, weeks, and months.
Despite the trend towards drying, wet extremes will likely increase. Even in areas where rainfall is expected to decline, there is a projected rise in extreme rainfall bursts.
This includes intense hourly rainfalls that can lead to flash floods, as well as major rainfalls from “atmospheric rivers”, which are strong channels of concentrated moisture that stream through the atmosphere above us, delivering water for extreme rainfalls.
For parts of southern Australia, this means that there will be less rainfall on average but more extreme rainfall events. We can expect significant challenges ahead, given the low water availability and high flood risk.