Rivers feel profound impact of climate change

If things look bad in the Murray-Darling now, just wait: the climatic trend points to things getting far worse.

Climate change is having a profound impact on the Murray/Darling river system, scientists say. Picture: Hollie Adams

If things look bad in the Murray-Darling now, just wait: the climatic trend points to things getting far worse.

The expert panel that investigated last summer’s mass fish kills warned of clear evidence that climate change over the past century is placing even greater stress on the rivers than envisaged when the Murray-Darling Basin Plan was agreed in 2012.

The report, commissioned by Water Resources Minister David Littleproud and led by Melbourne University professor Robert Vertessy, found several factors contributed to millions of fish dying in the Darling near Menindee.

The drought produced low water flows, hot and dry conditions had led to algal blooms, and a windy cold snap had killed the algae and also “destratified” the water, depriving the fish of oxygen.

But the Vertessy report, released in March, identified a clear trend towards even more dramatic events.

The severe and prolonged drought and climate change “are revealing that the northern basin faces even greater pressures than understood when the current Basin Plan was agreed”, the report says.

 

“Future changes in the global climate system are likely to have an even more profound impact on the hydrology and ecology of the Murray-Darling Basin and increase the risk of fish deaths in the future. In the northern basin, the decreased rainfall over recent years means conditions are less favourable for saturating the ground and priming it for generating runoff during the following wet season.”

The Vertessy report noted the Bureau of Meteorology had determined the recent extreme weather had been “amplified by climate change”.

The annual mean temperature in the northern basin is estimated to have risen by about 1.5C since 1910, slightly higher than the national average over that period.

“There is a marked tendency across Australia, including the northern basin, for more extreme and persistent heatwaves,” the Vertessy report says.

The report contains a particularly alarming graph plotting the number of days each year where Australia’s area-averaged daily mean temperature was extreme, extreme days defined as those above the 99th percentile of each month from the years 1910 to 2017. It showed a huge and progressive increase in extremely hot days, particularly in the past few years.

If climate change predictions based on a high emissions scenario prove correct, the report says, the outlook for coming decades is even grimmer.

“Rainfall declines result in a corresponding decrease in river flows, with implications for water availability for the environment, industries and communities,” it says.