Evidence proves droughts and floods

Recent heavy rains and floods have devastated parts of the country, leading to three deaths, evacuations and destruction of property.

However, a Facebook post says there is no evidence the floods and droughts are getting worse.

The claim is made in a Facebook post (archived here) with text from a blog post posted on a climate change discussion website.

Other versions of the claim have been shared here, here, here and here.

“There is nothing unusual about today’s floods, fires, droughts, homelessness and hunger – they have always been part of human history. But satellite technology allows us to better track them and our media around the world revels in disaster reporting, bringing tragic and tearful scenes to every living room, every night,” the message read.

“Population growth means more people are affected by extreme weather, but there is no evidence that floods and droughts are getting worse.”

Experts say there is evidence showing recent major floods have been more frequent and intense.

However, experts said AAP Fact Check that there is evidence that recent floods and droughts have been more frequent and intense, both in Australia and overseas.

Dr Conrad Wasko, a flood and hydrology expert at the University of Melbourne, said there is evidence of large floods increasing while small floods are decreasing.

“It’s probably due to changing patterns of precipitation and drier soils,” he said of the decrease in small floods.

He added: “‘Great’ floods, however, have been demonstrated historically and are projected into the future by climate models, to increase.”

Prof Rory Nathan, also from the University of Melbourne, described it as a ‘double whammy of climate adversity’.

Writing in The Age earlier this month, he said: “In a warming climate, not only will our big floods increase, but our small floods will get smaller. Larger floods mean threats to life and property will increase over time, but smaller floods will reduce essential inflows to the dams on which our water supplies depend.

Dr. Wasko pointed to two studies that show large floods have increased.

A 2016 study, which examined the frequency of floods along the east coast of Australia from 1860 to 2012, found “a statistically significant and increasing trend in the frequency of major floods since the late 19th century , which contributes to a 50% increase in frequency”. (page 2).

The researchers determined that the increase in flood frequency was caused by land use change, anthropogenic climate change and natural climate variability (page 8), but that further research was needed to clarify the level influence of each of these factors.

A more recent study, co-authored by Dr Wasko, looked at streamflow data from 2,776 gauging stations located around the world to investigate the magnitude of flooding in response to extreme rainfall events.

The study found that the magnitude of frequent floods – those that occur on average once a year – generally decreases, but that the magnitude of rarer – and more extreme – floods increases.

“We suggest this is because, for these rarer events, increased rainfall outweighs decreased soil moisture. Our results indicate a worst-of-both-worlds scenario where small floods, responsible for the filling our water supplies, are decreasing, while major floods that pose a risk to life and infrastructure, are increasing,” the study concludes (page 1).

Dr Kim Reid, a researcher at Monash University’s School of Earth, Atmosphere and Environment, said AAP Fact Check there is evidence that northwestern Australia has experienced an increase in heavy rainfall due to climate change.

“The exact cause is uncertain but is potentially due to increased monsoon rainfall due to a warmer atmosphere containing more moisture,” she said.

The Bureau of Meteorology’s 2020 State of the Climate Report also indicates that there has been an increase in the intensity of heavy rainfall in Australia, particularly in the north (page 8).

Drought at Wallandbeen in the NSW Southern Tablelands
Southern Australia has seen an increase in droughts, an expert told AAP FactCheck.

Dr Ailie Gallant, an associate professor also at Monash University, explained that Australia has always been affected by droughts, many of which can be linked to El Niño and La Niña cycles and influences from the Indian Ocean.

“The poster says there is ‘no evidence that floods and droughts are getting worse’. It is true that Australia experiences strong influences from natural climate variability, however, Australia also experiences the effects of climate change on top of that variability,” she said in an email.

Dr Gallant said there are strong drought trends in parts of the country, particularly the south-west of Western Australia, which has been on a drought trend for more than 40 years.

“This trend is directly linked to human-induced climate change. Here, it is clear that climate change outweighs natural variability.

Dr Reid also noted that southern Australia has seen an increase in droughts, but said “there is too much uncertainty to conclude whether droughts are increasing or decreasing in eastern and central Australia. ‘Australia”.

A 2017 paper, which pieced together 800 years of seasonal rainfall patterns across Australia, found that “the major droughts of the late 20th and early 21st centuries in southern Australia are likely unprecedented over the past 400 years”.

Dr. Benjamin Cook, an adjunct researcher in paleo biology and environment at Columbia University, said AAP Fact Check via email: “There is ample evidence that climate change is increasing the severity of droughts in many (but not all) regions.”

Dr Cook said more severe droughts have also been seen in parts of western North America, the Mediterranean, Central America, Chile-Argentina and southern Africa.

The verdict

The claim that there is no evidence that floods and droughts have worsened is false. Several experts have said AAP Fact Check there is evidence showing that both have been more frequent or more intense in recent decades.

Experts said climate change had had an impact. However, other factors such as natural climate variability, El Niño and La Niña cycles and the Indian Ocean have also contributed.

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