Originally published by Scimex

The global intensity of water cycle extremes — which includes the extent, duration, and severity of droughts and floods — increased from 2002-2021 according to international research that used satellite data to quantify wet and dry extreme events around the world, including from Australia. The authors found that there was an excess of intense droughts and extreme wet events during 2015–2021, which were also the hottest seven years on record. The team found that Australia’s Millennium Drought appeared as three smaller events in the data, but it would have ranked as the 14th most intense drought if it were counted as a singular event. This drought ended in 2011-12 and was followed by the 3rd most intense wet event recorded during the observations, they say. The researchers say that the intensity of water cycle extremes is highly correlated with global temperature, more so than with other climate indices, suggesting that continued warming could cause more frequent, widespread, and severe droughts and floods.

Journal/conference: Nature Water

Link to research (DOI): 10.1038/s44221-023-00040-5

Organisation/s: NASA Goddard Space Flight Center, USA

Funder: This study was funded by NASA’s GRACE-FO Science Team and NASA’s Energy and Water Cycle Study (NEWS) programme. Computing resources supporting this work were provided by NASA’s High-End Computing (HEC) programme through the NASA Center for Climate Simulation (NCCS) at Goddard Space Flight Center. GRACE and GRACE-FO were jointly developed and operated by NASA, DLR and the GFZ German Research Centre for Geosciences.

Media release

From: Springer Nature

Climate: Satellite observations show worldwide water extremes increasing since 2002

The global intensity of water cycle extremes — a metric that combines extent, duration, and severity of droughts and floods — increased from 2002-2021 according to a study in Nature Water. This intensity is highly correlated with global temperature, more so than with other climate indices, suggesting that continued warming could cause more frequent, widespread, and severe droughts and floods.

Droughts and flooding have been predicted to become more frequent and severe with climate change, but this has been difficult to measure and quantify. Previous studies focused primarily on rainfall data.

Matthew Rodell and Bailing Li apply a new approach using observations from the NASA/German Gravity Recovery and Climate Experiment (GRACE) and GRACE Follow-On (GRACE-FO) satellites, to identify and quantify wet and dry extreme events around the world between 2002–2021. The authors find that there was an excess of intense droughts and extreme wet events during 2015–2021, which were also the hottest seven years on record. During this period the frequency of the most extreme events was 4 per year, compared with 3 per year over the previous 13 years. Monthly dry and wet event intensities were well correlated with global mean temperature. The most intense extreme event of the past two decades was a wet event covering all of central Africa, which began in 2019 and is still ongoing. Three of the most intense droughts on record also began in recent years: in the southwestern USA, southern Europe, and southern Brazil.

The authors conclude that understanding past and ongoing extreme weather events, and projecting future events, can help improve preparedness and mitigate impacts.

Expert Reaction

These comments have been collated by the Science Media Centre to provide a variety of expert perspectives on this issue. Feel free to use these quotes in your stories. Views expressed are the personal opinions of the experts named. They do not represent the views of the SMC or any other organisation unless specifically stated.

Dr Kimberley Reid is an atmospheric scientist from the ARC Centre of Excellence for Climate Extremes at Monash University

“Global rainfall data is notoriously scarce, so I think that innovative methods of tracking changes to the water cycle such as through the GRACE satellite mission are really cool. 
 
However, there appear to be errors in the statistical analyses related to the climate sections of the study, so I would be cautious about overinterpreting the link to climate change.
 
First, it’s not mentioned in the methods that the authors “detrended” the global temperature and event intensity data before calculating the correlations. And based on the results, I suspect they didn’t. Since the correlation results are used as the main evidence for the climate change link, I would hesitate to give that conclusion much credence. That doesn’t mean climate change isn’t potentially affecting the water cycle, it just means the events mentioned in the study may have a variety of causes.
 
Why you must detrend: Statistical theory dictates that you must remove the trend in the data before doing a correlation analysis because if you don’t, all you are showing is that the two variables are both influenced by time – not that they influence each other. A real-world analogy is that the number of years I’ve been alive, and the number of premierships won by the Melbourne Football Club have a high correlation because they both increase with time, even though I have never kicked a footy on the MCG before and therefore probably didn’t cause any wins.
 
Second, 20 years is not a long enough dataset to draw conclusions about relationships with El Niño and La Niña (ENSO). El Niño and La Niña occur every 2-7 years on average, so in 20 years, there may only be, at most, 10 occurrences, which isn’t enough to make conclusions about cause and effect. Other studies have shown that El Niño and La Niña are still a major influence on floods and droughts in Australia although rising global temperatures are likely to start to have a bigger impact later in the century.
 
Those two issues should have been picked up on during peer review, but the processes designed to maintain scientific rigour are not infallible, unfortunately.”

Last updated: 10 Mar 2023 12:59pm

Name:Kimberley Reid
Email:kim.reid@monash.edu
Twitter:kjreid95

Declared conflicts of interest:

None

Dr Melanie Zeppel is Lead Data Scientist at Carbon Link and a Senior Research Fellow at Macquarie University

“This Nature Water study confirms what scientists have been reporting for years, that the warmer temperatures, and additional moisture in the atmosphere under climate change, have resulted in more extreme, and more frequent floods and droughts. 
 
The impact on Australians, particularly vulnerable populations, such as the Northern Rivers, towns across Northern NSW and Qld, and Western Sydney, are intense, with many homes now uninsurable. These findings really highlight how crucial it is that Australia to reduce our carbon emissions. The impetus to reduce CO2 emissions is to aim to decrease the occurrence of these extreme droughts and floods, avoiding future loss of property, and human suffering from floods, as well as from the bushfires that follow prolonged drought, such as the 2019/2020 bushfires. 

The report is particularly frustrating, as it confirms what scientists, working in this field, have been publishing for at least 15 years, that the extreme droughts and floods will continue unless atmospheric greenhouse gas concentrations are reduced.
 
Loss of home due to fire or flood, is particularly tragic in times of reduced housing availability. Vulnerable populations, such as the elderly, women and children, low socio-economic groups, and people living in regional areas, are most impacted, meaning the need to reduce our greenhouse gas emissions (CO2, methane, for example) remains stronger than ever.”

Last updated: 10 Mar 2023 12:59pm

Name:Melanie Zeppel
Mobile:+61 421 318 563
Email:mzeppel@carbonlink.com.au
Twitter:MelanieZeppel

Declared conflicts of interest:

None

Dr Milton Speer is a Visiting Fellow in the School of Mathematical and Physical Sciences at the University of Technology Sydney

“The just published article in Nature, “Changing intensity of hydroclimatic extreme events revealed by GRACE and GRACE-FO”, confirms the impacts of extreme rainfall amounts (both low and high) that have been occurring in recent decades.
 
The extreme flood-rain events have been occurring on time scales from hours to monthly, seasonal and longer in the middle and sub-tropical latitudes of both hemispheres, for example, Australia, the USA, Europe and China. Global warming (GW) has pushed the once reliable rain-producing frontal systems poleward at these latitudes. This process has been known for decades. 
 
However, GW also provides more available atmospheric moisture at these latitudes and when combined with favourable phases of climate drivers such as La Nina in eastern Australia, high rainfall extremes can eventuate. Importantly these conditions can remain for months or years because a key feature of accelerated GW has been reported as an increase in multi-year phases of climate drivers such as La Nina. This exacerbates the effects of La Nina because of saturated ground and subsequent rainfall run-off enabling rivers and dams to regularly produce floods as recently experienced in Australia.
 
Nevertheless, the resulting impacts of flood rainfall amounts are very complicated and can vary regionally even within the same phase of a climate driver. One example is the effect of topography. In equatorial east Africa, La Nina means that moisture-laden easterly winds can penetrate some valleys and not others, depending on their orientation.”

Last updated: 10 Mar 2023 12:58pm

Name:Dr Milton Speer
Phone:+64 29 124 6123 (Fri-Sun)
Mobile:+61 434 881 633 (Mon-Tue)
Email: Milton.Speer@uts.edu.au

Declared conflicts of interest:

None