To limit future flooding in Sydney, COP26 pledges need action

It’s easy to get lost in the numbers from COP26. For two weeks in November 2021, world leaders gathered to discuss emissions reduction targets for the next five years.

But amongst the personalities and press conferences, we may forget that our greenhouse gas emissions have real and disastrous implications.

The focus of COP26 was on what changes we need to make to keep global warming limited a possible increase of 1.5°C, 2°C or even 3°C relative to pre-industrial levels.

However, Australia’s climate is already changing with a pronounced increase in heat waves on land and in our oceans. For extreme rainfall events, the trends are more varied and it’s difficult to determine the role of climate change in damaging weather systems that lead to flooding.

Extreme rainfall is notoriously difficult to accurately simulate in climate models, relative to other climate extremes like heat waves. This is because of the chaotic nature and relatively small scale of storms. Global climate models can only simulate objects that are at least 250km across, while a thunderstorm is usually about 25km.

However, by switching the focus from individual storms to the large-scale flow of water vapour around the globe– in which thunderstorms are usually embedded –we can produce more robust predictions.

When tropical air moves south

Large-scale extreme rainfall and floods in Australia typically occur when wet tropical air intrudes south. Like using a firehose on a veggie patch, the southern states aren’t accustomed to such vast amounts of water.

We are seeing a perfect example of such a weather event over huge areas of Australia. Alice Springs recorded 100.2mm of rain in less than 24 hours on the 10th of November and there are flood warnings for most states and territories.

Another recent example of such an event occurred in March this year.

Between 17 and 24 March 2021, Eastern Australia experienced heavy rainfall and widespread flooding. Sydney received 85 per cent of its average March rainfall in 24 hours during an Atmospheric River that channelled tropical moisture over an already soaked NSW.

Tragically, three people died and the damage bill has exceeded $A650 million with thousands of insurance claims still unresolved.

The daily maximum amount of water vapour that passed over Sydney during this event was high but not unusual. However, the 10-day average water vapour flowing over Sydney was the third-highest since 1980.

The persistent and widespread rainfall that preceded the Atmospheric River was due to a blocking high-pressure system in the Tasman Sea which drove moist air from the Tasman and Coral Seas over the east coast.

Like a saturated sponge, when the heavy rain from the Atmospheric River fell, the soil couldn’t absorb any more water and NSW flooded.

Researchers from the Australian Research Council Centre of Excellence for Climate Extremes and University of Melbourne teamed up with a colleague from Indiana University and the Lawrence Berkeley National Lab in the USA to understand how the likelihood of these kinds of events might change in a warmer climate.

Projecting vapour transport

We used the latest generation of global climate models and advanced statistical modelling to project water vapour transport over Sydney under both high and moderate emissions scenarios.

Our new study, published in Geophysical Research Letters, found that the probability of events that are of similar magnitude to the weather system that led to the March 2021 flooding will likely increase by 80 per cent by the end of the 21st Century under both high and moderate emissions scenarios.

As well as an increase in the likelihood of events similar to the March 2021 event, we also found that the number of consecutive days with high water vapour transport over Sydney was projected to increase. This puts Sydney at a higher risk of regular flooding under moderate and high emissions scenarios.

The ‘moderate’ emissions scenario used in this study is equivalent to 2.5 to 3°C of global warming by the end of this century, while the ‘high’ emissions scenario equates to about 5°C of global warming relative to pre-industrial levels.

Limiting the chance of widespread, damaging extreme rainfall and flooding is why it is so important to reduce our greenhouse gas emissions.

Climate Action Tracker has warned that the short term COP26 pledges for the next decade won’t limit warming to 1.5°C or even 2°C, but are only enough to limit global temperatures rises to 2.4°C, which would have devastating consequences for the climate. It is crucially important then that we keep pressure on our leaders to deliver meaningful and sustained cuts to emissions.

By Kim Reid, Dr Andrew King and Professor Todd Lane, University of Melbourne