Election FactCheck: Are Larger, More Frequent Storms Predicted Due To Climate Change?

The Conversation - Kevin Walsh

Was the Prime Minister right about storms and global warming? AAP Image/Lukas Coch

Certainly larger and more frequent storms are one of the consequences that the climate models and climate scientists predict from global warming. But you cannot attribute any particular storm to global warming, so let’s be quite clear about that.
Prime Minister Malcolm Turnbull, speaking to reporters in Tasmania on June 9, 2016.

In the aftermath of the deadly East Coast Low that swamped eastern Australia, dumping massive amounts of rain in early June, the prime minister toured flood-affected Launceston and announced emergency relief funding.
Turnbull told reporters that larger and more frequent storms were forecast by climate scientists but cautioned that no individual storm could be attributed to global warming.
Is he right?

Checking the source
The Conversation asked the prime minister’s office for sources to support his statement but did not hear back before publication deadline. Nevertheless, we can test his statement against recent published and peer-reviewed research on this question.
The science shows that, just like real estate, climate change is all about location. Different parts of Australia will be affected in different ways by climate change.
And global warming will have different effects on different types of weather systems.
Let’s break Turnbull’s statement into two parts: is it true that we can expect larger and more frequent storms as a consequence of global warming? And is it possible to attribute a specific storm to global warming?

Can we expect larger and more frequent storms as a result of global warming?
Yes – but not for all regions or types of storms.
There are many types of storms that affect different parts of Australia, among them East Coast Lows, mid-latitude cyclones (a category that includes cyclones that happen in the latitudes between Australia and Antarctica), tropical cyclones, and associated extreme rainfall events. Each will be affected in a different way by climate change, and the effect will vary by region and by season.
On East Coast Lows: Acacia Pepler, who is studying extreme rainfall and East Coast Lows in relation to climate change, recently wrote in The Conversation that her research showed that:

… East Coast Lows are expected to become less frequent during the cool months May-October, which is when they currently happen most often. But there is no clear picture of what will happen during the warm season. Some models even suggest East Coast Lows may become more frequent in the warmer months. And increases are most likely for lows right next to the east coast – just the ones that have the biggest impacts where people live.

For all low-pressure systems near the coast, “most of the models we looked at had no significant change projected in the intensity of the most severe East Coast Low each year,” Pepler wrote.
On mid-latitude cyclones: Another study predicted that the overall wind hazard from mid-latitude cyclones in Australia will decrease – except in winter over Tasmania.
On tropical cyclones: Northern Australia is expected to get fewer cyclones in future – but their maximum wind speeds are expected to become stronger.
On rainfall: Scientists tend to be quite confident that climate change will be accompanied by an increase in extreme rainfall for most storms in future. One of the main reasons for this is that increased temperatures will cause increased evaporation. While the total amount of water held in the atmosphere will also increase slightly in future, the total amount of rain has to go up too.

Is it true you can’t attribute any particular storm to global warming?
Turnbull is correct. We cannot say for sure that a particular flooding rainfall event was solely “caused” by climate change, any more than we can say for certain that a particular car accident was solely caused by speeding (even if excessive speed was a likely or even major contributing factor).
Evidence for the effects of global warming on extreme rainfall events that have already occurred is currently equivocal for most regions.
According to a collection of studies published in 2015:

A number of this year’s studies indicate that human-caused climate change greatly increased the likelihood and intensity for extreme heat waves in 2014 over various regions. For other types of extreme events, such as droughts, heavy rains, and winter storms, a climate change influence was found in some instances and not in others.

One recent study in that report found:

evidence for a human-induced increase in extreme winter rainfall in the United Kingdom.

Verdict
Malcolm Turnbull was essentially correct on both points.
It’s true that scientists predict more frequent and intense storms for some parts of Australia as the climate changes. The evidence appears to be strong that extreme rainfall will increase. Some increases in extreme wind speeds are possible – but not in all regions or all seasons.
Turnbull was right to say you cannot attribute any particular storm to global warming. –Kevin Walsh

Review
This is a good FactCheck that summarises the broad conclusions from a range of studies examining the nature of current and likely future storms across Australia.
As the author points out, Australian storms range from tropical cyclones in the northern tropical regions to temperate east coast lows and mid-latitude cyclones.
The consensus regarding tropical cyclones is that they will generally decrease in frequency in the Australian region. In northeast Australia, they are forecast to experience the most dramatic decrease in frequency of any ocean basin globally. Some northern hemisphere ocean basins will see an increase in their frequency.
The intensity of these types of storms is expected to increase. This will not only involve higher wind speeds but also higher storm surges and floods. That will mean greater coastal impacts and damage to coastal developments and infrastructure.
So the prime minister’s statement about more frequent storms resulting from climate change does not apply to tropical cyclones – however, he was right to say that larger and more frequent storms are one of the predicted consequences of climate change. This consequence is predicted to apply to other storm categories, but not tropical cyclones.
And yes, climate scientists are hesitant to attribute the occurrence of any single storm to global warming. – Jonathan Nott

Links
Scientists are using drones to help predict coastal erosion
Explainer: what causes winter storms?
Explainer: the wild storms that lash Australia’s east coast
Why warmer storms could lead to more flooding than expected