Storms swept through southeastern Australia last week. Source: FreeImages.com- Lucretious
Last week Adelaide was hit by a powerful storm with strong winds, heavy rain, hail and even tornadoes. The storm left a trail of devastation but the biggest impact was probably the blackout across South Australia as the whole state was left without power for several hours. In some remote parts of the state it took days for the power to be restored.
There’s no doubt this was an extreme storm. It was described as a one-in-50 year event by the Bureau of Meteorology and the damage was widespread.
So was the storm made worse by climate change?
Fundamentally we know that the atmosphere is getting warmer. As the temperature of the air increases we know it can hold more moisture. For every degree Celsius of warming the amount of moisture that the atmosphere can hold increases by about 7%. This effect (known as the Clausius-Clapeyron relation) means that, because of climate change, intense rainfall events are getting heavier.
So that means that climate change did make this storm worse, right?
Well….. not necessarily.
We also need to consider if the “trigger” for the event - in this case a cold front associated with a deep low pressure system - is also changing in some way because of climate change.
There’s no doubt this was an extreme storm. It was described as a one-in-50 year event by the Bureau of Meteorology and the damage was widespread.
So was the storm made worse by climate change?
Fundamentally we know that the atmosphere is getting warmer. As the temperature of the air increases we know it can hold more moisture. For every degree Celsius of warming the amount of moisture that the atmosphere can hold increases by about 7%. This effect (known as the Clausius-Clapeyron relation) means that, because of climate change, intense rainfall events are getting heavier.
So that means that climate change did make this storm worse, right?
Well….. not necessarily.
We also need to consider if the “trigger” for the event - in this case a cold front associated with a deep low pressure system - is also changing in some way because of climate change.
The cold front as it approached Adelaide last Wednesday. Source: Bureau of Meteorology.
Over the last few decades, there has been a southward shift in storms that track across and to the south of Australia. This poleward shift in the storm track has led to a long-term drying trend in southwest Western Australia and has itself been linked to climate change.
What this all means is that we have a very messy picture of the role of climate change in this event. On the one hand the thermodynamic effect suggests that climate change is making this kind of event worse. On the other hand, the dynamic effect of climate change is to reduce the likelihood of storms affecting Adelaide as the storms tend to pass further to the south of Australia.
How has extreme rainfall changed in Adelaide?
Looking since the start of the 20th century we don’t really see much of a trend in extreme rainfall in Adelaide. Any trends are strongly dependent on start dates and the exact definition of extreme rainfall being used.
What this all means is that we have a very messy picture of the role of climate change in this event. On the one hand the thermodynamic effect suggests that climate change is making this kind of event worse. On the other hand, the dynamic effect of climate change is to reduce the likelihood of storms affecting Adelaide as the storms tend to pass further to the south of Australia.
How has extreme rainfall changed in Adelaide?
Looking since the start of the 20th century we don’t really see much of a trend in extreme rainfall in Adelaide. Any trends are strongly dependent on start dates and the exact definition of extreme rainfall being used.
There are very weak trends in maximum 1-day precipitation totals in South Australia. Source: Bureau of Meteorology.
We can’t say with confidence that extreme rainfall has increased around Adelaide over the last century.
But heavy rain wasn’t the only problem
As I said earlier it was the extreme winds that knocked out the transmission lines and caused the blackout across South Australia. And the hailstorms and tornadoes within the broader storm system contributed to the overall damage.
We don’t have great historical observations of tornadoes or hail, and even wind measurements have issues, so it’s hard to say how they’ve changed. Given the impacts of the event were brought about by the combination of these different components of the storm, it’s even harder to say whether climate change affected the storm and worsened its effects.
Can we properly investigate the influence of climate change on the storm?
Unfortunately it’s difficult.
Normally when we’re interested in a heatwave or a heavy rainfall event we use climate model simulations in two groups (one which includes human effects on the climate and one which doesn’t) and then we compare the frequency and the intensity of that kind of event between the two groups.
Because the storm was associated with extremes in many variables (mean sea level pressure, wind, rain) it would be hard to know where to start.
Also, most of our climate models are on too coarse a scale (typically 100-200 kilometres for most global models) to investigate even intense low pressure systems like this one (let alone tornadoes or hailstorms which occur on much smaller spatial scales).
In Australia we have a regional model called Weather@Home which we’ve used to study local-scale extremes before, like heatwaves in Adelaide, Melbourne and Brisbane. We might be able to use the Weather@Home model to look at this event, but we do not have the simulations and data available right now to do the analysis.
The effect of climate change on this storm is unclear
At the moment we can’t say whether climate change made this storm worse, in fact it’s quite possible that climate change has decreased the likelihood of storms like this one affecting Adelaide as the track has shifted southwards.
A full analysis to determine the effect of climate change on this storm is not straightforward to do. But it may be possible to use the Weather@Home model over the next few months to investigate some aspects of this storm like the intense low pressure and intense rainfall.
Until we have a proper analysis of the event we won’t be able to say what effect, if any, climate change has had on this storm.
I made a brief comment on the Adelaide storm and climate change for The Conversation.
But heavy rain wasn’t the only problem
As I said earlier it was the extreme winds that knocked out the transmission lines and caused the blackout across South Australia. And the hailstorms and tornadoes within the broader storm system contributed to the overall damage.
We don’t have great historical observations of tornadoes or hail, and even wind measurements have issues, so it’s hard to say how they’ve changed. Given the impacts of the event were brought about by the combination of these different components of the storm, it’s even harder to say whether climate change affected the storm and worsened its effects.
Can we properly investigate the influence of climate change on the storm?
Unfortunately it’s difficult.
Normally when we’re interested in a heatwave or a heavy rainfall event we use climate model simulations in two groups (one which includes human effects on the climate and one which doesn’t) and then we compare the frequency and the intensity of that kind of event between the two groups.
Because the storm was associated with extremes in many variables (mean sea level pressure, wind, rain) it would be hard to know where to start.
Also, most of our climate models are on too coarse a scale (typically 100-200 kilometres for most global models) to investigate even intense low pressure systems like this one (let alone tornadoes or hailstorms which occur on much smaller spatial scales).
In Australia we have a regional model called Weather@Home which we’ve used to study local-scale extremes before, like heatwaves in Adelaide, Melbourne and Brisbane. We might be able to use the Weather@Home model to look at this event, but we do not have the simulations and data available right now to do the analysis.
The effect of climate change on this storm is unclear
At the moment we can’t say whether climate change made this storm worse, in fact it’s quite possible that climate change has decreased the likelihood of storms like this one affecting Adelaide as the track has shifted southwards.
A full analysis to determine the effect of climate change on this storm is not straightforward to do. But it may be possible to use the Weather@Home model over the next few months to investigate some aspects of this storm like the intense low pressure and intense rainfall.
Until we have a proper analysis of the event we won’t be able to say what effect, if any, climate change has had on this storm.
I made a brief comment on the Adelaide storm and climate change for The Conversation.
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