18 September 2017
As many people in southern Australia are dealing with the effects of frosts on their crops, gardens and car windscreens, they may be tempted to think global warming could have some upsides, and winter and spring frosts will become a thing of the past.
While it seems reasonable to expect that a rising global temperature will lead to more hot extremes and fewer cold extremes, paradoxically global warming could be increasing the odds of some types of cold extremes in some places. How can this be?
Cold extremes in a warming world
Global warming is only one aspect of the changes that human influences are having on the climate. Climate change doesn’t just make the world incrementally warmer, it affects the atmospheric and ocean circulation as well, which in turn affect the weather.
Frosts and cold outbreaks are associated with particular conditions, including clear skies, little or no wind and low humidity, and weather systems that bring cold air. The strength or frequency of the atmospheric and ocean circulation features linked to frosts could be changing as a result of climate change. If so, the effect of changes to circulation could offset or counter the warming effect.
Climate change or natural variability?
We’ve actually seen an increased incidence of frosts in some areas of southern Australia in recent decades, despite a warming average climate. The question is, is this simply due to natural variability, or could there be a human influence?
In the northern hemisphere, a link has been proposed between climate change and the circulation pattern that brought cold extremes, such as the 2013 ‘snowmageddon’ that hit north-east USA. These events have been linked to wobbles and kinks in the jet stream that allowed the very cold air from the ‘polar vortex’ over the Arctic to come south. Recent research has found that the jet stream may be more likely to wobble and kink like this due to the effect of climate change on the circulation, and could have a link to changes in Arctic sea ice. This is an interesting possibility and is an open area of research in the northern hemisphere.
Is something similar at play in the southern hemisphere that is affecting frosts in southern Australia?
We know that increases in pressure in the mid-latitudes of the Southern Hemisphere – which mean a greater prevalence of clear nights – are driven to some extent by climate change. But are the weather patterns that bring cold air to frost-prone regions also occurring more often? And if so, is that change due to climate change?
Understanding future frost and cold extremes
Understanding how and where frosts are likely to occur, and how this may change in a changing climate, is important for planning for a range of sectors. Frosts can have a significant economic impact on agriculture, for example, where entire crops can be wiped out by a single frost event.
Hub researchers are examining a severe frost event that occurred in the south-west Western Australian grain belt in September 2016. One million tonnes of grain were lost during more than 18 frost nights in the month hit the region, with some sites seeing the worst September frosts on record. Researchers hope to determine if climate change played a role in this event.
Attribution studies such as this (that is, studies that determine the influence of climate change on particular events) are important because they contribute to our understanding of the effect that climate change is already having on our exposure to damaging climate extremes, which is essential for all sectors of our society to appropriately manage our changing climate risks.
Many thanks to Michael Grose and Pandora Hope from Project 2.2 Enhancing Australia’s capacity to manage climate variability and climate extremes in a changing climate for their assistance with this article.