what sea salt in Antarctic snowfall reveals about bushfires worse than the Black Summer

We can read patterns of fire weather in Australia in the ice cores of Antarctica. Leah-Anne Thompson/Shutterstock^[4]

What makes bushfire weather?

If the weekend is forecast to be hot, dry and windy, we would say this is bushfire weather^[5]. Bushfire weather does not mean bushfires are inevitable, only they are more likely to start and more likely to be intense and fast-spreading once they ignite.

Australia’s Black Summer was caused by several climate factors combining^[6]. We had a multi-year drought^[7], which coincided with climate variability^[8] bringing strong and hot northwesterly winds, and all against a backdrop of increasing heat and falling humidity^[9] over land brought by climate change^[10].

Climate change is bringing^[11] more frequent and more severe bushfire weather. This trend is projected^[12] to continue into the future.

But quantifying the impacts of climate change on bushfire weather over the southeast Australian regions hardest hit by the Black Summer bushfires is difficult. This is because the climate in these mostly coastal or alpine regions varies a lot and we only have a relatively short record of observations.

Read more: Antarctic ice shows Australia's drought and flood risk is worse than thought^[13]

Our previous research^[14] on droughts – a key ingredient for extreme bushfire seasons – showed our roughly 120 years of rainfall observations does not fully cover the range of drought conditions which have occurred in the past 1,000 years. This is supported by recent palaeoclimate work^[15] reconstructing 2,000 years of eastern Australian climate variability and research using climate models to show evidence of megadroughts^[16] lasting 20 years or more in southeastern Australia.

Australia’s bushfire weather observations only extend back to 1950. In short, this means we have limited insights based on observations alone into how bad bushfire weather can be.

Answers in the ice

To reach back further in time, we looked to Antarctica. This is because Australia and Antarctica are linked through a “weather bridge^[17]”, meaning the enormous, powerful weather systems of the Southern Ocean affect both at the same time.

Year after year, these systems lift and carry different amounts of sea salt from the ocean surface and transport it to Antarctica, where it is laid down in layers of snow and ice.

The weather bridge between Australia and Antarctica. Weather systems, including cyclones and cold fronts, are represented by wind arrows. Colours represent temperatures: red = hot, blue = cold. Danielle Udy, CC BY-NC-ND^[18][19]

About 125 km from Casey Station in Australia’s Antarctic territory lies Law Dome. This small, coastal ice cap is very useful for climate scientists, because it is regularly hit by cold cyclones from the Southern Ocean. These intense winds bring sea salt and trigger high snow accumulation – about 1.5 metres a year.

Drilling ice cores from places such as Law Dome has given us rich data sets of past climatic events. We have used these changing levels of sea salt in the ice to reconstruct rainfall and drought^[20] over two millennia in eastern Australia. Our new research focuses on Australia’s bushfire weather.

What did we find?

Using this evidence, we found an important relationship between the ice core record and the Forest Fire Danger Index^[21], used by authorities to measure bushfire weather.

We then grouped^[22] similar weather patterns together over time, so we could get what we were really after: the connection between high and low pressure systems in the Southern Ocean, sea salt, and bushfire weather.

In the observational record, most of Australia’s bushfire disasters have been linked to intense summer cold fronts^[23], including Ash Wednesday (1983), the Canberra bushfires (2003), Black Saturday (2009) and the Black Summer. These cold fronts direct hot, dry and intense winds from the Australian desert to the coast. This increases the chance of bushfires starting and can turn bushfires already burning into enormous conflagrations.

Using our ice core and weather datasets, we found the same weather conditions are behind periods of lower levels of sea salt in the ice and elevated bushfire weather in southeast Australia. These conditions are when the strong westerly winds circling Antarctica move north towards Australia, bringing summer cold fronts. In technical terms, this is when the Southern Annular Mode^[24] is negative.

In the last 2,000 years, we found seven years where bushfire weather was the same or even worse than the 2019-2020 Black Summer, in the summers of 485, 683, 709, 760, 862, 885 and 1108 CE.

This is what an ice core looks like, still inside the drill. Joel Pedro, Author provided (no reuse)

Ice and bushfire

Climate change is making extreme bushfire weather steadily more likely^[25].

What our research shows is that natural climate variability can also produce bushfire weather similar or worse than the Black Summer.

Fire authorities should take these possible natural extremes into account, in addition to the increasing intensity of bushfire weather from climate change.

Read more: A 20-year 'mega-drought' in Australia? Research suggests it's happened before – and we should expect it again^[26]

Read more https://theconversation.com/of-ice-and-fire-what-sea-salt-in-antarctic-snowfall-reveals-about-bushfires-worse-than-the-black-summer-230982

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