Human activity intensified the severity of Australia’s 2019 wildfires

Human activity intensified the severity of Australia’s 2019 wildfires
Human activity intensified the severity of Australia’s 2019 wildfires

The 2019-2020 bushfires ravaged 2.6% of Australia’s surface area, the equivalent of just under a third of France. In question ? The biggest drought ever recorded in the country, which ignited the powder between 2017 and 2019. These fires stand out from the precedents experienced by the country. They spread from south to north, in an uncontrollable and unprecedented way. A recent Australian study, published in the journal Science Advancesshows that global warming has intensified the drought preceding these deadly fires.

Hydrologists and climatologists have had access to a large amount of meteorological data, making it possible to develop computer models using machine learning. These models provide information on the evolution of the water cycle in the country over several decades. The study also shows that future droughts could be even more intense.

A high-impact drought

Climate specialists wanted to understand how such a catastrophic fire could have occurred. Born in the state of New South Wales and Victoria, the fires ravaged the south-east of Australia, encompassing the Murray-Darling basin, a region essential to agriculture since it represents 40% of Australian agricultural production. For 18 months, certain areas of this region faced an intense drought, known as “Tinderbox”, named after a fire starter.

By assembling historical data, the models show that the so-called cool months, during winter (June and July in Australia), received only half the amount of precipitation they normally receive. . The humid, hot summer months received at most three-quarters. Agricultural losses were deplored, groundwater emptied, soils dried up and watersheds emptied.

Causes of Tinderbox Drought

According to analyses, the El Niño meteorological phenomenon cannot be held responsible for this severe drought. Although it causes ocean temperatures to rise abnormally, it only appeared at the end of the Tinderbox drought. However, another phenomenon has reduced the chances of rain in Australia: the positive Indian Ocean Dipole (IOD), a particular interaction between the atmosphere and the ocean. Warm water is brought back to the Indian coast, hot air under pressure rises above the country, follows the water cycle and falls into depression. The water near the Australian coast then cools, limiting the arrival of monsoons.

In addition, some precipitation was diverted. “By analyzing the wind data, we noticed an abnormal circulation that diverted the atmospheric flow from the region affected by the drought towards the north of the country”, explains Anjana Devanand, hydrologist at the Center of Excellence for Climate Extremes (CLEX), in Sydney (Australia). In addition, the Australian atmosphere reached record vapor pressure deficits, indicating its dehydration. This “atmospheric thirst” was attenuated by emptying the moist reserves of the soil or plants, thus intensifying the phenomenon. The year 2019 is therefore the hottest year ever recorded in Australia, with heat peaks reaching +2.5°C compared to normal.

Human activity has intensified the severity of the weather situation

Climate change is reducing the number of rainy days in the long term, especially in arid regions. Using machine learning technique, scientists quantified the human-caused contribution to precipitation deficits caused by the Tinderbox drought. “We generated more than 10,000 years of data, and we used this long record to estimate the probability of occurrence of precipitation deficits, as bad as the Tinderbox drought”tells us Anjana Devanand.

Although subject to great uncertainties, the probability that this drought would be so intense in the absence of humans is less than 1%. The natural drought from 2017 to 2019 would have been intensified by around 18% by the impact of anthropogenic global warming. This type of learning model makes it possible to predict intense droughts. In the future, the risk of similar droughts will only increase, underscoring the urgency of reducing greenhouse gas emissions.

By Marine Laplace

Opening image: Residents defend their property from bushfires in Taree, northern Sydney, November 2019 (credit: Peter Parks / AFP).

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