As the world’s attention was drawn to Australia’s lethal bushfires, climate change was most invoked as the spark. But while global warming most certainly played some role in producing the worst fires in Australia’s recent history, a number of perennial climate fluctuations also coalesced during the 2019–20 season, scientists say. It’s likely those conditions will combine again, but probably not this year or even the one after that.

Bushfires are a regular feature of the Austral summer, but this season has been exceptional on several accounts. First, of course, is the extent of the burn—an estimated 11 million hectares as of 1 February. The surface area of the state of Virginia is about the same. The season began much earlier than is typical, as fires broke out during the first week of spring. And unlike most years, the fires have been concentrated near major population centers in southeastern Australia, filling the air in Sydney and other urban areas with unhealthy levels of smoke. According to the Global Fire Emissions Database, the number of active fires detected in New South Wales, Australia’s most populous state, through early January was more than four times that in the previous record season of 2002–3.

The immediate cause for the extreme season was the parched lands that resulted from a drought that began three years ago. In a continent accustomed to periodic droughts, 2019 was the hottest and driest year ever recorded. The annual mean temperature was 1.5 °C above the 1961–90 average of 21.8°, surpassing the previous record of 1.33° set in 2013. (Coincidentally, 1.5° above preindustrial levels is the aspirational global ceiling set in the 2015 Paris climate agreement.) Australia had an all-time average high temperature of 40.3° on 17 December, but that was topped the next day with a reading of 40.7°. The nation’s average rainfall totaled 277.6 mm last year, well below the previous record low of 314.5 mm established in 1902.

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“It’s not news to climate scientists that we’ve been experiencing an earlier extreme fire danger season and more intense bushfires,” says David Karoly, leader of the Earth systems and climate change hub at the Commonwealth Scientific and Industrial Research Organisation (CSIRO), a government-funded agency. “There are numerous studies showing this in the observational data. Not every summer, but many summers are showing increases in the area burned and earlier occurrences of extreme fire. We don’t like saying we told you so.”

The highly flammable eucalyptus forests that predominate in Australia “virtually explode” and emit volatile organic compounds, says Karoly. “It is probably the worst tree in the world for wildfires.”

Climate influences Section: Choose Top of page ABSTRACT Climate influences << A rare event Climate change Mitigation and adaptation A major global-scale cause of the extreme weather conditions in 2019 was the Indian Ocean Dipole (IOD), which last year moved into its strongest positive phase since 1997. The IOD is an oscillation that occurs in the sea-surface temperature between the western and eastern tropical Indian Ocean, analogous to the El Niño–Southern Oscillation in the Pacific Ocean. The relatively cool sea-surface temperatures near Australia during a positive IOD typically are linked to southeast Australia’s reduced spring rainfall. Last year’s IOD persisted into the beginning of summer, before being dissipated by a delayed monsoon. Most often, hot and dry conditions occur when a positive IOD coincides with an El Niño; this year’s extremes occurred in the absence of one. “The fact we’ve had a really serious drought with only one of those factors is quite exceptional,” says Mark Howden, director of the Climate Change Institute at the Australian National University (ANU). Positive IODs don’t occur that often. From 1960, when records began, there were 10 such events through 2016, according to Australia’s Bureau of Meteorology. Scientists disagree on whether they will increase in number and strength as the climate continues to warm. “There are papers suggesting that the positive IOD will become stronger and more frequent, leading to drier conditions in winter and spring,” says Julie Arblaster, an associate professor of Earth atmosphere and environment at Monash University. Some analyses indicate that a doubling of strongly positive IODs will occur with a global rise of 1.5°, says Howden. But, according to Karoly, there’s little evidence of an increase in climate models or in observations covering the period from the 1980s or 1990s to the present. He says that’s mainly because the influence of increased greenhouse gases hasn’t been strong enough yet. A second global-scale influence on Australia is the Southern Annular Mode (SAM), the north–south movement of the westerly wind belt that circles around Antarctica. Historically, those westerlies and accompanying winter storms, known as the roaring 40s for their latitude, have brought rain to the south of Australia during the winter months, as the SAM shifts the westerlies to the north. Typically, the SAM moves poleward in summer and allows the subtropical ridge that sits over the continent to slide southward, which brings warm, dry weather. The high-pressure ridge itself has strengthened with climate change throughout the midlatitudes. Human-induced climate change has caused the SAM to move more frequently into a positive phase during the winter, drawing the westerlies and the rains away from the southern coast, says Karoly. “I would argue the roaring 40s should be called the roaring 42s or 43s because they are shifted further south in winter,” he says. Last year’s dry winter set the stage for the disastrous fires. The same phenomenon led to a South African drought in which Cape Town nearly ran out of municipal water in 2018, he says. It has also helped to create the worst drought in Chile in 60 years. AUSTRALIAN GOVERNMENT BUREAU OF METEOROLOGY

A rare event Section: Choose Top of page ABSTRACT Climate influences A rare event << Climate change Mitigation and adaptation One highly unusual occurrence, a breakdown of the Antarctic polar vortex due to sudden stratospheric warming, worsened fire conditions in Australia last year. Such events occur with some frequency in the Northern Hemisphere winter, but they are rare in the Antarctic. Prior to last year, the only other sudden warming was observed in 2002, though weakenings of the vortex have occurred more frequently. By coincidence, a paper Arblaster coauthored in the November 2019 issue of Nature Geoscience describing the effects of those weakenings on Australia’s climate was in review when the actual event was in progress. Like its counterpart in the Arctic, the southern polar vortex annually weakens in late spring as the atmosphere warms. But last August’s sudden breakdown occurred months ahead of schedule. The effect was to increase the likelihood of spring hot and dry extremes across subtropical eastern Australia, as Arblaster and her colleagues described in their statistical analysis of observations over 40 years. Last year’s collapse of the vortex caused the SAM and the surface westerlies to move northward and prevented the typical moisture-laden southeast trade winds from the Tasman Sea from reaching Australia’s southeastern coast, says Karoly. It’s far too early to draw a connection between climate change and the sudden warming, researchers agree. Howden points to the extreme low humidity that accompanied the drought and heat waves as an apparent signal of climate change. The relative amount of moisture in the atmosphere has dropped across the midlatitudes that Australia occupies, compared with historical levels. At the synoptic scale, the current fire season has featured a persistent high-pressure system, centered over the Tasman Sea, that circulated hot, dry air from Australia’s interior desert to the southeastern part of the country, says Howden. When approaching cold fronts increased the pressure gradient in front of the high pressure, winds picked up and added fuel to the fires. Turbulent winds from multiple directions followed passage of the fronts and made the fires difficult to manage. According to Howden, some analyses have suggested that this pattern is likely to grow in frequency as the climate continues to transform.

Climate change Section: Choose Top of page ABSTRACT Climate influences A rare event Climate change << Mitigation and adaptation Australia is the most arid continent, and it features the greatest variability in rainfall from year to year—twice that of North America. Neville Nicholls, emeritus professor of Earth atmosphere and environment at Monash University, says that every influence on the nation’s climate has been altered by climate change, even if the exact mechanism for how that is happening in each case isn’t yet known. Australian daytime temperatures over land have increased by 1.25° in the past 25 years, or 5° per century. “That’s a massive increase, and it overshadows the variations in temperature we get from year to year through what we used to call natural climate variations due to SAM or the IOD or El Niño,” he says. “We can no longer say that anything is a purely natural phenomenon.” Among the developed nations, Australia is the most vulnerable to the impacts of climate change. Climate Science for Australia’s Future, published in July 2019 by the National Climate Science Advisory Committee, says warming has caused increased intensity and frequency of extreme heat events and droughts, longer fire seasons, hotter and more acidic oceans, and rising sea levels that amplify the effects of high tides and storm surges on coastal communities and infrastructure. Australia’s population is tightly concentrated in cities along its eastern coast. Average April–October rainfall in southeast Australia has fallen 11% compared with the 1900–1998 period, according to the State of the Climate 2018 report from Australia’s Bureau of Meteorology and CSIRO. Nine of the country’s top 10 hottest years have occurred since 2005. “I’ve never been through a summer like this in my 70 years,” says Nicholls. “In Melbourne, we have had massive hailstones, smoke from fires hundreds of kilometers away, and a massive dust storm in central Australia” that coated the city during rains. “The warming trend is so deep that definitely sometime in the next decade or so the record temperatures we set last year will be broken again,” he says. Howden points to a “mind-boggling storm” that on 20 January slammed the ANU Canberra campus and produced fist-sized hail that smashed car windows. Just 11 days later, a state of emergency was declared for the capital city as a bushfire approached from the south and the high reached 42°. The impact of the bushfires on the ecosystem has been devastating, with some estimates putting the death toll of animals at a billion or more. For some species, fires destroyed most of their habitats. “The thing that really struck home to people was the pictures of the animals and the sheer number of wildlife that could have been affected,” says Howden. “They’ve seen pictures of fires before and houses burnt down before, but pictures of koalas with singed feet and kangaroos barely escaping fire fronts really hit them hard.”