The hundreds of bushfires that have been raging across Australia for the last month have already accounted for 5 million hectares burned, at least 20 deaths, and the destruction of more than 1,000 homes. They will likely continue to burn for the next couple of months. Many people are asking whether climate change has caused this extreme event.
While it is neither appropriate nor possible to answer that question yet, a relatively new branch of climate science called extreme event attribution serves to answer related questions, such as “How has or will climate change increase the frequency of such events?” or “To what extent did climate change play a role in this event?” Answering these questions is beyond the scope of this blog, but I can highlight some of the large-scale (climate) factors in place at the beginning of the event and how climate change has contributed to them.
Extreme heat and drought
The two key ingredients for bushfire are extreme heat and drought. Australia’s Bureau of Meteorology (BoM) recently stated that 2019 was the hottest and driest year on record. Mean temperatures across Australia have been rising steadily since the mid-20th century and are currently about 1.3°C above the 1961-1990 average, which is notably higher than the 1.1°C global increase for air temperatures over land. In its latest report released 2018, the IPCC noted that anthropogenic climate change is the cause for most of this warming.
High mean temperatures prime the climate system to reach extreme temperatures more easily. Importantly, very warm air over a region distorts the flow pattern at upper levels, so rain-laden weather systems are steered away. As the land surface dries out, the air warms up even more because there is little evaporative cooling to counter the heating. The processes spawn a positive feedback loop in that the response reinforces rather than reduces the forcing.
July through November rainfall was the lowest on record for the southern half of Australia. By December 2019, over three-quarters of Australia was either in the Very Much Below Average or Lowest on Record Category for annual rainfall according to the BoM.
The Indian Ocean Dipole
While climate change can explain the basic bushfire tendency in this straightforward way, there is more to understanding how the warming and drying began. Climate change does not just methodically increase temperatures; air currents and ocean circulations are also affected.
One very relevant large-scale coupled atmosphere ocean circulation that influences climate over Africa, Asia, and Australia is the Indian Ocean Dipole (IOD). This phenomenon can basically be described as the temperature difference between the western and eastern portions of the Indian Ocean. The so-called positive phase of the IOD has been in place since early 2019: Warmer than normal water to the west accelerates the easterly trade winds, which causes upwelling of cooler water off the west coast of Australia. The effect is similar to what happens off the west coast of South America during La Niña, the negative phase of the El Niño-Southern Oscillation (ENSO). The cool water suppresses precipitation over western Australia while the warm water over the western part of the Indian Ocean enhances rainfall over western Africa and India. Positive phases of the IOD have become more common. Having occurred 4 out of every 30 years early in the last century, they now occur about 10 times every 30 years. Computer simulations show that the observed increase is from climate change and will likely continue through the 21st century.
The Polar Vortex
While the IOD was in effect drying out the western part of Australia, one other climate driver, the Polar Vortex, affected the eastern half. Climate change has been heating up the Antarctic region faster than at mid latitudes, just as it has been heating up the Arctic region. This Antarctic Acceleration is affecting the south polar vortex in the same way Arctic Acceleration is impacting the northern one. It is weakening, and just like its Northern Hemisphere counterpart it is subject to splitting and shifting at times. Late last year, a northward shift of the vortex caused strong westerlies that normally exist south of Australia to extend over the continent, carrying the dry air from the west with it.
As horrific as this bushfire event is, the aforementioned ENSO phenomenon has been relatively dormant. If it were in its positive phase (El Niño), even more drying would have occurred. There is some indication that climate change will likely increase the frequency and intensity of ENSO events in the future, so the possibility unfortunately exists for all three climate factors to phase together for an even more extreme event.
The bottom line
To summarize, it would take a lot of computer simulations to quantify the impact that climate change may have had on the Australia bushfire event in progress. But breaking down the ingredients in terms of temperature, precipitation, and wind—and using what is already known about climate change impacts (albeit from a slew of prior computer simulations)—we can certainly understand qualitatively how climate change may have played a role.