Human-caused climate change made
southeastern Australia’s devastating wildfires during 2019–2020 at least 30 percent more likely to occur, researchers report in a new study published online
March 4.

A prolonged heat wave that
baked the country in 2019-2020 was the primary factor raising the fire risk, said
climate scientist Geert Jan van Oldenborgh, with the Royal Netherlands
Meteorological Institute in De Bilt. The study also linked the extremity of
that heat wave to climate change, van Oldenborgh said March 3 during a news
conference to explain the findings. Such an intense heat wave in the region is
about 10 times more likely now than it was in 1900, the study found.

Van Oldenborgh also noted
that climate simulations tend to underestimate the severity of such heat waves,
suggesting that climate change may be responsible for even more of the region’s
high fire risk. “We put the lower boundary at 30 percent, but it could well be
much, much more,” he said.

This week, the southeastern
Australia region was declared free of wildfires for the first time in over 240 days, according to a
statement March 2 by the New South Wales Rural Fire Service on Twitter. The
fires have burned through an estimated 11 million hectares, killing at least 34
people and destroying about 6,000 buildings since early July. About 1.5 billion
animals also died in the blazes. Researchers are still tallying the damage and assessing the potential for recovery for many
native plant and animal species (SN: 2/11/20).

The climate attribution study was conducted by the World Weather Attribution group, an international consortium of researchers who investigate how much of a role climate change might be playing in natural disasters. Given the quick turnaround time, the study has not yet been peer reviewed. “We wanted to bring the scientific evidence [forward] at a time when the public is talking about the event,” said climate modeler Friederike Otto of the University of Oxford. Then the group examined how climate change altered the Fire Weather Index, an estimation of the risk of wildfires.

The climate simulations show
that the probability of a high Fire Weather Index during the 2019–2020 season increased by at least 30 percent, relative
to the fire risk in 1910. That is primarily due to the increase in extreme
heat; the study was not able to determine the impact of climate change on
extreme drought conditions, which also helped fuel the blazes.

Researchers previously have suggested
that an El Niño-like atmosphere-ocean weather pattern known as the Indian Ocean Dipole, which was in a strong positive phase in 2019, may have played a role
in exacerbating the dry conditions (SN: 1/9/20). Global warming may make
such extreme positive phases of this pattern more common. The new study
confirmed that the 2019 positive phase made drought conditions more extreme,
but could not confirm this particular phase’s relationship to climate change.

“It is always rather
difficult to attribute an individual event to climate change,” but this study
is nicely done, says Wenju Cai, a climate scientist at CSIRO who is based in
Melbourne, Australia. The link identified to climate change is reasonable, if
not particularly surprising, he says.

The year 2019 was Australia’s
hottest and driest since modern recordkeeping began in the country in 1910.
Summers Down Under also appear to be lengthening: The Australia Institute, a
Canberra-based think tank, released a report March 2 that found that Australian
summers during the years 1999 to 2018 lasted longer by a month, on average, than they did 50 years ago.

Temperature observations
going back to 1910 show that the region’s temperatures have risen by about 2
degrees Celsius on average, van Oldenborgh and colleagues report. The climate
simulations underrepresented that warming, however, showing an increase of only
1 degree Celsius in that time.

Climate modelers previously have struggled to reconcile the disparity between recorded temperatures and simulated heat waves: Simulations tend to underestimate the severity of the extreme events. The team noticed a similar underestimation in its simulations of the 2019 heat waves in Europe (SN: 7/2/19). Conditions not generally factored into regional climate simulations, such as land-use changes, may be responsible for the disparity. Changes in vegetation cover, for example, can have an impact on how hot or dry a region gets.