Australia is burning. What's the cause?

Here's what you need to know from the science.

Hello everyone,

You’ve probably heard that hundreds of bushfires have been devastating massive chunks of Australia, near the size of England in scale. Living through this in my hometown, Sydney has been blanketed with apocalyptic red skies and thick smoke haze reminiscent of a sci-fi movie. I remember a few days in early December where ash was falling from the sky in inner Sydney.

Thousands of firefighters and volunteers have bravely done what they can against a tsunami of fire. Tragically, at least 28 people have died, thousands of homes lost and hundreds of millions of animals killed. It’s one of those events that make you realize how connected we are to the environment.

With my own background as an Australian climate scientist, I dedicated much of this week to investigate the facts for you about these fires, climate change and how they got so bad - with some help from Metafact experts.

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What started the Australian bushfires?

Mostly ‘Dry Lightning Storms’

The scale of the fires is immense. Every state and territory has been impacted. To give you perspective, they are more than 10-times the size of the 2018 Californian wildfires. So how did they start?

Careless cigarette disposal and arson were detected in some areas, but according to experts, most of the fires were lit by ‘dry lightning storms’.

"I can confidently say the majority of the larger fires that we have been dealing with have been a result of fires coming out of remote areas as a result of dry lightning storms," says New South Wales Rural Fire Service (RFS) Inspector Ben Shepherd.

So what are ‘dry lightning storms’?

Dry lightning occurs when a storm forms from high temperatures or along a weather front (as usual) but, unlike normal thunderstorms, the rain evaporates before it reaches the ground, so lightning strikes dry vegetation and sparks bushfires.

Dr Nick Earl et al. writing in The Conversation

According to a recent analysis, about 1% of the land burnt in NSW/Victoria was officially classed as arson.

Why did the fires become so large and widespread?

Australia’a vast Eucalypt forests lit up under the most severe fire conditions ever recorded

On January 9, Australia’s Bureau of Meteorology released official climate data showing 2019 to be “Australia's warmest year on record, with the annual national mean temperature 1.52°C above average”. It also showed 2019 to be “Australia's driest year on record. Nationally-averaged rainfall 40% below the average for the year at 277.6 mm.” Having the hottest & driest ever recorded contributed to Australia’s Forest Fire Danger Index to be the highest ever recorded (since 1950 when national records began).

What is the Forest Fire Danger Index? In the 1950s, a fire scientist from the CSIRO named Alan McArthur devised a method to examine the risks of bushfires in forested areas of Australia. Now known as the Forest Fire Danger Index (FFDI), it combines temperature, wind, humidity, rainfall, soil moisture and drought to forecast fire risk.

Is the Fire Index Changing? According to the Bureau of Meteorology and CSIRO the most extreme 10% of FFDI days "has increased in recent decades across many regions of Australia, especially in southern and eastern Australia. There has been an associated increase in the length of the fire weather season. Climate change, including increasing temperatures, is contributing to these changes.”

What was the fuel? Hot dry climate conditions allow fires to spread quickly. But I can throw a flame into a hot dry desert, and it won’t likely spread much. 70% of Australia is either arid or semi-arid land little vegetation (the white areas in the map below). These areas have little fuel to which a fire needs to burn and spread. But 74% of Australia’s forests are Eucalypt forests covering 14% of the continent. These forests (green in the map below) surround Sydney and dominate eastern Australia so when unusually dry hot conditions persist through spring and summer, they are a ticking time-bomb.

Source: Department of Environment.

What caused the record temperatures?

Globally, 2019 was the 2nd hottest year on record according to just-released figures by NASA. This follows a strong consistent climate warming taking place that is ‘Near Certain’ to be caused by human emissions increasing greenhouse gases in the atmosphere.

In Australia, the warming trend separated from natural climate variations by the 1980s. For extreme events like fires, it’s the maximum temperature extremes that are the worry and 2019 was the warmest on record (2.09 °C above average).

Source: Bureau of Meteorology, plotted by Kate Doyle

What caused the record dry in 2019?

Unlike the clear warming occurring across Australia, there is no evidence for Australian-wide trends for rainfall (see below). 2019 however, sticks out as an exceptionally low rainfall year (see the red bar below) - but this is not a climate trend!

Source: Bureau of Meteorology, plotted by Kate Doyle

There is a clear expert consensus that recent climate warming makes droughts worse, by inducing more evaporation and less soil moisture - but there is little evidence so far that climate warming is changing rainfall pattern changes in Australia (outside South-Western Australia).

Dr Andrew King is an Australian rainfall expert from the University of Melbourne who shared an insightful answer last year on whether climate change is a) changing rainfall patterns and/or b) making droughts worse in Australia:

In southwestern Australia, where there's been a substantial decline in winter rainfall in the last 60 years, it is likely that droughts are getting worse and human-caused climate change is contributing to this through the southward shift in rain-bearing frontal systems. In eastern Australia, rainfall is highly variable due to strong relationships with several climate modes including the El Niño-Southern Oscillation. Here, in rainfall at least, it is hard to see a clear signal towards more droughts.

The other issue is the variable we use to measure drought as rainfall deficits only provide one metric. Other measures incorporate evaporation or soil moisture and these may have different characteristics, but are less well measured. In eastern Australia, where there is little evidence of rainfall decline in most places, greater evaporation under warmer conditions may be contributing to slightly worse droughts.

Overall, it is harder to attribute drought events to climate change than it is for extreme heat events, but there is likely a slight overall intensification of drought conditions in some areas by some measures.

So climate change may have contributed to worsening the record dryness of 2019 - but there is little evidence it caused it. So what did?

Enter the Indian Ocean…

During the last months of 2019, East Africa had wide-spread flooding that killed hundreds and displaced millions in Kenya, Somalia, Burundi, Tanzania, South Sudan, Uganda, Djibouti and Ethiopia. What has this got to do with the lack of Australian rain? Alot.

Buried in the details of the Bureau of Meteorology Climate statement they make a very important observation that 2019 had “one of the strongest positive Indian Ocean Dipole events on record”. So what’s the “Indian Ocean Dipole” and why is that important to the drought?

Rainfall patterns across the world are controlled by oceans. Moisture in the atmosphere comes from the ocean, so if you get trade winds blowing moist air onto land, then rains will come and vice versa. Warm surface oceans bring more moisture to the atmosphere, while colder ocean brings dry conditions.

The Indian Ocean Dipole (IOD) was discovered in 1999 by Japanese scientists as an irregular oscillation of sea surface temperature changes between the western and eastern Indian Ocean. Driven by wind changes, there a 3 different IOD events: positive, neutral and negative.

Ocean and atmospheric conditions during a positive IOD event: source

Positive IOD events are where the eastern equatorial Indian Ocean becomes unusually cool, and the western equatorial Indian Ocean warms, resulting in reduced rainfall in Indonesia and Australia and heavy rainfall in eastern Africa.” writes Professor Nerilie Abram, an expert on the IOD from Australian National University.

From September to November, the IOD reached it’s highest ever positive phase. Despite being thousands of kilometers away, the Indian ocean was a big contributing factor to the record low rainfall in Australia and big floods in east Africa during late 2019.

Source: Bureau of Meteorology plotted by BBC

Is the Indian Ocean dipole changing from climate warming?

The big question is whether the IOD is changing from climate change? Likely, as Professor Nerilie Abram explains on Metafact today:

Observationsmodels and palaeoclimate data all suggest that human-caused climate warming is resulting in more frequent and more intense positive Indian Ocean Dipole (IOD) events.

There are weakeness in observations, models and palaeoclimate data of the IOD. But the fact that all three types of evidence indicate that positive IOD events are now occurring more often, and that we are seeing more of the extreme forms of these events, makes it likely that we are seeing the IOD changing because of climate warming.

Ongoing warming is expected to see extreme positive IOD events become even more frequent (a tripling of how often they occur in the 21st Century compared to the 20th Century) if we continue on a high greenhouse gas emission pathway without effective climate change mitigation policies.

The vulnerability of Australia to climate change is immense, yet the opportunity to lead the global shift towards a low carbon economy is even greater in my view. Ten years ago, I wrote a book about it. Yet since then on the global stage, Australia’s politicians, have been noncommittal in leading the world on this multi-generational challenge. Some experts say these bushfires might change things - I’m skeptical.

As I write this in Sydney, the good news is that the rain is pouring down! Some relief has started to come thankfully.

May the facts be with you,

Ben McNeil

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