By now, everyone is familiar with the apocalyptic scenes of bushfires that have been ravaging Australia for several months in 2019, destroying forests, wildlife and homes. The scenes from Eastern Australia this week have been especially horrific, with entire town beings evacuated to beaches, some people taking refuge in lakes and ocean waters. Bushfires are not new to Australia, but the intensity and extent of the current bushfires is unprecedented. What are the underlying causes of these intense bushfires in Australia and elsewhere?
In this diary, we take a deeper look at some of the climate and weather forecasting science for Australia and the factors that influence weather and bushfires in the Australasia region. We also examine some connections between bushfires and global warming.
Please note that I am not a climate scientist; most of this information is culled from articles written by climate experts and weather organizations. The information I have gathered is by no means comprehensive or of sufficient depth. At times, I am certainly oversimplifying things. Hopefully, others in our DK community can correct any mistakes and add to our collective knowledge. Also take a look at other diaries at this site which cover the news and the politics behind the bushfires.
What Causes Bushfires?
Simply put, it is caused by large areas of dried vegetation. A year or two of normal to wet weather conditions followed by a year or two of hot dry conditions leads to dead trees and dried vegetation, which ignite easily. Bushfires can be ignited by lightning, power lines, other bushfires, some birds (!) and accidentally or intentionally by humans. High winds can cause the fires to quickly spread across the landscape.
Similar reasons are behind the California fires.
Climate of Australia
Australia is a large continent and climate differs in different regions of the country. By and large, Australia is a hot dry country, with low average rainfall, especially in the interior and the west. Two thirds of Australia is desert or semi-desert and experiences very high rates of evaporation; only about 10 per cent of rainfall survives as surface run-off to feed the rivers and lakes. Most rivers have low water flow.
The tropical areas of northern Australia have a wet summer because of the monsoon. During "the wet", typically October to April, humid north-westerly winds bring showers and thunderstorms. Winters stay dry.
The southern parts of Australia get most of their rain in Winter and spring (May through Oct), brought by westerly winds and rain-bearing cold fronts. Summers in southern Australia are generally dry and hot with coastal sea breezes.
The New South Wales region, that covers the South Eastern part of the East coast and includes Sydney, experiences most of its rainfall in summer and autumn with smaller amounts in winter and spring. Most of Queensland, which covers the North Eastern region has similar weather patterns
What Causes Dry and Wet Conditions in Australia
Weather in Australia (and elsewhere) is influenced by a large number of atmospheric and oceanic factors. Here we focus on 3 phenomena, that have the largest influence on Australia’s weather -
- The Indian Ocean Dipole (IOD)
- El Niño–Southern Oscillation (ENSO)
- The Southern Annular Mode (SAM)
1. The Indian Ocean Dipole
The Indian Ocean Dipole (IOD) is an irregular oscillation of sea-surface temperatures between the western and eastern regions of the Indian Ocean, that cause weather pattern changes across the regions.
The IOD has three phases: neutral, positive and negative.
When the IOD is in a positive phase, sea surface temperatures (SSTs) are warmer than normal in the western Indian Ocean area and cooler in the Eastern region. This causes -
- An increase in cloud formation and above-average rainfall in western region around eastern Africa
- An increase of the normal westerly winds and sometimes to shift to easterly winds across the Indian Ocean
- Below-average rainfall over the Indonesian/Australian region
- Movement of warm surface water from east to west
- Upwelling of colder water in the eastern regions
There is a positive feedback loop in there that causes the imbalance to build up over time. Positive feedback loops play a key role in many weather phenomenon.
During an IOD negative phase, surface temperatures are warmer than normal in the eastern Indian Ocean area and cooler in the Western region.
This causes -
- An increase in cloud formation and above-average rainfall in eastern region around Indonesia/Australia
- An increase in the westerly winds across the Indian Ocean
- Below-average rainfall over the east African region
- Movement of warm surface water from west to east
- Upwelling of colder water in the western regions
IOD events usually start around May or June, peak between August and October and then rapidly decay by December.
The following chart shows the IOD Index for the past 150 years. Larger swings in index values can be seen in the past 25 years. 2018 and 2019 have experienced large positive IOD index values, which have caused drought conditions over the past 2 years.
Here is a video explaining the IOD -
2. El Niño Southern Oscillation (ENSO)
The El Niño–Southern Oscillation (ENSO) is an irregular oscillation of sea-surface temperatures between the western and eastern regions of the Pacific Ocean, that affects weather across the regions.
The ENSO has three phases: neutral, positive and negative. The positive and negative phases last several months each and typically occur every few years with varying intensity per period.
During a neutral ENSO phase, trade winds push warm surface water to the west and help draw up deeper, cooler water in the east. The area north of Australia experiences cloud formation and rainfall. Under "normal" conditions the western tropical Pacific is 8 to 10°C warmer than the eastern tropical Pacific.
During a La Niña phase, trade winds strengthen, increasing the temperature of the warm water north of Australia. Cloudiness and rainfall north of Australia are enhanced, typically leading to above average winter–spring rainfall for eastern and central parts of the country, and a wetter start to the northern wet season.
During the El Niño phase, trade winds weaken (or reverse) and warmer surface water builds up in the central Pacific. This typically leads to below average winter–spring rainfall for eastern parts of the country, and a drier start to the northern wet season.
The following chart shows the ENSO Index for the past 30 years. ENSO have been relatively quiet the past few years. El Niño phases are indicated by positive values.
El Niño years are generally correlated with positive IOD phases; similarly La Niña years are are generally correlated with negative IOD phases. The correlation is not perfect; several IOD positive and negative phases have occurred in neutral El Niño years. But the combination tends to make rainfall anomalies worse for Australia.
3. The Southern Annular Mode (SAM)
The Southern Annular Mode is a north-south movement of the strong westerly winds that blow in the mid to high latitudes of the southern hemisphere.
The SAM has three phases: neutral, positive and negative. Each positive or negative SAM event tends to last for around one to two weeks. The time interval between positive and negative events is random, but is typically between a week and a few months.
In winter, normally the belt of westerly winds is close to Australia. Southern Australia typically get rainfall from cold fronts and troughs which pass over the southern reaches of Australia.
In a positive SAM phase, the belt of westerly winds contracts towards Antarctica. This reduces rainfall for southern Australia and brings more rainfall to the east.
In a negative SAM phase, the belt of westerly winds expands towards Australia. This increases rainfall for southern Australia and decreases rainfall for eastern Australia.
In summer, the normal position of the belt of westerly winds is further south of Australia compared to winter.
In a positive SAM phase, the belt of westerly winds contracts towards Antarctica. This further reduces rainfall for southern Australia and brings more rainfall to the east. This effect for eastern parts of Australia is much more widespread in summer as the east coast normally receives its highest rainfall in the summertime.
In a negative SAM phase, the belt of westerly winds expands towards Australia. This increases rainfall for western Australia and decreases rainfall for eastern Australia.
The following plot shows the SAM index since 1957. Note that the index is currently in negative territory. The trend is towards positive values.
SAM tends to be correlated with ENSO — El Niño tends to favor negative SAM during the spring to summer months, while La Niña tends to favor positive SAM during the spring to summer months. Both relationships typically enhance the respective dry and wet signals of El Niño and La Niña.
Among the 3 oscillation phenomenon, IOD and SAM tend to have a stronger effect on Australia’s weather than ENSO.
Australia’s weather over the past few years
2018 was Australia's third-warmest year on record (since 1910). Australia's area-averaged mean temperature for 2018 was 1.14 °C above the 1961–1990 average. The 11-year mean temperature for 2008–2018 was the highest on record at 0.77 °C above average. Only one of Australia's warmest ten years occurred before 2005, and nine of the last ten years have been warmer than average.
2019 was Australia’s warmest year on record. The national average maximum temperature was 2.09°C above the long-term average and 0.5°C above the previous record from 2013. Australia recorded its hottest day on record on 18 December, 2019 with an average maximum temperature of 41.9C (107.4F), beating the previous record by 1C that had been set the previous day.
A positive Indian Ocean Dipole has led to low rainfall nationally in 2018 and 2019. Average rainfall for 2018 was 11% below the 1961–1990 average of 465.2 mm. The positive IOD in spring significantly exacerbated drought conditions across parts of South-East Australia.
Spring 2019 was the driest and second-hottest on record.
2016 and 2017 were relatively wet, creating conditions for dangerous bushfires after the dry spells in 2018 and 2019.
In addition,
- Oceans around Australia have warmed by around 1°C since 1910, contributing to longer and more frequent marine heatwaves.
- Sea levels are rising around Australia, increasing the risk of inundation.
- The oceans around Australia are acidifying (the pH is decreasing).
blogs.scientificamerican.com/… summarizes the situation as follows —
Southeastern Australia has been in drought since 2017. Rainfall here is normally highly variable from year to year, but there have now been three winters in a row where the winter rains failed. This is a situation that has never been seen before in the historical record of Australia’s rainfall, even during infamous decade-long droughts such as the Millennium Drought. The severity of the current drought has caused large swathes of vegetation to die. It has even dried out wet rain forests, allowing fierce fires to take hold in places that would not normally burn.
Near Term Forecasts and Prognosis
The positive Indian Ocean Dipole (IOD) is weakening and will likely end in early January, while the negative Southern Annular Mode (SAM) is likely to rapidly decay in the new year. ENSO is expected to stay neutral.
But, the short-term temperature and rainfall outlook is negative. Rainfall conditions in the East will improve, but is likely to stay below average.
Bushfires and Climate Change
One might correctly point out that ENSO, IOD and SAM have been doing their dance for centuries, even though our knowledge about them is new and evolving. Australia is known for large climate variations from year and year and bushfires are common. So, what does climate change and global warming have to do with bushfires?
- The scientific evidence is well known for how anthropogenic greenhouse gas emissions are causing global warming and long-term climate change.
- Climate change is making Australian wildfires larger and more frequent.
- Increasing temperatures cause increased evaporation that dries the soil and vegetation faster.
- Global warming is raising ocean surface temperatures, which increases the intensity and frequency of oceanic oscillation cycles like IOD, ENSO and SAM.
- IOD, ENSO and SAM cycles are creating alternate cycles of above-normal rain and extended dry conditions.
- Multiple years of negative IODs and SAMs followed by multiple years of positive cycles creates conditions ripe for widespread bushfires.
- The trend will continue as worldwide temperatures continue to rise due to anthropogenic greenhouse gas emissions.
Worse yet, these bushfires will contribute to global warming. Analysis by NASA shows that CO2 emissions by these fires since August is equivalent to almost half of Australia’s annual greenhouse gas emissions. The burnt landscape is unlikely to recover enough to absorb much of the emitted carbon.
A more recent estimate puts the bushfire caused emissions at two-thirds of the annual emission of Australia -
The firestorm in Australia has been predicted by climatologists. The 2014 IPCC report for Australasia clearly stated -
Warming is projected to continue through the 21st century (virtually certain) along with other changes in climate.
Annual average rainfall is expected to decrease in southwestern Australia (high confidence) and elsewhere in most of far southern Australia and the northeast South Island and northern and eastern North Island of New Zealand (medium confidence), and to increase in other parts of New Zealand (medium confidence).
Tropical cyclones are projected to increase in intensity but remain similar or decrease in numbers (low confidence), and fire weather is projected to increase in most of southern Australia (high confidence) and many parts of New Zealand (medium confidence).
This was predicted in 2008 in an Aussie govt-commissioned report.
Here is an interesting chart on the contributions by various sources to atmospheric CO2 and how oceans have absorbed some of it to lessen the impact, but it cannot continue to do so without devastating consequences.
Long term solutions
I won’t get into the the plethora of solutions for combating climate change in this diary. But here are a couple of tweets that show that the investment cost is relatively small and the long term benefits of course are much larger.
Epilogue
The loss of life and livelihoods, of flora and fauna, and the effect on health and the economy caused by these bushfires will be felt for months if not years to come. And there will be more such episodes in the future.
Short-term solutions will probably include better forest management, controlled fires, creating firewalls, and more fire-fighting equipment and personnel.
But there is no doubt that unless Australia and the rest of the world find the political will to reduce greenhouse gases emissions, the bushfire situation in Australia and elsewhere will only get worse, along with the other deleterious effects of global warming.
Large scale change will only occur if we throw the climate-science-denying bums out. That’s where you and I come in; we need to keep educating our electorate about climate science, about the urgency of of the situation, about the science behind it and about various mitigation techniques that are at hand that will help avoid disaster and create jobs and wealth for everyone, not just the fossil fuel industry or Republican politicians.
P.S.
Hopefully, this diary will help improve your understanding of climate science and help smack down nonsensical arguments from your conservative friends who claim that climate change is a hoax or that bushfires are caused by sun spot activity, exploding horse manure or by leftist arsonists.
References
- The culprit behind east Australia's big dry — www.smh.com.au/…
- Unusually strong positive Indian Ocean Dipole (IOD) event develops, powers severe droughts in Australia, and supports warmer October in Europe. — www.severe-weather.eu/…
- Australia’s Angry Summer: This Is What Climate Change Looks Like — blogs.scientificamerican.com/…
- Indian Ocean influences on Australian climate — www.bom.gov.au/…
- Climate Model Summary for January to May 2020 — www.bom.gov.au/…
- Long-range sea surface temperature outlooks — www.bom.gov.au/…
- Southern Annular Mode: The climate 'influencer' you may not have heard of — www.abc.net.au/…
- Understanding the variability of Australian fire weather between 1973 and 2017 — journals.plos.org/…
- Climate Change 2014: Impacts, Adaptation, and Vulnerability. Part B: Regional Aspects — www.ipcc.ch/…