Christian Jakob, Monash University and Michael Reeder, Monash UniversityEight days ago, it rained over the western Pacific Ocean near Japan. There was nothing especially remarkable about this rain event, yet it made big waves twice.
First, it disturbed the atmosphere in just the right way to set off an undulation in the jet stream – a river of very strong winds in the upper atmosphere – that atmospheric scientists call a Rossby wave (or a planetary wave). Then the wave was guided eastwards by the jet stream towards North America.
Along the way the wave amplified, until it broke just like an ocean wave does when it approaches the shore. When the wave broke it created a region of high pressure that has remained stationary over the North American northwest for the past week.
This is where our innocuous rain event made waves again: the locked region of high pressure air set off one of the most extraordinary heatwaves we have ever seen, smashing temperature records in the Pacific Northwest of the United States and in Western Canada as far north as the Arctic. Lytton in British Columbia hit 49.6℃ this week before suffering a devastating wildfire.
What makes a heatwave?
While this heatwave has been extraordinary in many ways, its birth and evolution followed a well-known sequence of events that generate heatwaves.
Heatwaves occur when there is high air pressure at ground level. The high pressure is a result of air sinking through the atmosphere. As the air descends, the pressure increases, compressing the air and heating it up, just like in a bike pump.
Sinking air has a big warming effect: the temperature increases by 1 degree for every 100 metres the air is pushed downwards.
High-pressure systems are an intrinsic part of an atmospheric Rossby wave, and they travel along with the wave. Heatwaves occur when the high-pressure systems stop moving and affect a particular region for a considerable time.
When this happens, the warming of the air by sinking alone can be further intensified by the ground heating the air – which is especially powerful if the ground was already dry. In the northwestern US and western Canada, heatwaves are compounded by the warming produced by air sinking after it crosses the Rocky Mountains.
How Rossby waves drive weather
This leaves two questions: what makes a high-pressure system, and why does it stop moving?
As we mentioned above, a high-pressure system is usually part of a specific type of wave in the atmosphere – a Rossby wave. These waves are very common, and they form when air is displaced north or south by mountains, other weather systems or large areas of rain.
Rossby waves are the main drivers of weather outside the tropics, including the changeable weather in the southern half of Australia. Occasionally, the waves grow so large that they overturn on themselves and break. The breaking of the waves is intimately involved in making them stationary.
Importantly, just as for the recent event, the seeds for the Rossby waves that trigger heatwaves are located several thousands of kilometres to the west of their location. So for northwestern America, that’s the western Pacific. Australian heatwaves are typically triggered by events in the Atlantic to the west of Africa.
Another important feature of heatwaves is that they are often accompanied by high rainfall closer to the Equator. When southeast Australia experiences heatwaves, northern Australia often experiences rain. These rain events are not just side effects, but they actively enhance and prolong heatwaves.
What will climate change mean for heatwaves?
Understanding the mechanics of what causes heatwaves is very important if we want to know how they might change as the planet gets hotter.
We know increased carbon dioxide in the atmosphere is increasing Earth’s average surface temperature. However, while this average warming is the background for heatwaves, the extremely high temperatures are produced by the movements of the atmosphere we talked about earlier.
So to know how heatwaves will change as our planet warms, we need to know how the changing climate affects the weather events that produce them. This is a much more difficult question than knowing the change in global average temperature.
How will events that seed Rossby waves change? How will the jet streams change? Will more waves get big enough to break? Will high-pressure systems stay in one place for longer? Will the associated rainfall become more intense, and how might that affect the heatwaves themselves?
Explainer: climate modelling
Our answers to these questions are so far somewhat rudimentary. This is largely because some of the key processes involved are too detailed to be explicitly included in current large-scale climate models.
Climate models agree that global warming will change the position and strength of the jet streams. However, the models disagree about what will happen to Rossby waves.
From climate change to weather change
There is one thing we do know for sure: we need to up our game in understanding how the weather is changing as our planet warms, because weather is what has the biggest impact on humans and natural systems.
To do this, we will need to build computer models of the world’s climate that explicitly include some of the fine detail of weather. (By fine detail, we mean anything about a kilometre in size.) This in turn will require investment in huge amounts of computing power for tools such as our national climate model, the Australian Community Climate and Earth System Simulator (ACCESS), and the computing and modelling infrastructure projects of the National Collaborative Research Infrastructure Strategy (NCRIS) that support it.
We will also need to break down the artificial boundaries between weather and climate which exist in our research, our education and our public conversation.
Ian Wright, Western Sydney UniversityThe wild storms that recently raged across eastern Victoria caused major property and environmental damage, and loss of lives. They’ve also triggered serious water contamination incidents.
Yarra Valley Water issued an urgent health warning to not to drink tap water — not even if it’s boiled — in three affected suburbs: Kalista, Sherbrooke and The Patch.
So what caused this incident? Yarra Valley Water says the severe weather led to an equipment failure, with potentially unsafe water entering the drinking water system.
I spoke to the water authority about the nature of the contamination, and they did not provide any more detail. But based on my three decades of experience in the water industry, I can offer some insight into how disasters create contamination crises, and Australia’s vulnerabilities.
Does boiling water help?
Despite recent health warnings, it’s worth pointing out that Australia’s water supply is generally safe and reliable, with few exceptions. Still, this is hardly the first time disasters have disrupted water supply, whether from droughts, storms and floods, or bushfires.
For example, the Black Summer bushfires damaged water supply infrastructure for many communities, such as in Eden and Boydtown on the south coast of New South Wales. The Bega Valley Shire Council issued a boil water notice, as the loss of electricity stopped chlorinating the water supply, which is needed to maintain safe disinfection levels.
In inland and remote communities, drinking water contamination can be more common and very difficult to resolve.
For example, many remote Western Australian towns have chronic water quality problems, with drinking water often failing to meet Australian standards. And in 2015, the WA Auditor General reported the water in many Indigenous communities contains harmful contaminants, such as uranium and nitrates.
The source of this contamination is often naturally occurring chemical compounds in the local geology of ground water supplies.
One of the biggest contamination incidents in Australia occurred in August and September in 1998. A series of extreme wet weather events after a long drought triggered the contamination of Sydney’s drinking water with high levels of protozoan parasites, which can cause serious diseases such as gastroenteritis or cryptosporidiosis. It resulted in boil water alerts across much of the Sydney metropolitan area.
But what makes this latest incident in Victoria so concerning is that authorities have warned even boiling will not reduce contamination. This suggests contamination may be due to the presence of a harmful chemical, or high levels of sediment particles.
Sediment in water — measured as “turbidity” — can be hazardous because these particles can hold other contaminants, or even shield pathogens from disinfection.
Yarra Valley Water’s advice for the affected suburbs is to avoid using water in any cooking, making ice, brushing teeth or mixing baby formula, and for people to take care not to ingest water in the shower or bath. Emergency drinking water is being supplied by Yarra Valley Water in some locations.
So why do disasters threaten our drinking water?
This latest incident is another reminder that our drinking water is vulnerable to disruption from extreme weather.
This is almost certain to continue, and worsen, as the the Bureau of Meterology’s State of the Climate 2020 report predicts more extreme weather — including drought, heatwaves, bushfires, storms, and floods — in Australia’s future.
As these disasters become more frequent and extreme under climate change, impacts on water supplies across Australia are likely to become more destructive.
A good example of how this can unfold was the impact on Canberra’s water supply after the destructive 2003 bushfires.
Fire burned most of the region’s Cotter River catchments, which hold three dams. After fires went out, massive storms eroded the weakened ground, and washed ash, soil and organic debris into the storage reservoirs. It took years for the water supply system to fully recover.
Physical damage to water infrastructure is also a big risk, as modern water supplies are large and complex. For example, a fallen tree could break open the roof of a sealed water storage tank, exposing water to the elements.
Interruptions of electrical supplies after extreme weather are also common, leading to failures of water supply technology. This, for instance, could stop a water pump from operating, or break down the telemetry system which helps control operations.
As difficult as these hits to Australia’s water security are, and will be in future, it’s even more problematic in the developing world, which may not have the resources to recover.
How can we withstand these challenges?
To maintain optimal water quality, we must protect the integrity of water catchments — areas where water is collected by the natural landscape.
For example, damaging logging operations along steep slopes in Melbourne’s biggest water catchment threatens to pollute the city’s drinking water because it increases the risk of erosion during storms.
There’s also merit in Australian cities investing in advanced treatment of wastewater for reuse, rather than build infrequently used desalination plants for when there’s drought.
Australia could follow the US state of California which has ambitious targets to reuse more than 60% of its sewage effluent.
And it’s completely safe — Australia has developed guidelines to ensure recycled water is treated and managed to operate reliably and protect public health.
Why does some tap water taste weird?
If you’re concerned about water quality from the tap and haven’t received any alerts, you might just not like its taste. If in doubt, contact you local water supplier.
This story is part of a series The Conversation is running on the nexus between disaster, disadvantage and resilience. It is supported by a philanthropic grant from the Paul Ramsay foundation. You can read the rest of the stories here.
Anthony Richardson, RMIT UniversityLast week’s storm system wreaked havoc across Victoria. Some 220,000 households and businesses lost power, and residents in the hills on Melbourne’s fringe were warned yesterday it might not be restored for three weeks.
The extreme weather severely damaged the poles and powerlines that distribute electricity, particularly in the Mount Dandenong area. Senior AusNet official Steven Neave said of the region this week, “we basically have no network left, the overhead infrastructure is pretty much gone. It requires a complete rebuild”.
That leaves about 3,000 customers without electricity for weeks, in the heart of winter. The loss of power also cut mobile phone and internet services and reportedly allowed untreated water to enter drinking supplies.
So, could this disaster have been avoided? And under climate change, how can we prepare for more events like this?
An uncertain future
The Mount Dandenong area is heavily forested, and the chance of above-ground power infrastructure being hit by falling trees is obviously high.
Without electricity, people cannot turn on lights, refrigerate food or medications, cook on electric stoves or use electric heaters. Electronic banking, schooling and business activities are also badly disrupted. For vulnerable residents, in particular, the implications are profound.
Such disruptions are hard to avoid, at least while the electricity network is above ground. Good management, however, can prevent some trees coming down in storms.
The more pertinent question is: how can we prepare for such an event in the future?
Scientists warn such extreme weather will increase in both frequency and severity as climate change accelerates. The Australian Energy Market Operator is acutely aware of this, warning climate change poses “material risks to individual assets, the integrated energy system, and society”.
However, it’s challenging to predict exactly how future heatwaves, storms, bushfires and floods will affect the power network. As AEMO notes, many climate models related to storms and cyclones involve an element of unpredictability. So, plans to make the electricity system more resilient must address this uncertainty.
As researchers have noted, there is no “one future” to prepare for – we must be ready for many potential eventualities.
Yallourn – the bigger problem?
Meanwhile, in Victoria’s LaTrobe Valley, a situation at the Yallourn coal-fired power station which may have even greater consequences for electricity supplies.
A coal mine wall adjacent to the station is at risk of collapse after flooding in the Morwell River caused it to crack. If the wall is breached and the mine is flooded, as happened in 2012, there will be no coal to power the station and almost a quarter of Victoria’s power supply could be out for months.
Victoria’s energy needs are increasingly supplied by renewables. However, losing Yallourn’s generation capacity would reduce the capacity of the network to adapt to other possible disruptions.
Look beyond the immediate crisis
The Victorian government has offered up to A$1,680 per week, for up to three weeks, to help families without power buy supplies and find alternative accommodation.
Welfare groups say the assistance could be improved. They have called for changes to make it quicker and easier for people to access money, cash injections to frontline charities and more temporary accommodation facilities for displaced people and their pets.
While no doubt needed, these are all reactive responses targeted at those without electricity. When any system is disrupted, however, the effects can be widespread and felt long after the initial problem has been addressed.
Take dairy farmers in Gippsland, for example, who could not milk their cows without electricity. Cows must be milked regularly or else they stop producing milk – they cannot be “switched back on” when electricity is restored. Longer-term assistance may well be required for farmers facing such ripple effects.
And as welfare groups have noted, power companies should support affected customers over the long-term, with electricity discounts, deferrals and payment plans.
A call for backup
So, what else can be done to prepare for future power disruptions? Those with backup options, such as portable fuel-powered generators, or off-grid household batteries connected to solar panels, will undoubtedly be more resilient in such events.
These are examples of “system redundancy”, providing alternative electricity until the network is restored.
But it costs money to invest in household batteries or a generator that may never be used. Resilience is often a function of wealth, and the less well-off risk being left behind.
Certainly, governments can act to make society as a whole more resilient to power outages. For example, mobile phone towers have backup battery life of just 24 hours. As Victoria’s Emergency Management Commissioner Andrew Crisp said this week, extending that is something authorities “need to look at”.
Power and communications infrastructure could be moved underground to protect it from storms. While such a move would be expensive, it has been argued not doing so will lead to greater long-term costs under a changing climate.
The recent challenges at Yallourn and Callide show the risks inherent in a centralised electricity network dominated by coal.
Certainly, integrating renewable energy sources into the power network comes with its own challenges. However, expanding energy storage such as batteries, or shifting to small, community-level microgrids will go a long way to improving the resilience of the system.
This story is part of a series The Conversation is running on the nexus between disaster, disadvantage and resilience. It is supported by a philanthropic grant from the Paul Ramsay Foundation. Find the series here.
Gregory Moore, The University of MelbourneThe savage storms that swept Victoria last week sent trees crashing down, destroying homes and blocking roads. Under climate change, stronger winds and extreme storms will be more frequent. This will cause more trees to fall and, sadly, people may die.
These incidents are sometimes described as an act of God or Mother Nature’s fury. Such descriptions obscure the role of good management in minimising the chance a tree will fall. The fact is, much can be done to prevent these events.
Trees must be better managed for several reasons. The first, of course, is to prevent damage to life and property. The second is to avoid unnecessary tree removals. Following storms, councils typically see a spike in requests for tree removals – sometimes for perfectly healthy trees.
A better understanding of the science behind falling trees – followed by informed action – will help keep us safe and ensure trees continue to provide their many benefits.
Why trees fall over
First, it’s important to note that fallen trees are the exception at any time, including storms. Most trees won’t topple over or shed major limbs. I estimate fewer than three trees in 100,000 fall during a storm.
Often, fallen trees near homes, suburbs and towns were mistreated or poorly managed in preceding years. In the rare event a tree does fall over, it’s usually due to one or more of these factors:
1. Soggy soil
In strong winds, tree roots are more likely to break free from wet soil than drier soil. In arboriculture, such events are called windthrow.
A root system may become waterlogged when landscaping alters drainage around trees, or when house foundations disrupt underground water movement. This can be overcome by improving soil drainage with pipes or surface contouring that redirects water away from trees.
You can also encourage a tree’s root growth by mulching around the tree under the “dripline” – the outer edge of the canopy from which water drips to the ground. Applying a mixed-particle-size organic mulch to a depth of 75-100 millimetres will help keep the soil friable, aerated and moist. But bear in mind, mulch can be a fire risk in some conditions.
Root systems can also become waterlogged after heavy rain. So when both heavy rain and strong winds are predicted, be alert to the possibility of falling trees.
2. Direct root damage
Human-caused damage to root systems is a common cause of tree failure. Such damage can include roots being:
- cut when utility services are installed
- restricted by a new road, footpath or driveway
- compacted over time, such as when they extend under driveways.
Trees can take a long time to respond to disturbances. When a tree falls in a storm, it may be the result of damage inflicted 10-15 years ago.
3. Wind direction
Trees anchor themselves against prevailing winds by growing roots in a particular pattern. Most of the supporting root structure of large trees grows on the windward side of the trunk.
If winds come from an uncommon direction, and with a greater-than-usual speed, trees may be vulnerable to falling. Even if the winds come from the usual direction, if the roots on the windward side are damaged, the tree may topple over.
The risk of this happening is likely to worsen under climate change, when winds are more likely to come from new directions.
4. Dead limbs
Dead or dying tree limbs with little foliage are most at risk of falling during storms. The risk can be reduced by removing dead wood in the canopy.
Trees can also fall during strong winds when they have so-called “co-dominant” stems. These V-shaped stems are about the same diameter and emerge from the same place on the trunk.
If you think you might have such trees on your property, it’s well worth having them inspected. Arborists are trained to recognise these trees and assess their danger.
Trees are worth the trouble
Even with the best tree management regime, there is no guarantee every tree will stay upright during a storm. Even a healthy, well managed tree can fall over in extremely high winds.
While falling trees are rare, there are steps we can take to minimise the damage they cause. For example, in densely populated areas, we should consider moving power and communications infrastructure underground.
By now, you may be thinking large trees are just too unsafe to grow in urban areas, and should be removed. But we need trees to help us cope with storms and other extreme weather.
Removing all trees around a building can cause wind speeds to double, which puts roofs, buildings and lives at greater risk. Removing trees from steep slopes can cause the land to become unstable and more prone to landslides. And of course, trees keep us cooler during summer heatwaves.
Victoria’s spate of fallen trees is a concern, but removing them is not the answer. Instead, we must learn how to better manage and live with them.
Quite possibly. The weather can affect the performance of your internet connection in a variety of ways.
This can include issues such as physical damage to the network, water getting into electrical connections, and wireless signal interference. Some types of connection are more vulnerable to weather than others.
The behaviour of other humans in response to the weather can also have an effect on your connection.
How rain can affect your internet connection
Internet connections are much more complicated than the router and cables in our homes. There are many networking devices and cables and connections (of a variety of types and ages) between our homes and the websites we are browsing.
An internet connection may involve different kinds of physical link, including the copper wiring used in the old phone network and more modern fibre optic connections. There may also be wireless connections involved, such as WiFi, microwave and satellite radio.
Rain can cause physical damage to cables, particularly where telecommunication networks are using old infrastructure.
ADSL-style connections, which use the old phone network, are particularly vulnerable to this type of interference. Although many Australians may be connected to the National Broadband Network (NBN), this can still run (in part) through pre-existing copper wires (in the case of “fibre to the node” or “fibre to the cabinet” connections) rather than modern optical fibres (“fibre to the home”).
Much of the internet’s cabling is underground, so if there is flooding, moisture can get into the cables or their connectors. This can significantly interfere with signals or even block them entirely, by reducing the bandwidth or causing an electrical short-circuit.
But it isn’t just your home connection that can be impacted. Wireless signals outside the home or building can be affected by rainfall as water droplets can partially absorb the signal, which may result in a lower level of coverage.
Even once the rain stops, the effects can still be felt. High humidity can continue to affect the strength of wireless signals and may cause slower connection speeds.
Copper cables and changed behaviour
If you are using ADSL or NBN for your internet connection, it is likely copper phone cables are used for at least some of the journey. These cables were designed to carry voice signals rather than data, and on average they are now more than 35 years old.
Only around 18% of Australian homes have the faster and more reliable optical-fibre connections.
There is also a behaviour factor. When it rains, more people might decide to stay indoors or work from home. This inevitably leads to an increase in the network usage. When a large number of people increase their internet usage, the limited bandwidth available is rapidly consumed, resulting in apparent slowdowns.
This is not only within your home, but is also aggregated further up the network as your traffic is joined by that from other homes and eventually entire cities and countries.
Heatwaves and high winds
In Australia, extreme cold is not usually a great concern. Heat is perhaps a more common problem. Our networking devices are likely to perform more slowly when exposed to extreme heat. Even cables can suffer physical damage that may affect the connection.
Imagine your computer fan is not running and the device overheats — it will eventually fail. While the device itself may be fine, it is likely the power supply will struggle in extremes. This same issue can affect the networking equipment that controls our internet connection.
Satellite internet services for rural users can be susceptible to extreme weather, as the satellite signals have to travel long distances in the air.
Radio signals are not usually affected by wind, but hardware such as satellite dishes can be swayed, vibrated, flexed or moved by the wind.
Most of the time, human behaviour is the main cause
For most users, the impact of rain will be slight – unless they are physically affected by a significant issue such as submerged cables, or they are trying to use WiFi outside during a storm.
So, can weather affect your internet connection? Absolutely.
Will most users be affected? Unlikely.
So if your favourite Netflix show is running slow during in rainy weather, it’s most likely that the behaviour of other humans is to blame — holed up indoors and hitting the internet, just like you.
Nerilie Abram, Australian National University; Martin De Kauwe, UNSW, and Sarah Perkins-Kirkpatrick, UNSWSenator Matt Canavan sent many eyeballs rolling yesterday when he tweeted photos of snowy scenes in regional New South Wales with a sardonic two-word caption: “climate change”.
Prime Minister Scott Morrison has previously insisted there is “no dispute in this country about the issue of climate change, globally, and its effect on global weather patterns”. But Canavan’s tweet would suggest otherwise.
The reality is, as the climate warms, record-breaking cold weather is becoming less common. And one winter storm does not negate more than a century of human-caused global warming. Here, we take a closer look at the cold weather misconception and two other common climate change myths.
Myth #1: A cold snap means global warming isn’t happening
Canavan’s tweet is an example of a common tactic used by climate change deniers that deliberately conflates weather and climate.
Parts of Australia are currently in the grip of a cold snap as icy air from Antarctica is funnelled up over the eastern states. This is part of a normal weather system, and is temporary.
Climate, on the other hand, refers to weather conditions over a much longer period, such as several decades. And as our climate warms, the probability of such weather systems bringing record-breaking cold temperatures reduces dramatically.
Just as average temperatures in Australia have risen markedly over the past century, so too have winter temperatures. That doesn’t mean climate change is not happening. In a warming world, extremely cold winter temperatures can still occur, but less often than they used to.
In fact, human-caused climate change means extreme winter warmth now occurs more often, and across larger parts of the country. Record-breaking hot events in Australia now far outweigh record breaking cold events.
Myth #2: Global warming is good for us
Yes, climate change may bring isolated benefits. For example, warmer global temperatures may mean fewer people die from extreme cold weather, or that shorter shipping routes open up across the Arctic as sea ice melts.
But the perverse benefits that may flow from climate change will be far outweighed by the damage caused.
Extreme heat can be fatal for humans. And a global study found 37% of heat-related deaths are a direct consequence of human-caused climate change. That means nearly 3,000 deaths in Brisbane, Sydney and Melbourne between 1991 and 2018 were due to climate change.
Extreme heat and humidity may make some parts of the world, especially those near the Equator, essentially uninhabitable by the end of this century.
Global warming also kills plants, animals and ecosystems. In 2018, an estimated one-third of Australia’s spectacled flying foxes died when temperatures around Cairns reached 42℃. And there is evidence many Australian plants will not cope well in a warmer world – and are already nearing their tipping point.
Heatwaves also damage oceans. The Great Barrier Reef has suffered three mass bleaching events in just five years. Within decades the natural wonder is unlikely to exist in is current form – badly hurting employment and tourism.
Myth #3: More CO₂ means Earth will definitely get greener
In January last year, News Corp columnist Andrew Bolt caused a stir with an article that suggested rising carbon dioxide (CO₂) emissions were “greening the planet” and were therefore “a good thing”.
During photosynthesis, plants absorb CO₂. So as the concentration of CO₂ in the atmosphere increases, some researchers predict the planet will become greener and crop yields will increase.
Consistent with this hypothesis, there is indirect evidence of increased global photosynthesis and satellite-observed greening. There is also indirect evidence of increased “carbon sinks”, whereby CO₂ is drawn down from the atmosphere by plants, then stored in soil.
Rising temperatures lead to an earlier onset of spring, as well as prolonged summer plant growth – particularly in the Northern Hemisphere. Researchers think this has triggered an increase in the land carbon sink.
However, there’s also widespread evidence some trees are not growing as might be expected given the increased CO₂ levels in our atmosphere. For example, a study of how Australian eucalypts might respond to future CO₂ concentrations has so far found no increase in growth.
Increased plant growth may also cause them to use more water, causing significant reductions in streamflow that will compound water availability issues in dry regions.
Overall, attempts to reconcile the various lines of evidence of how climate change will alter Earth’s land vegetation have proved challenging.
So, are we doomed?
After all this bad news, you might be feeling a bit dejected. And true, the current outlook isn’t great.
Earth has already warmed by about 1℃, and current policies have the world on track for at least 3℃ warming this century. But there is still reason for hope. While every extra bit of warming matters, so too does every action to reduce greenhouse gas emissions.
And there are promising signs of increasing ambition to reduce greenhouse gas emissions on the global front – from the United States, the United Kingdom, the European Union, Japan and others.
Unfortunately, Australia is far behind our international peers, instead pushing the burden of action onto future generations. We now need the political leadership to set our country, and the world, on a safer and more secure path. Ill-informed tweets by senior members of the government only set back the cause.
Nerilie Abram, Professor; ARC Future Fellow; Chief Investigator for the ARC Centre of Excellence for Climate Extremes; Deputy Director for the Australian Centre for Excellence in Antarctic Science, Australian National University; Martin De Kauwe, Senior lecturer, UNSW, and Sarah Perkins-Kirkpatrick, ARC Future Fellow, UNSW
Tess Parker, Monash University and Ailie Gallant, Monash UniversityAt the tail end of winter in 2015, the ground in the Wimmera in northwestern Victoria had been a little dry but conditions weren’t too bad for farmers. The crop season was going well.
The start of September looked promising. It was cool, and there were decent rains. One Wimmera lentil grower said, “As long as it doesn’t get too hot, we should actually be OK.”
A few weeks later, summer weather had arrived early. At the start of October, the soils were baked dry. Lentils and other pulse crops were devastated.
This kind of event, where drier-than-normal conditions transform into severe or extreme drought in the space of weeks, is called a “flash drought”. While flash droughts are still not well understood, our research studies how they occur in Australia – which may help move us toward being able to warn of flash drought in advance.
The different kinds of drought
Scientists typically talk about drought as a lack or deficit of available moisture to meet various needs, such as in agriculture or for water resources. We often classify different types of drought depending on where there is a lack of water, or what its effects are:
- meteorological drought is a deficit of rain or other precipitation
- agricultural drought is a deficit of moisture in the soil and evaporating or transpiring into the air
- hydrological drought is a deficit of water in runoff and surface storage such as dams
- socioeconomic drought is a lack of water that affects the supply and demand of economic goods and services.
Different types of drought can occur at the same time, or a drought may evolve from one type to another. Droughts can last from months to decades, and can cover areas from a local region to most of the continent.
Recently, a new characterisation of drought has been added to the drought spectrum: “flash” drought.
What causes flash droughts?
Flash droughts are droughts that begin suddenly and then rapidly become more intense. Droughts only occur when there is insufficient rainfall, but flash droughts intensify rapidly over timescales of weeks to months because of other factors such as high temperatures, low humidity, strong winds and clear skies.
These conditions make the air “thirsty”, which meteorologists call “increased evaporative demand”. This means more water evaporates from the surface and transpires from plants, and moisture in the soil is rapidly depleted.
Under these conditions, evaporation and transpiration increase for as long as moisture is available at the surface. When this moisture is depleted and there is no rain to replenish it, the lack of water limits evaporation and transpiration – and vegetation becomes stressed as drought emerges.
Why haven’t we heard about flash drought before?
Flash droughts have always existed, and were first described in 2002. However, some particularly devastating flash droughts over the past decade have led to a surge of interest among researchers.
One such drought happened in the US Midwest. In May 2012, 30% of the continental United States was experiencing abnormally dry conditions. By August, that had extended to more than 60%. Although other rapidly developing droughts had been seen before, the widespread impacts of this event caught the attention of the US public and government.
Flash droughts are also increasingly a focus of attention in China and Australia. One of the few studies of flash drought in Australia examined an event when conditions in the country’s east suddenly changed from wet in December 2017, to dry in January 2018.
Anecdotal reports from farmers in the northern Murray–Darling Basin indicated removal of livestock from properties, and sheep numbers at record lows. By June 2018, there were reports of trees dying and a desert-like landscape, with little grass cover.
What happened in the Wimmera?
Our recent study of flash drought in Australia used several different measurements to capture a range of conditions related to drought.
- precipitation describes the supply of moisture from the atmosphere to the surface
- evaporative demand is the atmospheric demand for moisture from the surface
- evaporative stress is the supply of moisture from the surface relative to the demand from the atmosphere
- soil moisture is the wetness or dryness of the land surface.
The index we used to determine the atmospheric demand shows that the speed of development and the intensity of flash drought are driven by high temperatures, low humidity, strong winds and clear skies. All of these increase the demand for moisture from the surface.
After a drier than normal winter, southeast Australia experienced a cool and wet start to September 2015, with some rain in the first week of the month. Humidity and surface air pressure were roughly average, and surface sunshine below average, suggesting normal evaporative demand.
A warm spell began in mid-September, and intensified into a severe heatwave by early October, with temperatures over 35℃ persisting for several days in some areas. Throughout this period the overlying air became very dry. A persistent high-pressure system brought clear skies and increased sunshine.
By the end of October, the Wimmera was in severe or extreme drought conditions, devastating pulse and grain crops. Analysts estimated wheat production fell by 23%, with a loss of A$500 million in potential yields.
Flash drought in Australia
Flash droughts in Australia occur in all seasons. In the Wimmera, flash droughts are most frequent in summer and autumn. They can end as rapidly as they start, but in some cases may last many months.
In several instances, flash droughts in the Wimmera have started in summer or autumn, and the region has remained in drought through the following winter, and sometimes into spring. In this way, flash drought can be the catalyst for the common droughts lasting 6-12 months typical of southeast Australia.
Explainer: El Niño and La Niña
But there is some potential good news. We have long known that seasonal-scale droughts in Australia are strongly related to the El Niño-Southern Oscillation (ENSO), which gives us some ability to predict them.
ENSO strongly affects rainfall, which means it can also be linked to flash droughts in winter and spring.
Further, sub-seasonal forecasting, which predicts the climatic conditions weeks to a month in advance, has improved considerably in recent years. Given flash droughts occur on these timescales, we can be optimistic that prediction of flash droughts may be possible.
Alison O’Donnell, The University of Western Australia; Edward Cook, Columbia University, and Pauline Grierson, The University of Western AustraliaDrought over the last two decades has dealt a heavy blow to the wheatbelt of Western Australia, the country’s most productive grain-growing region. Since 2000, winter rainfall has plummeted by almost 20% and shifted grain-growing areas towards the coast.
Our recent research, however, found these dry conditions are nothing out of the ordinary for the region.
In fact, after analysing rings in centuries-old tree trunks, we found the region has seen far worse “megadroughts” over the last 700 years. Australia’s instrumental climate records only cover the last 120 or so years (at best), which means these historic droughts may not have previously been known to science.
Our research also found the 20th century was the wettest of the last seven centuries in the wheatbelt. This is important, because it means scientists have likely been underestimating the actual risk of drought – and this will be exacerbated by climate change.
What we can learn from ancient trees
We estimate the risk of extreme climate events, such as droughts, cyclones and floods, based on what we know from instrumental climate records from weather stations. Extending climate records by hundreds or even thousands of years means scientists would be able to get a much better understanding of climate variability and the risk of extreme events.
Thankfully we can do just that in many parts of the world using proxy records — things like tree rings, corals, stalagmites and ice cores in Antarctica. These record evidence of past climate conditions as they grow.
For example, trees typically create a new layer of growth (“growth ring”) around their trunks, just beneath the bark, each year. The amount of growth generally depends on how much rain falls in the year. The more it rains, the more growth and the wider the ring.
We used growth rings of native cypress trees (Callitris columellaris) near a large salt lake at the eastern edge the wheatbelt region. These trees can live for up to 1,000 years, perhaps even longer.
We can examine the growth rings of living trees without cutting them down by carefully drilling a small hole into the trunk and extracting a column (“core”) of wood about the size of a drinking straw. By measuring the ring widths, we developed a timeline of tree growth and used this to work out how much rain fell in each year of a tree’s life.
This method allowed us to reconstruct the last 668 years of autumn-winter rainfall in the wheatbelt.
A history of megadroughts
One of the most pressing questions for the wheatbelt is whether the decline in autumn-winter rainfall observed in recent decades is unusual or extreme. Our extended record of rainfall lets us answer this question.
Yes, rainfall since 2000 was below the 668-year average — but it was not extremely low.
The last two decades may seem particularly bad because our expectations of rainfall in the wheatbelt are likely based on memories of higher rainfall. But this frequent wet weather has actually been the anomaly. Our tree rings revealed the 20th century was wetter than any other in the last 700 years, with 12% more rain in the autumn-winter seasons on average than the 19th century.
Before the 20th century, the wheatbelt saw five droughts that were longer and more severe than any we’ve experienced in living memory, or have recorded in instrumental records. This includes two dry periods in the late 18th and 19th centuries that persisted for more than 30 years, making them “megadroughts”.
While the most recent dry period has persisted for almost two decades so far, rainfall during this period is at least 10% higher than it was in the two historical megadroughts.
This suggests prolonged droughts are a natural and relatively common feature of the wheatbelt’s climate.
So how does human-caused climate change play into this?
It’s likely both natural climate variability and human-caused climate change contributed to the wheatbelt’s recent decline in rainfall. Unfortunately, it’s also likely their combined influence will lead to even less rainfall in the near future.
What happens now?
Our findings have important implications for assessing the risk of drought. It’s now clear we need to look beyond these instrumental records to more accurately estimate the risk of droughts for the wheatbelt.
But currently, proxy climate records like tree rings aren’t generally used in drought risk models, as there aren’t many of them in the regions scientists want to research.
Improving risk estimates leads to better informed decisions around preparing for and managing the effects of droughts and future natural disasters.
Our findings are a confronting prospect for the future of farming in the wheatbelt.
Australian farmers have shown tremendous innovation in their ability to adapt in the face of drought, with many shifting from livestock to crops. This resilience will be critical as farmers face a drier, more difficult future.
Alison O’Donnell, Research Fellow in Dendroclimatology, The University of Western Australia; Edward Cook, Ewing Lamont Research Professor, Director Of Tree-Ring Lab, Columbia University, and Pauline Grierson, Director, West Australian Biogeochemistry Centre, The University of Western Australia