What is a 1 in 100 year weather event? And why do they keep happening so often?

Andy Pitman, UNSW; Anna Ukkola, UNSW, and Seth WestraPeople living on the east coast of Australia have been experiencing a rare meteorological event. Record-breaking rainfall in some regions, and very heavy and sustained rainfall in others, has led to significant flooding.

In different places, this has been described as a one in 30, one in 50 or one in 100 year event. So, what does this mean?

What is a 1 in 100 year event?

First, let’s clear up a common misunderstanding about what a one in 100 year event means. It does not mean the event will occur exactly once every 100 years, or that it will not happen again for another 100 years.

For meteorologists, the one in 100 year event is an event of a size that will be equalled or exceeded on average once every 100 years. This means that over a period of 1,000 years you would expect the one in 100 year event would be equalled or exceeded ten times. But several of those ten times might happen within a few years of each other, and then none for a long time afterwards.

Ideally, we would avoid using the phrase “one in 100 year event” because of this common misunderstanding, but the term is so widespread now it is hard to change. Another way to think about what a one in 100 year event means is that there is a 1% chance of an event of at least that size in any given year. (This is known as an “annual exceedance probability”.)

How common are 1 in 100 year events?

Many people are surprised by the feeling that one in 100 year events seem to happen much more often than they might expect. Although a 1% probability might sound pretty rare and unlikely, it is actually more common than you might think. There are two reasons for this.

First, for a given location (such as where you live), a one in 100 year event would be expected to occur on average once in 100 years. However, across all of Australia you would expect the one in 100 year event to be exceeded somewhere far more often than once in a century!

In much the same way, you might have a one in a million chance of winning the lottery, but the chance someone wins the lottery is obviously much higher.

Second, while a one in 100 year flood event might have a 1% chance of occurring in a given year (hence it’s referred to as a “1% flood”), the chance is much higher when looking at longer time periods. For example, if you have a house designed to withstand a 1% flood, this means over the course of 70 years there’s a roughly 50% chance the house would be flooded at some point during this time! Not the best odds.

How well do we know how often flood events occur?

Incidents like these 1% annual exceedance probability events are often referred to as “flood planning levels” or “design events”, because they are commonly used for a range of urban planning and engineering design applications. Yet this presupposes we can work out exactly what the 1% event is, which sounds simpler than it is in practice.

First of all, we use historical data to estimate the one in 100 year event, but Australia has only about 100 years of reliable meteorological observations, and even shorter records of river flow in most locations. We know for sure this 100-year record does not contain the largest possible events that could occur in terms of rainfall, drought, flood and so on. We have data from indirect paleoclimate evidence pointing to much larger events in the past.

So a 1% event is by no means a “worst case” scenario, and some of the evidence from paleoclimate data suggests the climate has been very different in the deep past.

Second, estimating the one in 100 year event using historical data assumes the underlying conditions are not changing. But in many parts of the world, we know rainfall and streamflow are changing, leading to a changing risk of flooding.

Moreover, even if there was no change in rainfall, changes to flood risk can occur due to a host of other factors. Increased flood risk can result from land clearing or other changes in the vegetation in a catchment, or changes in catchment management.

Increased occurrence of flooding can also be associated with poor planning decisions that locate settlements on floodplains. This means a one in 100 year event estimated from past observations could under- or indeed overestimate current flood risk.

A third culprit for influencing how often a flood occurs is climate change. Global warming is unquestionably heating the oceans and the atmosphere and intensifying the hydrological cycle. The atmosphere can hold more water in a warmer world, so we would expect to see rainfall intensities increasing.

Extreme rainfall events are becoming more extreme across parts of Australia. This is consistent with theory, which suggests we will see roughly a 7% increase in rainfall per degree of global warming.

Australia has warmed on average by almost 1.5℃, implying about 10% more intense rainfall. While 10% might not sound too dramatic, if a city or dam is designed to cope with 100mm of rain and it is hit with 110mm, it can be the difference between just lots of rain and a flooded house.

So what does this mean in practice?

Whether climate change “caused” the current extreme rainfall over coastal New South Wales is difficult to say. But it is clear that with temperatures and heavy rainfall events becoming more extreme with global warming, we are likely to experience one in 100 year events more often.

We should not assume the events currently unfolding will not happen again for another 100 years. It’s best to prepare for the possibility it will happen again very soon.

Andy Pitman, Director of the ARC Centre of Excellence for Climate System Science, UNSW; Anna Ukkola, ARC DECRA Fellow, UNSW, and Seth Westra, Associate Professor, School of Civil, Environmental and Mining Engineering

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Frequent extreme bushfires are our new reality. We need to learn how to live with smoke-filled air

Gabriel da Silva, University of Melbourne

As fires ravaged large sections of the Australian bush last summer, cities and towns all along the coast were blanketed in toxic smoke. Air pollutants were measured at unheard of levels across the country.

Hazardous air descended on cities hundreds of kilometres away from the fires themselves. This air was the most dangerous to breathe on the planet.

The bushfire royal commission was tabled on October 30, with some sobering findings about fires and air pollution. Unfortunately, it showed that as a nation we were not prepared to deal with this public health emergency.

These disasters are inevitable under climate change, and while we need to urgently act on climate change to protect future generations, we also need to make changes now to mitigate the risks that already face us.

Australia must get better at communicating how to identify and then stay safe in hazardous air. A national set of air quality categories would go a long way to achieving this.

Over 400 deaths attributed to bushfire smoke

The royal commission heard that air pollution from the summer fires likely caused more than 400 deaths. Thousands of additional hospital admissions put added strain on our hospitals. All up the added burden to our health system was estimated at almost A$2 billion.

Even in the absence of extreme natural disasters, air pollution is one of Australia’s biggest public health concerns. Pollution from all sources causes thousands of deaths per year. This includes emissions from coal-fired power stations, diesel cars and wood-fired heaters.

Better preparing ourselves to deal with bushfire smoke will have flow-on benefits in tackling these problems.

Different state, different health advice

The royal commission found “there is an urgent need for national consistency in the categorisation of air quality”. At the moment, every state has their own system to categorise air quality and communicate it to the public.

But there are major discrepancies with how different states identify the worst air quality.

Air quality is the sum impact of the concentration of various unhealthy chemicals in the air. These include ozone, nitrogen and sulfur oxides, and fine particulate matter. To communicate this to the public, most countries convert these chemical concentrations into an Air Quality Index (AQI).

In the US, there is a standardised AQI categorisation for the whole country.

In Australia, the situation is very different. Every state has its own bands, with their own colour codes. These bands trigger at different pollutant levels and carry different health advice. The Royal Commission told us this needs to be standardised, and now.

For example, in NSW the worst air quality category is “Hazardous”, which triggers at an AQI of 200. South Australia, however, only recognises “Very Poor” as the worst class of air quality, with an AQI of 150 and above.

During the summer bushfires, AQI values as high as 5,000 were measured. It’s clear the highest bands of air pollution are no longer appropriate.

We need a national air quality system

We have faced a similar problem before. After Victoria’s Black Saturday fires in 2009, we recognised that our fire danger ratings were inadequate.

The Black Saturday royal commission found we needed a higher category for the most dangerous fire conditions. The “Catastrophic” category (“CODE RED” in Victoria) was added. It carried clear advice about what to do in such dangerous conditions, instructing people to safely leave as early as possible.

Fire danger rating sign in front of a grass fire — The ‘CODE RED’ or ‘Catastrophic’ fire danger rating was added after the Black Saturday fires.
Shutterstock

Something similar now needs to happen with air quality ratings.

When facing future extreme bushfires, we need a way to identify when catastrophic conditions have led to air so unhealthy that everyone should take precautions, such as staying indoors and wearing masks. We then need to get clear health advice out to the public.

A national air quality rating system could achieve this, and would also help address other important recommendations of the Royal Commission: That we need improved means of getting reliable information out to the public, along with better community education around what to do when air quality plummets.

There’s work to do

An Australian AQI should be featured on national weather reports and forecasts, providing important health information to the public every day of the year. At the same time it would familiarise Australians with air quality measures and actions that need to be taken to protect ourselves from unhealthy air.

But there is work to do. First, we need to develop a new set of air quality categories that work for the entire country, and reflects both the everyday hazards of industrial pollution and the extreme dangers of bushfires. These categories also need to be matched with sound health advice.

And if we are going to report these measures more widely then we also need to get better at measuring and predicting air quality across the nation — two other important royal commission recommendations.

Achieving all of this won’t be easy. But if we can get it right then we will be much better placed to deal with smoke risk the next time severe bushfires inevitably happen.

Gabriel da Silva, Senior Lecturer in Chemical Engineering, University of Melbourne

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Extreme heat and rain: thousands of weather stations show there’s now more of both, for longer

Jim Salinger, University of Tasmania and Lisa Alexander, UNSW

A major global update based on data from more than 36,000 weather stations around the world confirms that, as the planet continues to warm, extreme weather events such as heatwaves and heavy rainfall are now more frequent, more intense, and longer.

The research is based on a dataset known as HadEX and analyses 29 indices of weather extremes, including the number of days above 25℃ or below 0℃, and consecutive dry days with less than 1mm of rain. This latest update compares the three decades between 1981 and 2010 to the 30 years prior, between 1951 and 1980.

Globally, the clearest index shows an increase in the number of above-average warm days.

For Australia, the team found a country-wide increase in warm temperature extremes and heatwaves and a decrease in cold temperature extremes such as the coldest nights. Broadly speaking, rainfall extremes have increased in the west and decreased in the east, but trends vary by season.

In New Zealand, temperate regions experience significantly more summer days and northern parts of the country are now frost-free.

Extreme temperatures

Unusually warm days are becoming more common throughout Australia. When we compare 1981-2010 with 1951-80, the increase is substantial: more than 20 days per year in the far north of Australia, and at least 10 days per year in most areas apart from the south coast. The increase occurs in all seasons but is largest in spring.

This increase in temperature extremes can have devastating impacts for human health, particularly for older people and those with pre-existing medical conditions. Excessive heat is not only an issue for people living in cities but also for rural communities that have already been exposed to days with temperatures above 50℃.

New Zealanders are also experiencing more days with temperatures of 25℃ or more. The climate stations show the frequency of unusually warm days has increased from 8% to 12% from 1950 to 2018, with an average of 19 to 24 days a year above 25℃ across the country. Unusually warm days, defined as days in the top 10% of historic records for the time of year, are also becoming more common in both countries.

During the summers of 2017-18 and 2018-19, marine heatwaves delivered 32 and 26 (respectively) days above 25℃ nationwide in New Zealand, well above the average of 20 days. This led to accelerated glacial melting in the Southern Alps and major disruption to marine ecosystems, with die-offs of bull kelp around the South Island coast and salmon in aquaculture farms in the Marlborough Sounds.

More heat, more rain, less frost

In many parts of New Zealand, cold extremes are changing faster than warm extremes.

Between 1950 and 2018, frost days (days below 0℃) have declined across New Zealand, particularly in northern parts of the country which has now become frost-free, enabling farmers to grow subtropical pasture grasses. At the same time, crops that require winter frosts to set fruit are no longer successful, or can only be grown with chemical treatments (currently under review) that simulate winter chilling.

Across New Zealand, the heat available for crop growth during the growing season is increasing, which means wine growers have to shift varieties further south.

In Australia, the situation is more complicated. In many parts of northern and eastern Australia, there has also been a large decrease in the number of cold nights. But in parts of southeast and southwest Australia, frost frequency has stabilised, or even increased in places, since the 1980s.

These areas have seen a large decrease in winter rainfall in recent decades. The higher number of dry, clear nights in winter, favourable for frost formation, has cancelled out the broader warming trend.

In Australia, extreme rainfall has become more frequent in many parts of northern and western Australia, especially the northwest, which has become wetter since the 1960s. In eastern and southern Australia the picture is more mixed, with little change in the number of days with 10mm or more of rain, even in those regions where total rainfall has declined.

In New Zealand, more extremely wet days contribute towards the annual rainfall total in the east of the North Island, with a smaller increase in the west and south of the South Island. For Australia, there are significant drying trends in parts of the southwest and northeast, but little change elsewhere.

Extremes of temperature and precipitation can have dramatic effects, as seen during two marine heatwaves in New Zealand and the hottest, driest year in Australia during 2019.

Jim Salinger, Honorary Associate, Tasmanian Institute for Agriculture, University of Tasmania and Lisa Alexander, Chief Investigator ARC Centre of Excellence for Climate System Science and Associate Professor Climate Change Research Centre, UNSW

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Weather bureau says hottest, driest year on record led to extreme bushfire season

It’s the first time since overlapping records began that Australia experienced both its lowest rainfall and highest temperatures in the same year.
dan HIMBRECHTS/AAP

David Jones, Australian Bureau of Meteorology; Karl Braganza, Australian Bureau of Meteorology, and Skie Tobin, Australian Bureau of Meteorology

The Bureau of Meteorology’s annual climate statement released today confirms 2019 was the nation’s warmest and driest year on record. It’s the first time since overlapping records began that Australia experienced both its lowest rainfall and highest temperatures in the same year.

The national rainfall total was 37mm, or 11.7%, below the 314.5 mm recorded in the previous driest year in 1902. The national average temperature was nearly 0.2°C above the previous warmest year in 2013.

Globally, 2019 is likely to be the second-warmest year, with global temperatures about 0.8 °C above the 1961–1990 average. It has been the warmest year without the influence of El Niño.

Across the year, Australia experienced many extreme events including flooding in Queensland and large hail in New South Wales. However, due to prolonged heat and drought, the year began and ended with fires burning across the Australian landscape.

Part of Menindee Lakes on the Darling River, which is under pressure from low water flow as a result of the prolonged drought.
Dean Lewins/AAP

The effect of the long dry

Bushfire activity for the 2018–19 season began in late November 2018, when fires burned along a 600km stretch of the central Queensland coast. Widespread fires later followed across Victoria and Tasmania throughout the summer.

Persistent drought and record temperatures were a major driver of the fire activity, and the context for 2019 lies in the past three years of drought.

The dry conditions steadily worsened over 2019, resulting in Australia’s driest year on record, with area-average rainfall of just 277.6mm (the 1961–1990 average is 465.2 mm).

Almost the entire continent experienced rainfall in the lowest 10th percentile over the year.

Record low rainfall affected the central and southern inland regions of the continent and the north-eastern Murray–Darling Basin straddling the NSW and Queensland border. Many weather stations over central parts of Australia received less than 30mm of rainfall for the year.

Every capital city recorded below average annual rainfall. For the first time, national rainfall was below average in every month.

Record heat dominates the nation

2019 was Australia’s warmest year on record, with the annual mean temperature 1.52°C above the 1961–1990 average, surpassing the previous record of 1.33°C above average in 2013.

January, February, March, April, July, October, November, and December were all amongst the ten warmest on record for Australian mean temperature for their respective months, with January and December exceeding their previous records by 0.98°C and 1.08°C respectively.

Maximum temperatures recorded an even larger departure from average of +2.09°C for the year. This is the first time the nation has seen an anomaly of more than 2 °C, and about half a degree warmer than the previous record in 2013.

The year brought the nation’s six hottest days on record peaking at 41.9°C
(December 18), the hottest week 40.5 °C (week ending December 24), hottest month 38.6 °C (December 2019), and hottest season 36.9 °C (summer 2018–19).

The highest temperature for the year was 49.9 °C at Nullarbor (a new national December record) on December 19 and the coldest temperature was –12.0°C at Perisher Valley on June 20.

Keith West in southeast South Australia recorded a maximum 49.2°C on December 20, while Dover in far southern Tasmania saw 40.1°C on March 2, the furthest south such high temperatures have been observed in Australia.

Accumulating fire danger over 2019

The combination of prolonged record heat and drought led to record fire weather over large areas throughout the year, with destructive bushfires affecting all states, and multiple states at once in the final week of the year.

Many fires were difficult to contain in regions where drought has been severe, such as northern NSW and southeast Queensland, or where below average rainfall has been persistent, such as southeast Australia.

The Forest Fire Danger Index, a measure of fire weather severity, accumulated over the month of December was the highest on record for that month, and the highest for any month when averaged over the whole of Australia.

Record-high daily index values for December were recorded at the very end of December around Adelaide and the Yorke Peninsula in South Australia, East Gippsland in Victoria and the Monaro in NSW. These regions which experienced significant fire activity.

Don’t forget the floods

Amidst the dry, 2019 also included significant flooding across Queensland and the eastern Top End.

Heavy rain fell from January into early February, with damaging floods around Townsville and parts of the western Peninsula and Gulf Country.

Tropical cyclone Trevor brought further heavy rainfall in April in the eastern Northern Territory and Queensland. Floodwaters eventually reached Lake Eyre/Kati Thanda which, amidst severe local rainfall deficiencies in South Australia, experienced its most significant filling since 2010–11.

There was a notable absence of rainfall on Australia’s snow fields during winter and spring which meant less snow melt. Snow cover was generous, particularly at higher elevations.

A Townsville resident removes damaged items from a house after the Townsville floods in early 2019.
Dan Peled/AAP

What role did climate change play in 2019?

The climate each year reflects random variations in weather, slowly evolving natural climate drivers such as El Niño, and long-term trends through the influence of climate change.

A strong and long-lived positive Indian Ocean Dipole – another natural climate driver – affected Australia from May until the end of the year, and played a major role in suppressing rainfall and raising temperatures for much of the year.

Spring brought an unusual breakdown of the southern polar vortex which allowed westerly winds to affect mainland Australia. This reduced rainfall, raising temperature and contributing to the increased fire risk.

Climate change continues to cause long-term changes to Australia’s climate. Conditions in 2019 were consistent with trends of declining rainfall in parts of the south, worsening fire seasons and rising temperatures.

David Jones, Climate Scientist, Australian Bureau of Meteorology; Karl Braganza, Climate Scientist, Australian Bureau of Meteorology, and Skie Tobin, Climatologist, Australian Bureau of Meteorology

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Extreme weather caused by climate change has damaged 45% of Australia’s coastal habitat

Bleached staghorn coral on the Great Barrier Reef. Many species are dependent on corals for food and shelter.
Damian Thomson, Author provided

Russ Babcock, CSIRO; Anthony Richardson, The University of Queensland; Beth Fulton, CSIRO; Eva Plaganyi, CSIRO, and Rodrigo Bustamante, CSIRO

If you think climate change is only gradually affecting our natural systems, think again.

Our research, published yesterday in Frontiers in Marine Science, looked at the large-scale impacts of a series of extreme climate events on coastal marine habitats around Australia.

We found more than 45% of the coastline was already affected by extreme weather events caused by climate change. What’s more, these ecosystems are struggling to recover as extreme events are expected to get worse.

There is growing scientific evidence that heatwaves, floods, droughts and cyclones are increasing in frequency and intensity, and that this is caused by climate change.

Life on the coastline

Corals, seagrass, mangroves and kelp are some of the key habitat-forming species of our coastline, as they all support a host of marine invertebrates, fish, sea turtles and marine mammals.

Our team decided to look at the cumulative impacts of recently reported extreme climate events on marine habitats around Australia. We reviewed the period between 2011 and 2017 and found these events have had devastating impacts on key marine habitats.

Healthy kelp (left) in Western Australia is an important part of the food chain but it is vulnerable to even small changes in temperature and particularly slow to recover from disturbances such as the marine heatwave of 2011. Even small patches or gaps (right) where kelp has died can take many years to recover.
Russ Babcock, Author provided

These include kelp and mangrove forests, seagrass meadows, and coral reefs, some of which have not yet recovered, and may never do so. These findings paint a bleak picture, underscoring the need for urgent action.

During this period, which spanned both El Niño and La Niña conditions, scientists around Australia reported the following events:

2011: The most extreme marine heatwave ever occurred off the west coast of Australia. Temperatures were as much as 2-4℃ above average for extended periods and there was coral bleaching along more than 1,000km of coast and loss of kelp forest along hundreds of kilometres.

Seagrasses in Shark Bay and along the entire east coast of Queensland were also severely affected by extreme flooding and cyclones. The loss of seagrasses in Queensland may have led to a spike in deaths of turtles and dugongs.

2013: Extensive coral bleaching took place along more than 300km of the Pilbara coast of northwestern Australia.

2016: The most extreme coral bleaching ever recorded on the Great Barrier Reef affected more than 1,000km of the northern Great Barrier Reef. Mangrove forests across northern Australia were killed by a combination of drought, heat and abnormally low sea levels along the coast of the Gulf of Carpentaria across the Northern Territory and into Western Australia.

2017: An unprecedented second consecutive summer of coral bleaching on the Great Barrier Reef affects northern Great Barrier Reef again, as well as parts of the reef further to the south.

Heritage areas affected

Many of the impacted areas are globally significant for their size and biodiversity, and because until now they have been relatively undisturbed by climate change. Some of the areas affected are also World Heritage Areas (Great Barrier Reef, Shark Bay, Ningaloo Coast).

Seagrass meadows in Shark Bay are among the world’s most lush and extensive and help lock large amounts of carbon into sediments. The left image shows healthy seagrass but the right image shows damage from extreme climate events in 2011.
Mat Vanderklift, Author provided

The habitats affected are “foundational”: they provide food and shelter to a huge range of species. Many of the animals affected – such as large fish and turtles – support commercial industries such as tourism and fishing, as well as being culturally important to Australians.

Recovery across these impacted habitats has begun, but it’s likely some areas will never return to their previous condition.

We have used ecosystem models to evaluate the likely long-term outcomes from extreme climate events predicted to become more frequent and more intense.

This work suggests that even in places where recovery starts, the average time for full recovery may be around 15 years. Large slow-growing species such as sharks and dugongs could take even longer, up to 60 years.

But extreme climate events are predicted to occur less than 15 years apart. This will result in a step-by-step decline in the condition of these ecosystems, as it leaves too little time between events for full recovery.

This already appears to be happening with the corals of the Great Barrier Reef.

Gradual decline as things get warmer

Damage from extreme climate events occurs on top of more gradual changes driven by increases in average temperature, such as loss of kelp forests on the southeast coasts of Australia due to the spread of sea urchins and tropical grazing fish species.

Ultimately, we need to slow down and stop the heating of our planet due to the release of greenhouse gases. But even with immediate and effective emissions reduction, the planet will remain warmer, and extreme climatic events more prevalent, for decades to come.

Recovery might still be possible, but we need to know more about recovery rates and what factors promote recovery. This information will allow us to give the ecosystems a helping hand through active restoration and rehabilitation efforts.

We will need new ways to help ecosystems function and to deliver the services that we all depend on. This will likely include decreasing (or ideally, stopping) direct human impacts, and actively assisting recovery and restoring damaged ecosystems.

Several such programs are active around Australia and internationally, attempting to boost the ability of corals, seagrass, mangroves and kelp to recover.

But they will need to be massively scaled up to be effective in the context of the large scale disturbances seen in this decade.

Mangroves at the Flinders River near Karumba in the Gulf of Carpentaria. The healthy mangrove forest (left) is near the river while the dead mangroves (right) are at higher levels where they were much more stressed by conditions in 2016. Some small surviving mangroves are seen beginning to recover by 2017.
Robert Kenyon, Author provided

Russ Babcock, Senior Principal Research Scientist, CSIRO; Anthony Richardson, Professor, The University of Queensland; Beth Fulton, CSIRO Research Group Leader Ecosystem Modelling and Risk Assessment, CSIRO; Eva Plaganyi, Senior Principal Research Scientist, CSIRO, and Rodrigo Bustamante, Research Group Leader , CSIRO

This article is republished from The Conversation under a Creative Commons license. Read the original article.

How climate change can make catastrophic weather systems linger for longer

Steve Turton, CQUniversity Australia

Many parts of Australia have suffered a run of severe and, in some cases, unprecedented weather events this summer. One common feature of many of these events – including the Tasmanian heatwave and the devastating Townsville floods – was that they were caused by weather systems that parked themselves in one place for days or weeks on end.

It all began with a blocking high – so-called because it blocks the progress of other nearby weather systems – in the Tasman Sea throughout January and early February.

This system prevented rain-bearing cold fronts from moving across Tasmania, and led to prolonged hot dry northwesterly winds, below-average rainfall and scorching temperatures.

Meanwhile, to the north, an intense monsoon low sat stationary over northwest Queensland for 10 days. It was fed on its northeastern flank by extremely saturated northwesterly winds from Indonesia, which converged over the greater northeast Queensland area with strong moist trade winds from the Coral Sea, forming a “convergence zone”.

Ironically, these trade winds originated from the northern flank of the blocking high in the Tasman, deluging Queensland while leaving the island state parched.

Unusually prolonged

Convergence zones along the monsoon trough are not uncommon during the wet season, from December to March. But it is extremely rare for a stationary convergence zone to persist for more than a week.

Could this pattern conceivably be linked to global climate change? Are we witnessing a slowing of our weather systems as well as more extreme weather?

There does seem to be a plausible link between human-induced warming, slowing of jet streams, blocking highs, and extreme weather around the world. The recent Tasman Sea blocking high can be added to that list, along with other blocking highs that caused unprecedented wildfires in California and an extreme heatwave in Europe last year.

There is also a trend for the slowing of the forward speed (as opposed to wind speed) of tropical cyclones around the world. One recent study showed the average forward speeds of tropical cyclones fell by 10% worldwide between 1949 and 2016. Meanwhile, over the same period, the forward speed of tropical cyclones dropped by 22% over land in the Australian region.

Climate change is expected to weaken the world’s circulatory winds due to greater warming in high latitudes compared with the tropics, causing a slowing of the speed at which tropical cyclones move forward.

Obviously, if tropical cyclones are moving more slowly, this can leave particular regions bearing the brunt of the rainfall. In 2017, Houston and surrounding parts of Texas received unprecedented rainfall associated with the “stalling” of Hurricane Harvey.

Townsville’s floods echoed this pattern. Near the centre of the deep monsoon low, highly saturated warm air was forced to rise due to colliding winds, delivering more than a year’s worth of rainfall to parts of northwest Queensland in just a week.

The widespread rain has caused significant rises in many of the rivers that feed into the Gulf of Carpentaria and the Great Barrier Reef lagoon, and some runoff has made it into the Channel Country and will eventually reach Lake Eyre in South Australia. Unfortunately, little runoff has found its way into the upper reaches of the Darling River system.

Satellite images before (right) and after (left) the floods in northwest Queensland.
Courtesy of Japan Meteorological Agency, Author provided

Huge impacts

The social, economic and environmental impacts of Australia’s recent slow-moving weather disasters have been huge. Catastrophic fires invaded ancient temperate rainforests in Tasmania, while Townsville’s unprecedented flooding has caused damage worth more than A$600 million and delivered a A$1 billion hit to cattle farmers in surrounding areas.

Townsville’s Ross River, which flows through suburbs downstream from the Ross River Dam, has reached a 1-in-500-year flood level. Some tributaries of the dam witnessed phenomenal amounts of runoff, reliably considered as a 1-in-2,000-year event

Up to half a million cattle are estimated to have died across the area, a consequence of their poor condition after years of drought, combined with prolonged exposure to water and wind during the rain event.

Farther afield, both Norfolk Island and Lord Howe Island – located under the clear skies associated with the blocking high – have recorded exceptionally low rainfall so far this year, worsening the drought conditions caused by a very dry 2018. These normally lush subtropical islands in the Tasman Sea are struggling to find enough water to supply their residents’ and tourists’ demands.

Many parts of Australia have tolerated widespread extreme weather events this year, including some records. This follows a warm and generally dry 2018. In fact, 9 of the 10 warmest years on record in Australia have occurred since 2005, with only 1998 remaining from last century with reliable records extending back to 1910. Steady warming of our atmosphere and oceans is directly linked to more extreme weather events in Australia and globally.

If those extreme weather events travel more slowly across the landscape, their effects on individual regions could be more devastating still.

Steve Turton, Adjunct Professor of Environmental Geography, CQUniversity Australia

This article is republished from The Conversation under a Creative Commons license. Read the original article.

US Record Cold and Climate Change

Ice melt in Greenland and Antarctica predicted to bring more frequent extreme weather

File 20190204 193206 1wihd6q.jpg?ixlib=rb 1.1 — A new climate model combines data on ice loss from both polar regions for the first time.
Mark Brandon, CC BY-ND

Nick Golledge, Victoria University of Wellington

Last week, rivers froze over in Chicago when it got colder than at the North Pole. At the same time, temperatures hit 47℃ in Adelaide during the peak of a heatwave.

Such extreme and unpredictable weather is likely to get worse as ice sheets at both poles continue to melt.

Our research, published today, shows that the combined melting of the Greenland and Antarctic ice sheets is likely to affect the entire global climate system, triggering more variable weather and further melting. Our model predictions suggest that we will see more of the recent extreme weather, both hot and cold, with disruptive effects for agriculture, infrastructure, and human life itself.

We argue that global policy needs urgent review to prevent dangerous consequences.

Accelerated loss of ice

Even though the goal of the Paris Agreement is to keep warming below 2℃ (compared to pre-industrial levels), current government pledges commit us to surface warming of 3-4℃ by 2100. This would cause more melting in the polar regions.

Already, the loss of ice from ice sheets in Antarctica and Greenland, as well as mountain glaciers, is accelerating as a consequence of continued warming of the air and the ocean. With the predicted level of warming, a significant amount of meltwater from polar ice would enter the earth’s oceans.

The West Antarctic Ice Sheet is considered more vulnerable to melting, but East Antarctica , once thought to be inert, is now showing increasing signs of change.
Nick Golledge, CC BY-ND

We have used satellite measurements of recent changes in ice mass and have combined data from both polar regions for the first time. We found that, within a few decades, increased Antarctic melting would form a lens of freshwater on the ocean surface, allowing rising warmer water to spread out and potentially trigger further melting from below.

In the North Atlantic, the influx of meltwater would lead to a significant weakening of deep ocean circulation and affect coastal currents such as the Gulf Stream, which carries warm water from the tropics into the North Atlantic. This would lead to warmer air temperatures in Central America, Eastern Canada and the high Arctic, but colder conditions over northwestern Europe on the other side of the Atlantic.

Recent research suggests that tipping points in parts of the West Antarctic Ice Sheet may have already been passed. This is because most of the ice sheet that covers West Antarctica rests on bedrock far below sea level – in some areas up to 2 kilometres below.

Bringing both poles into one model

It can be a challenge to simulate the whole climate system because computer models of climate are usually global, but models of ice sheets are typically restricted to just Antarctica or just Greenland. For this reason, the most recent Intergovernmental Panel of Climate Change (IPCC) assessment used climate models that excluded ice sheet interactions.

Global government policy has been guided by this assessment since 2013, but our new results show that the inclusion of ice sheet meltwater can significantly affect climate projections. This means we need to update the guidance we provide to policy makers. And because Greenland and Antarctica affect different aspects of the climate system, we need new modelling approaches that look at both ice sheets together.

When the edges of the West Antarctic Ice Sheet start to recede, they retreat into deeper and deeper water and the ice begins to float more easily.
Mark Brandon, CC BY-ND

Seas rise as ice melts on land

Apart from the impact of meltwater on ocean circulation, we have also calculated how ongoing melting of both polar ice caps will contribute to sea level. Melting ice sheets are already raising sea level, and the process has been accelerating in recent years.

Our research is in agreement with another study published today, in terms of the amount that Antarctica might contribute to sea level over the present century. This is good news for two reasons.

First, our predictions are lower than one US modelling group predicted in 2016. Instead of nearly a metre of sea level rise from Antarctica by 2100, we predict only 14-15cm.

Second, the agreement between the two studies and also with previous projections from the IPCC and other modelling groups suggests there is a growing consensus, which provides greater certainty for planners. But the regional pattern of sea level rise is uneven, and islands in the southwest Pacific will most likely experience nearly 1.5 times the amount of sea level rise that will affect New Zealand.

While some countries, including New Zealand, are making progress on developing laws and policies for a transition towards a low-carbon future, globally policy is lagging far behind the science.

The predictions we make in our studies underline the increasingly urgent need to reduce greenhouse gas emissions. It might be hard to see how our own individual actions can save polar ice caps from significant melting. But by making individual choices that are environmentally sustainable, we can persuade politicians and companies of the desire for urgent action to protect the world for future generations.

Nick Golledge, Associate Professor of Glaciology, Victoria University of Wellington

This article is republished from The Conversation under a Creative Commons license. Read the original article.

When extreme weather wipes out wildlife, the fallout can last for years

Sean Maxwell, The University of Queensland and James Watson, The University of Queensland

The recent heatwaves have proved deadly to many Australian animals, from feral horses to flying foxes.

And it’s not just heatwaves that can cause mass die-offs. Last year, flooding rain wiped out entire Antarctic penguin colonies, while drought has previously caused mass mangrove diebacks around the Gulf of Carpentaria.

These events generate headlines, but what about the aftermath? And are these catastrophic events part of a wider pattern?

Our research describes how species have responded to extreme weather events over the past 70 years. These responses can tell us a great deal about how species are likely to cope with change in the frequency and intensity of extreme events in coming years.

We reviewed 517 studies, dating back to 1941 and conducted throughout the world, that examined how birds, mammals, fish, amphibians, reptiles, invertebrates or plants have responded to droughts, cyclones, floods, heatwaves, and cold snaps.

We found more than 100 cases of dramatic population declines. In a quarter of these cases, population numbers showed no sign of recovery long after the event. And in most cases, extreme events reduced populations of common species that play an important role in maintaining ecosystem integrity.

For example, extreme drought in the 2000s drove massive population declines of invertebrate freshwater species across Australia’s Murray-Darling Basin, and populations of buffalo, waterbuck, and kudu along the Zambezi River in Zimbabwe suffered severe and persistent declines following droughts in the 1980s.

We also found 31 cases of populations completely disappearing after an extreme event. Large populations of lizards and spiders were eradicated after Hurricane Lilli struck the Bahamas in 1996, for example. These populations had begun to recover one year after Lilli, but in half of all the cases of local population extinction, the species was still absent years or decades after exposure to an extreme event.

Negative responses were the most commonly reported, and also included habitat loss, declines in species numbers, and declines in reproductive fitness after an extreme event. These impacts clearly pose a serious risk to the longevity of many species, and to threatened species in particular. Kosciuszko National Park, for example, is a stronghold for the endangered northern corroboree frog, but 42% of its breeding sites in the park were rendered unusable by severe drought conditions throughout the 2000s.

Is there an upside?

Alongside the many negative impacts, we also found a larger‐than‐expected number of positive or neutral responses to extreme events (21% of all responses). This is a reminder that natural disturbances from extreme events often play a crucial role in the natural dynamics of an ecosystem.

Unfortunately, however, in many cases it was invasive species that benefited from extreme events. Flooding in southern Minnesota in 2004, for example, led to the rapid incursion of invasive green sunfish into streams, and cyclones accelerated the invasion of sweet pittosporum in Jamaican rainforests in the 1990s.

Cases of extreme events benefiting threatened species were uncommon, but included rainforest frogs becoming less susceptible to a fungal pathogen, chytrid fungus, after cyclones reduced rainforest canopy cover.

We also identified a range of “ambiguous” responses, including changes in diet or foraging behaviour, and changes in the types of species inhabiting a study area. Changes in invertebrate communities were particularly prevalent (87 cases). In 18 of these cases, the changes were long-lasting. However, most of the studies we reviewed lasted less than one year, and did not monitor for long-term recovery following an extreme event. This limits our ability to assess the long-term implications of extreme events on the species composition of ecosystems.

Avoiding future impacts

The one failsafe option for helping species cope with extreme events is to retain intact habitats, as these are the places where species are most resilient to extreme events. Intact habitats are contiguous areas of water or native vegetation that often span various altitudes, temperatures and rainfall patterns. These places can also act as important refuges for species that rely on long breaks between extreme events to recover.

Where intact habitat protection is not possible, restoring land or seascapes can also help species to adapt to extreme events. For example, long-term restoration efforts (that is, those that will be effective for at least 15 years) in brackish marshes help plant and animal communities cope with drought events.

Ecological restoration that helps species to adapt to extreme events can also benefit humans too. For instance, coastal communities can use oyster reefs or seagrass beds to guard against flooding.

Climate change has already increased the intensity and frequency of extreme events across the world, and the trend is expected to accelerate in the future. Recognising the importance of planning for extreme events is essential for helping species cope with climate change. Building resilience to extreme events may also provide an opportunity to reduce the vulnerability of humans too.

Governments, local councils, and local communities are under increasing pressure to plan for extreme climate events. We now need similar recognition of the importance of extreme events in threatened species planning efforts. Right now, this planning is virtually non-existent, and that needs to change.

Sean Maxwell, Postdoctoral fellow, The University of Queensland and James Watson, Professor, The University of Queensland

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Trust Me, I’m An Expert: Australia’s extreme weather

File 20180927 48647 vfqlcj.jpg?ixlib=rb 1.1 — Australia’s cyclone season lies ahead.
NASA / ESRSU / Seán Doran, CC BY-NC-SA

Madeleine De Gabriele, The Conversation and Wes Mountain, The Conversation

It’s easy to write off Australia’s extreme weather as business as usual. We deal with floods, droughts, cyclones and other wild events every year. But as climate change raises global temperatures, are the droughts happening more often? Are the floods getting worse?

The October episode of Trust Me, I’m An Expert looks back through colonial evidence and prehistoric records, and forward to the Bureau of Meteorology’s Cyclone Weather Outlook for the year ahead.

The full episode will be released on October 8, but today you can catch a little of our interview with the Bureau of Meteorology’s Andrew Watkins. Keep an eye out for the full episode, where we ask: are we in uncharted territory, or is this life as usual on a changeable continent?

Trust Me I’m An Expert is a monthly podcast from The Conversation, where we bring you stories, ideas and insights from the world of academic research.

You can download previous episodes of Trust Me here. And please do check out other podcasts from The Conversation – including The Conversation US’ Heat and Light, about 1968 in the US, and The Anthill from The Conversation UK, as well as Media Files, a brand new podcast all about the media.

You can find all our podcasts over here.

Music:

SensualMusic4You – “By Your Side” (Youtube)

Madeleine De Gabriele, Deputy Editor: Energy + Environment, The Conversation and Wes Mountain, Deputy Multimedia Editor, The Conversation

This article is republished from The Conversation under a Creative Commons license. Read the original article.

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