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.




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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.




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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.




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The Conversation


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.

Managed retreat of settlements remains a tough call even as homes flood and coasts erode


Tayanah O’Donnell, Australian National UniversityIt is no joke that New South Wales residents are in the midst of their fourth “one in 100 year” event since January 2020. Much of the Australian east coast continues to experience heavy rainfall, strong winds and abnormally high tides. All will make the current floods worse.

As climate tipping points are reached and the Earth’s systems begin to buckle under the strain, the need for considered adaptation strategies is overwhelmingly clear. One of these strategies is for human settlements to retreat from areas most at risk, whether from floods or bushfires. While something needs to be done to ensure future generations do not suffer catastrophic consequences, managed retreat is a complex tool.

These strategic decisions in the next five to ten years will be challenging. And these decisions really matter: where and how do we build residential areas that can cope with a climate-changed world?




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What is managed retreat?

Managed retreat can be defined as “purposeful, co-ordinated movement of people and assets out of harm’s way”. Managed retreat more often refers to the retreat of existing development out of harm’s way. Planned retreat is usually the preferred phrasing for new development that is planned for possible future relocation.

Both planned and managed retreat are focused on the permanent relocation of people and assets, as opposed to the evacuations we are seeing now.

Managed retreat is experiencing a resurgence in scientific literature as the impacts of climate change become increasingly frequent, severe and more obvious. These impacts bring with them a recognition of the need for adaptation even as we urgently reduce greenhouse gas emissions.

Of course, relocating away from high-risk locations is not an entirely new concept. However, managed retreat in response to a changing climate is not only complex, but also has a lot of political baggage. The complexity spans legal, financial, cultural and logistical factors among others: the political baggage seemingly associated with effective climate action in Australia often hinders governments’ abilities to respond properly.

Societies around the world need to grapple with the reality that managed retreat will become a suite of tools to respond to crisis. Insurers will not always be available, and the costs to governments (and therefore to you, the taxpayer) of responding to increasing rates of disasters, irrespective of insurance, will continue to grow exponentially.




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Responding to events after the fact is an unsustainable model of adaptation. There is, too, a need to acknowledge settlement needs and historical built environment legacies that have put significant state infrastructure in harm’s way.

Managing difficult trade-offs

We know trade-offs need to be made between what we protect and what we let go in suburban floodplain areas.

Legal machanisms to force people and assets to move can and must be thoughtful. The implementation of managed retreat in urbanised areas faces multiple hurdles. These include:




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It is wrong to see managed retreat as the panacea for climate risk and development in vulnerable locations. In many cases, once development is in place, it can be more appealing to some to protect an at-risk area rather than work towards managed retreat. Even where managed retreat has been successful, as in the case of the flood-prone township of Grantham, it was not without pain.

There are also other, more basic needs, such as having land available where people can relocate.

Working out highest and best use of land

There are ways that land can be used for its highest and best use at a point in time. For example, tools like easements can enable vulnerable land to be used, subject to event-based or time-based trigger-point thresholds. Once these thresholds are reached, the land is put to some other use. The advantage of these machanisms, especially for new development, is that owners are clear about the risks from the start.

This still leaves us with hard decisions about responding to at-risk current developments. Putting off these hard decisions and leaving them for future decision-makers will result in a huge injustice, because there will be catastrophe as Earth’s tipping points are passed. Development decisions made now will determine the impacts on our children and grandchildren.

Urban development decisions for both new and existing development in this coming decade demand courage and leadership. If we accept that Australian cities will continue to expand and increase in density, then we have some serious questions to ask ourselves. What kind of future do we want?

Some areas should simply not be developed.




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There is a risk that an over-reliance on managed retreat will over-simplify the challenge of working out what to do about development in at-risk locations. There is a clear need to separate out what to do about current and past developments, and how to approach new developments.

The latter is easy – do not rebuild residential homes in at-risk areas. Governments should repurpose these zones for uses that permit nature-based solutions to the need to adapt to climate change.

Current development is much more complex. In some cases, managed retreat – done thoughtfully and considerately – will be the only option.The Conversation

Tayanah O’Donnell, Honorary Senior Lecturer, Australian National University

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

Climate explained: how particles ejected from the Sun affect Earth’s climate


Earth’s magnetic field protects us from the solar wind, guiding the solar particles to the polar regions.
SOHO (ESA & NASA)

Annika Seppälä


CC BY-ND

Climate Explained is a collaboration between The Conversation, Stuff and the New Zealand Science Media Centre to answer your questions about climate change.

If you have a question you’d like an expert to answer, please send it to climate.change@stuff.co.nz


When the Sun ejects solar particles into space, how does this affect the Earth and climate? Are clouds affected by these particles?

When we consider the Sun’s influence on Earth and our climate, we tend to think about solar radiation. We are acutely aware of the skin-burning dangers of ultraviolet, or UV, radiation.

But the Sun is an active star. It also continuously releases what is known as “solar wind”, made up of charged particles, largely protons and electrons, that travel at speeds of hundreds of kilometres per hour.

Some of these particles that reach Earth are guided into the polar atmosphere by our magnetic field. As a result, we can see the southern lights, aurora australis, in the southern hemisphere, and the northern equivalent, aurora borealis.

Aurora Australis
Aurora australis observed above southern New Zealand.
Shutterstock/Fotos593

This visible manifestation of solar particles entering Earth’s atmosphere is a constant reminder there is more to the Sun than sunlight. But the particles have other effects as well.




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Solar particles and ozone

When solar particles enter the atmosphere, their high energies ionise neutral atmospheric nitrogen and oxygen molecules, which make up 99% of the atmosphere. This “energetic particle precipitation”, named because it’s like a rain of particles from space, is a major source of ionisation in the polar atmosphere above 30km altitude — and it sets off a chain of reactions that produces chemicals that facilitate the destruction of ozone.

The impact of solar particles on atmospheric ozone was first observed in 1969. Since the early 2000s, thanks to new kinds of satellite observations, we have seen growing evidence that solar particles play an important part in influencing polar ozone. During particularly active times, when the Sun releases large amounts of particles into space, up to 60% of ozone at altitudes above 50km can be depleted. The effect can last for weeks.

Lower down in the atmosphere, below 50km, solar particles are important contributors to the year-to-year variability in polar ozone levels, often through indirect pathways. Here, solar particles again contribute to ozone loss, but a recent discovery showed they also help curb some of the depletion in the Antarctic ozone hole.

How ozone affects the climate

Most of the ozone in the atmosphere resides in a thin layer at altitudes of 20-25km — the “ozone layer”.

But ozone is everywhere in the atmosphere, from the Earth’s surface to altitudes above 100km. It is a greenhouse gas and plays a key role in heating and cooling the atmosphere, which makes it critical for climate.

In the southern hemisphere, changes in polar ozone are known to influence regional climate conditions.

Satellite image of Earth's atmosphere
Solar particles ionise nitrogen and oxygen molecules in the atmosphere, which leads to other chemical reactions that contribute to ozone destruction.
Shutterstock/PunyaFamily

Its depletion above Antarctica had a cooling effect, which in turn pulled the westerly wind jet that circles the continent closer. As the Antarctic hole recovers, this wind belt can meander further north and affect rainfall patterns, sea-surface temperatures and ocean currents. The Southern Annular Mode describes this north-south movement of the wind belt that circles the southern polar region.

Ozone is important for future climate predictions, not only in the thin ozone layer, but throughout the atmosphere. It is crucial we understand the factors that influence ozone variability, be it man-made or natural like the Sun.

The Sun’s direct influence

The link between solar particles and ozone is reasonably well established, but what about any direct effects solar particles may have on the climate?

We have observational evidence that solar activity influences regional climate variability at both poles. Climate models also suggest such polar effects link to larger climate patterns (such as the Northern and Southern Annular Modes) and influence conditions in mid-latitudes.

The details are not yet well understood, but for the first time the influence of solar particles on the climate system will be included in climate simulations used for the upcoming Intergovernmental Panel on Climate Change (IPCC) assessment.




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Through solar radiation and particles, the Sun provides a key energy input to our climate system. While these do vary with the Sun’s 11-year cycle of magnetic activity, they can not explain the recent rapid increase in global temperatures due to climate change.

We know rising levels of greenhouse gases in the atmosphere are pushing up Earth’s surface temperature (the physics have been known since the 1800s). We also know human activities have greatly increased greenhouse gases in the atmosphere. Together these two factors explain the observed rise in global temperatures.

What about clouds?

Clouds are much lower in the atmosphere than where most solar particles penetrate. Particles know as galactic cosmic rays (coming from the centre of our galaxy rather than the Sun) may be linked to cloud formation.

It has been suggested cosmic rays could influence the formation of condensation nuclei, which act as “seeds” for clouds. But recent research at the CERN nuclear research facility suggests the effects are insignificant.

This doesn’t rule out some other mechanisms for cosmic rays to affect cloud formation, but thus far there is little supporting evidence.The Conversation

Annika Seppälä, Senior Lecturer in Geophysics

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