Climate change will make fire storms more likely in southeastern Australia


Giovanni Di Virgilio, UNSW; Andrew Dowdy, Australian Bureau of Meteorology; Jason Evans, UNSW; Jason Sharples, UNSW, and Rick McRae, ACT Emergency Services Agency

Temperatures across many regions of Australia are set to exceed 40℃ this week, including heatwaves forecast throughout parts of eastern Australia, raising the spectre of more devastating bushfires.

We have already heard warnings this fire season of the possibility of firestorms, created when extreme fires in the right conditions form their own weather systems.




Read more:
Firestorms and flaming tornadoes: how bushfires create their own ferocious weather systems


Firestorms are the common term for pyrocumulonimbus bushfires – fires so intense they create their own thunderstorms, extreme winds, black hail, and lightning.

While they are very rare, our research published earlier this year, found climate change is making it likely they will become more common in parts of southeast Australia.

We also identified certain regions in southern and eastern Australia, including near Melbourne’s fringe, that in the second half of this century will be far more vulnerable to these events than others.

How firestorms happen

The 2003 Canberra bushfires, devastating on a grand scale, saw a Canberra resident film a fire tornado for the first time ever. Six years later, the ferocious Black Saturday bushfires in Victoria created three separate pyrocumulonimbus events.

More recently, fire storms devastated California in November 2018.

Pyrocumulonimbus events begin with the intense heat of a very big and fast-burning wildfire, which causes a large and rapidly rising smoke plume. As the plume rises, low atmospheric pressure causes it to expand and cool. Moisture can condense into a type of cloud known as a pyrocumulus – not pyrocumulonimbus, yet. This type of cloud can be common in large fires.

However, with the right environmental conditions the plume goes much higher and pyrocumulonimbus clouds can form, towering up to 15km in some cases. As it rises, the plume cools, and the upper part of the clouds form ice particles that collide and can produce lightning.

These thunderstorms can create erratic and dangerously strong wind gusts. These can drive blizzards of embers that ignite spot fires beyond the fire font.

Lightning from the plume can start new fires, well ahead of the main fire. In one case, lightning generated in a pyrocumulonimbus cloud has been recorded starting new fires up to 100km ahead of the main fire.

How climate change makes firestorms more likely

One of the key elements to a firestorm forming is the precondition of the atmosphere above it. We wanted to investigate how a changing climate might affect the likelihood of firestorms happening.

Previous research has found there is more dynamic interaction between a large fire and the atmosphere when the air about 1.5km above the surface is relatively dry, and when there are larger temperature differences across increasing altitudes.

The larger the temperature difference, the more unstable the atmosphere may become. When higher altitudes get cold more quickly than normal, and are also very dry at low levels, it can become more likely that a pyrocumulonimbus event will develop during a large fire.

We used high-resolution climate modelling of projected lower atmospheric instability and dryness conditions to assess the risk of pyrocumulonimbus in southeastern Australia between 2060 and 2079, compared with 1990-2009. We then overlaid this information with the forest fire danger index to identify particularly dangerous fire days.

We were then able to identify how often dangerous fire weather days occurred at the same time as a dry and unstable atmosphere. Verifying our models against past observations, we then examined how often these two characteristics coincided in the future under climate change, should our greenhouse gas emissions remain on their current trajectory.

The results were startling. From 2060 onwards, we saw sharp increases in dangerous fire days across southeast Australia that coincided with atmospheric conditions primed to generate firestorms.

These extremely dangerous days also shifted across seasons, starting to appear in late spring, whereas historically Australian pyrocumulonimbus wildfires have typically been summer phenomena.

Across large areas of Victoria and South Australia, on average, we saw four or five more days every spring that were conducive to pyrocumulonimbus events.




Read more:
A surprising answer to a hot question: controlled burns often fail to slow a bushfire


These were sobering findings, even in a land of extremes like Australia. Our research suggests human-caused climate change has already resulted in more dangerous weather conditions for bushfires in recent decades for many regions of Australia. These trends are very likely to increase due to rising greenhouse gas emissions.The Conversation

Giovanni Di Virgilio, Research associate, UNSW; Andrew Dowdy, Senior Research Scientist, Australian Bureau of Meteorology; Jason Evans, Associate Professor, UNSW; Jason Sharples, Associate Professor, School of Physical, Environmental and Mathematical Sciences, UNSW Australia, UNSW, and Rick McRae, Researcher, Bushfire Cooperative Research Centre, ACT Emergency Services Agency

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

Climate explained: why coastal floods are becoming more frequent as seas rise



As sea levels rise, it becomes easier for ocean waves to spill further onto land.
from http://www.shutterstock.com, CC BY-ND

James Renwick, Victoria University of Wellington


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

I saw an article claiming that “king tides” will increase in frequency as sea level rises. I am sceptical. What is the physics behind such a claim and how is it related to climate change? My understanding is that a king tide is a purely tidal effect, related to Moon, Sun and Earth axis tilt, and is quite different from a storm surge.

This is a good question, and you are right about the tides themselves. The twice-daily tides are caused by the gravitational forces of the Moon and the Sun, and the rotation of the Earth, none of which is changing.

A “king” tide occurs around the time when the Moon is at its closest to the Earth and Earth is at its closest to the Sun, and the combined gravitational effects are strongest. They are the highest of the high tides we experience.

But the article you refer to was not really talking about king tides. It was discussing coastal inundation events.




Read more:
King tides and rising seas are predictable, and we’re not doing enough about it


When tides, storms and sea levels combine

During a king tide, houses and roads close to the coast can be flooded. The article referred to the effects of coastal flooding generally, using “king tide” as a shorthand expression. We know that king tides are not increasing in frequency, but we also know that coastal flooding and coastal erosion events are happening more frequently.

As sea levels rise, it becomes easier for ocean waves to penetrate on to the shore. The biggest problem arises when storms combine with a high tide, and ride on top of higher sea levels.

The low air pressure near the centre of a storm pulls up the sea surface below. Then, onshore winds can pile water up against the coast, allowing waves to run further inshore. Add a high or king tide and the waves can come yet further inshore. Add a bit of sea level rise and the waves penetrate even further.

The background sea level rise has been only 20cm around New Zealand’s coasts so far, but even that makes a noticeable difference. An apparently small rise in overall sea level allows waves generated by a storm to come on shore much more easily. Coastal engineers use the rule of thumb that every 10cm of sea level rise increases the frequency of a given coastal flood by a factor of three.

This means that 10cm of sea level rise will turn a one-in-100-year coastal flood into a one-in-33-year event. With another 10cm of sea level rise, it becomes a one-in-11-year event, and so on.

Retreating from the coast

The occurrence rates change so quickly because in most places, beaches are fairly flat. A 10cm rise in sea levels might translate to 30 or 40 metres of inland movement of the high tide line, depending on the slope of the beach. So when the tide is high and the waves are rolling in, the sea can come inland tens of metres further than it used to, unless something like a coastal cliff or a sea wall blocks its way.

The worry is that beaches are likely to remain fairly flat, so anything within 40 metres of the current high tide mark is likely to be eroded away as storms occur and we experience another 10cm of sea level rise. If a road or a house is on an erodible coast (such as a line of sand dunes), it is not the height above sea level that matters but the distance from the high tide mark.

Another 30cm of sea level rise is already “baked in”, guaranteed over the next 40 years, regardless of what happens with greenhouse gas emissions and action on climate change. By the end of the century, at least another 20cm on top of that is virtually certain.




Read more:
Our shameful legacy: just 15 years’ worth of emissions will raise sea level in 2300


The 30cm rise multiplies the chances of coastal flooding by a factor of around 27 (3x3x3) and 50cm by the end of the century increases coastal flooding frequency by a factor of around 250. That would make the one-in-100-year coastal flood likely every few months, and roads, properties and all kinds of built infrastructure within 200 metres of the current coastline would be vulnerable to inundation and damage.

These are round numbers, and local changes depend on coastal shape and composition, but they give the sense of how quickly things can change. Already, key roads in Auckland (such as Tamaki Drive) are inundated when storms combine with high tides. Such events are set to become much more common as sea levels continue to rise, to the point where they will become part of the background state of the coastal zone.

To ensure cities such as Auckland (and others around the world) are resilient to such challenges, we’ll need to retreat from the coast where possible (move dwellings and roads inland) and to build coastal defences where that makes sense. The coast is coming inland, and we need to move with it.The Conversation

James Renwick, Professor, Physical Geography (climate science), Victoria University of Wellington

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