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10 January 2020

Climate change has introduced a new word to the lexicon: pyrocumulonimbus

The cloud Nasa describes as a “fire breathing dragon” is becoming more common.

By Sanjana Varghese

The facts of the Australian wildfires that have burned for the last three months are becoming almost unbelievable. More than 20 million acres of land – an area twice the size of Denmark – has burned. A billion animals have died, along with at least 27 people. And now, scientists around the world are tracking the growth of a rare weather event above the bushfires: pyrocumulonimbus, a cross between a thunderstorm and a fire.

Nasa’s official website calls the pyrocumulonimbus formation a “fire-breathing dragon”. It is an airborne vortex that sucks up everything in its path and spits out embers kilometres away. Scientists had considered pyrocumulonimbus clouds to be extremely rare but more have been identified in the last ten years, as extreme weather events become more common.

Pyrocumulonimbus clouds form when a mass of very hot air moves upwards into an unstable atmospheric environment, which in turn allows the hot air to move higher still. This creates a powerful updraft that sucks in smoke particles, water vapour, ash, and scorched debris. The moisture then condenses, as it rises, to create a towering cloud column.

At worst, these clouds can extend up to ten miles high. Despite the presence of water, rain is unusual but lightning, which then starts new wildfires or contributes to existing ones, is common. Worse still, the storm can move rapidly, accumulating embers and materials from the fires at surface level and spitting them out kilometres away, spreading the conflagration still further. There is little indication of the direction in which new fires will spread once a pyrocumulonimbus cloud has started to form.

There have been intense and fast-moving bushfires in Australia, California and Spain in the past year, but they appear to have mostly lacked the specific conditions necessary to create pyrocumulonimbus clouds. However, these conditions appear to be becoming more common as temperatures rise around the world. Even in places not previously thought of as dry and arid, such as the forests of Canada, the risk of wildfires has increased.

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In Australia, pyrocumulonimbus clouds have already played a significant part in national disasters. On 7 February 2009, which came to be known as Black Saturday, fires that were intensified by the existence of pyrocumulonimbus killed 173 people. In 2003, a pyrocumulonimbus over bushfires in Canberra developed into a tornado of fire – a process known as pyro-tornadogenesis – that incinerated hundreds of acres of land in less than half a second.

Australia has always had bushfires, but the severity of the fires and their associated extreme weather is changing fast. Climate scientist Jason Evans, at the University of New South Wales, told the BBC there has been a “crazy increase in the frequency of these events”.

Nor are these events happening only in Australia. A report from Yale’s environmental magazine at the beginning of last year explained how scientists were seeing more and more incidents of pyrocumulonimbus clouds, previously considered a rare and extreme weather event. Pyrocumulonmibus clouds have recently been documented for the first time in Canada, Portugal and Argentina.

But an article published in Scientific Reports in July of last year found that there is a clear trend towards near-surface fire weather conditions for Australia specifically. The article also found that while these weather events are likely to be influenced by anthropogenic climate change, there are “large knowledge gaps” on how these events might change in the future.

The fact that these events have been so rare in the past means there is little research on what causes them – and what to do when they do form. Researchers around the world are now racing to explain this terrifying new phenomenon, as governments and emergency services personnel look for better ways to help people escape or survive it. As with all the extreme weather events associated with anthropogenic climate change, however, the answer is both very complex, and it is staring us in the face.

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