SCIENCE ET VIE ARTICLE. They account for just 3 per cent of fires but are responsible for more than 50 per cent of the land area burnt worldwide.
Just like the monster fire that ravaged California in November 2018, killing 85 people and destroying 60,000 hectares, uncontrollable mega-fires are becoming increasingly common. The causes include human activity, the condition of the forests, and, above all, the heat and drought linked to global warming.
The apocalyptic images have been seen around the world. With 85 people dead and 14,000 homes destroyed, 60,000 hectares reduced to ashes and nearly 10 billion dollars’ worth of damage, it is an understatement to say that the fire which, in particular, ravaged the town of Paradise, California, on 8 November 2018, was an extraordinary monster. In the wealthiest state of the world’s leading superpower – a state equipped with fire-fighting resources unrivalled anywhere on the planet – the fire spiralled out of control and raged for 17 days… becoming the deadliest wildfire in California in a century.
A spectacular exception? No. Scientists are unanimous: we must now prepare, on a global scale, for such ‘mega-fires’.
"In recent years, we have seen them time and again in California, Australia and the Mediterranean basin: in Spain, Portugal, Turkey and Greece… They are becoming increasingly common everywhere, even in the most sparsely populated boreal regions" , warns Emilio Chuvieco of the University of Alcalá (Spain), one of Europe’s leading experts in remote sensing of fires.
1 million lights a year
This is the average number of fires that burn each year across the globe (2003–2016). Or, to be more precise, the number of fires covering an area of more than 0.2 km², the threshold at which monitoring instruments detect them.
1,000 hectares burnt
This is the threshold above which fires are classified as ‘mega-fires’ in Europe. The term refers to the 2–3 per cent of the largest fires in a region. In the United States, fires covering 10,000 hectares or more are classified as ‘mega-fires’.
72 days
Duration of the longest recorded fire
In 2007, the largest fire on record raged in Australia for more than two months, consuming 40,026 km² – an area equivalent to that of Switzerland – and spreading at an average rate of 19 km per day.
EXTRAORDINARY FIRES
To gauge the increase in the number of these exceptional fires, scientists have drawn on satellite data, even though this data is still too recent and too coarse to provide answers to all the questions. "Thanks to the MODIS instrument, which measures energyAs for the polar-orbiting satellite, the data is fairly good up to 1999. However, it only passes over any given point twice a day. It is therefore necessary to cross-reference its data with that from geostationary satellites, even though the latter has a lower resolution.
There are also the burnt areas that can be seen in images from the Landsat satellite, launched in 1982. Extracting reliable data therefore requires complex algorithms and a great deal of cross-referencing of information" , warns Emilio Chuvieco. He nevertheless believes that the trend is clear: major fires are becoming more frequent.
The apparent paradox of this increase in the number of mega-fires is that, on a global scale, the area burnt has decreased. In a seminal article published in 2017, Niels Andela of NASA demonstrated that, over the course of some twenty years, the area affected by fires across the planet has shrunk by around 25 per cent: "However, this decline is most evident in semi-arid tropical regions, particularly in Africa, which accounts for 70 per cent of the world’s burnt areas. These relatively poor regions, which are traditionally sparsely populated and maintained each year "are becoming increasingly densely populated and cultivated as a result of slash-and-burn farming. Let us not forget that, in the space of 20 years, the Earth’s population has grown by 1.5 billion and agricultural production has increased by 40 per cent!" The result?
Savannahs and steppes are now occupied by permanent crops and villages, which explains the change in fire patterns.
But whilst these small, controlled fires are, as we have just seen, on the decline, we are, on the other hand, witnessing the proliferation and intensification of massive blazes which are completely out of control. "To tell the truth, there is as yet no agreed definition in the scientific community for the term 'mega-fire' ', explains Thomas Curt, Director of Research into Forest Fire Risk at the Institute for Research in Environmental and Agricultural Science and Technology. Sometimes, particularly in the United States, the term is used to refer to fires covering more than 10,000 hectares, but more often than not, it refers to the 2 or 3 per cent of the largest fires in a region. In Europe, for example, or in Mediterranean countries, a fire that destroys 1,000 hectares is already classified as a mega-fire."
And even though they account for a very small percentage of recorded fires, "Large-scale wildfires alone account for more than half of the area burnt and the emissions caused by wildfires" , says the researcher. He points out that a fire is a ‘non-linear’ phenomenon: once a certain threshold is exceeded, it changes in nature and exhibits new properties. "Mega-fires are, above all, ultra-dynamic phenomena that literally leap in all directions, giving them a speed of 2 to 5 km/h . To get away from them, walking isn’t enough any more… you have to run!" warns Thomas Curt.
Not to mention that they don’t confine themselves to the undergrowth of litter that has accumulated on the ground, but "reach the treetops and the tops of the trees, rising to spectacular heights, with flames 50 metres above the ground, whilst the trees are about fifteen metres tall" , the researcher explains.
RELATED TO EXTREME WEATHER
Finally, the energy released is in the region of 30 kWh per metre of the fire front, with temperatures reaching several hundred degrees, which prevent firefighters from ‘holding their ground’, even at a distance of 30 metres and whilst fully kitted out. In Paradise, their hoses were melting in places and their fire engines were falling apart under the intense heat! "This makes these fires unstoppable, and contributes to their extraordinary size and the damage they cause" , sums up Thomas Curt.
The fundamental question remains: how can we explain the global proliferation of these devastating fires? Isn’t the commonly cited explanation of global warming rather simplistic when it comes to such localised phenomena? "Clearly, there are many factors" , admits Niels Andela, who points out that a fire needs three elements to start: the right weather conditions, fuel and a source of ignition. Yet, in all three of these areas, humankind has gone overboard.
Consequently, with a human population that continues to grow and now prefers to settle on the outskirts of towns, as close to nature as possible, the number of fires is on the rise: cigarette ends, pieces of glass acting as magnifying glasses, engines, barbecues, power lines… are increasingly coming into contact with combustible material (dead leaves, undergrowth, dry trees). To name but one example, Paradise was literally built within a forest.
These are ultra-dynamic, unstoppable fires that spread at 5 km/h: to escape them, you have to run! - THOMAS CURT Institute for Research in Science and Technology for the Environment and Agriculture
© M.KONTENTE - DR
Secondly, fuel has also accumulated to an unprecedented level over the last century in regions prone to wildfires. This is partly due to rural depopulation, which has led to the disappearance of livestock herds and the cessation of traditional wood-gathering practices. But above all, in developed countries, this is due to a ‘zero-tolerance’ policy, coupled with the establishment of modern, well-equipped fire services capable of bringing almost all fires under control within hours of them starting. However, whilst these precautionary measures have succeeded in eliminating the small fires that naturally swept through these environments, they have, paradoxically, contributed to the accumulation of dead organic matter, decade after decade, in the majority of forests in the Northern Hemisphere. This has paved the way for massive fires should favourable weather conditions arise.
And that’s where the third ‘ingredient’ comes in: if global warming isn’t to blame for everything, "It is making an already bad situation significantly worse" , according to David Bowman (University of Tasmania). Not only does it raise average temperatures and (by concentrating rainfall into short periods) prolong dry spells, but it is also accompanied by an increase in extreme events, such as heatwaves and droughts, which are closely linked to the risk of wildfires.
Mega-fires are, in fact, extraordinary events in their own right: "A mega-fire is very similar to a hundred-year flood" , notes Thomas Curt. And their climate impact is particularly evident in uninhabited boreal regions, where fires are becoming more frequent regardless of any human activity!
Using a satellite database containing 23 million fires, David Bowman examined the weather conditions under which the 500 largest fires on the planet had broken out over the last 15 years.
The verdict is clear: 96 per cent of these incidents occurred during periods of exceptionally hot or dry weather, and there is a strong correlation with the ‘weather fire index’, a measure that takes into account temperature, drought, wind and other factors… The researchers also found that the number of days conducive to wildfires had increased globally by 18.7 per cent between 1979 and 2013.
The risk of extreme fires is increasing everywhere
NORTH AMERICA
The West Coast, particularly California, has been experiencing devastating wildfires for several years, which have had a particularly severe economic impact given the region’s wealth. The drought (now virtually chronic) bears a large part of the responsibility for this and is a hallmark of global warming.
SOUTH AMERICA
Weather conditions are becoming increasingly conducive to wildfires, particularly along Brazil’s Atlantic coast.
Concerns are particularly acute regarding the Amazon rainforest, on a continent where deforestation is a major problem. A destructive combination of factors.
EUROPE
Large-scale wildfires remain relatively rare for the time being (except in Spain and Portugal) in a densely populated and economically developed Europe. However, rising temperatures and longer dry spells could significantly increase the risk of wildfires.
Projected change in the number of days with the highest fire risk between now and 2041–2070 (as a percentage)
A DIFFERENT APPROACH TO FIRE MANAGEMENT
And the situation is set to get worse! By 2050, experts are forecasting a 35 per cent increase in the number of days conducive to wildfires – and the rise will be even greater in the Mediterranean, on Brazil’s Atlantic coast, in South Africa, in eastern Australia, the south-western United States and Mexico. That’s a staggering figure. "In France, we can see that the isotherms are moving northwards, Thomas Curt confirms, and at higher altitudes: the area at risk of fire is therefore increasing every year." And to warn: "What will happen if, for example, the Southern Alps become increasingly densely populated, at the same time as they become more flammable?"
However, the rise in mega-fires is not inevitable. Whilst the climate clearly plays in their favour, the other factors all offer opportunities for action.
Forest management strategies can therefore make a significant difference by avoiding the planting of conifers and eucalyptus trees, the establishment of human settlements in flammable areas (following the model used for flood-prone areas), the accumulation of excess biomass and the encroachment of scrub. As for fire management, "Studies suggest that allowing certain fires to burn, or even starting them – particularly in winter and spring, when the risk of them getting out of control is lowest – is a good strategy , as Emilio Chuvieco points out. "Even though, psychologically speaking, starting fires and burning land is difficult for us." One thing is certain, however: inaction is not an option.
Our overpopulated and overheated planet is entering a new era of fire: establishing rules to minimise its impact has become a new priority.