SLU's knowledge bank

Awaiting the next summer of fires

Last changed: 12 June 2023
Picture of a forest fire

SMHI, the Swedish Meteorological Institute, has forecasted that persistent high pressure systems may dominate the weather of Scandinavia for most of June, July and August (2023), and bring us a warm or very warm summer. Additionally, SMHI considers it possible that several thunderstorms originating in Central Europe may find their way to Scandinavia.

The coming summer looks to be fraught with long-lasting heat, very limited downpour, and plenty of lightning. These are all factors that affect the risk of forest fires, and if the forecast would come true, we might end up with a summer similar to that of 2018. That year, 7 000 fires destroyed well over 22 000 hectares (ha) of Swedish forests.

The conditions of fire

For a forest fire to occur, three prerequisites must be met. Firstly, there has to be fuel. One might think that fuel is always present in a forest, but it is not dead wood and logs that drive the initiation and development of a forest fire, but litter, moss and lichens. Deciduous and coniferous forests are widely different in this regard. The understory of a deciduous forest is difficult to ignite. It is dominated by grass and herbs rich in moisture, and the autumn leaves suffocate the mosses.

In particular the pine forests dry vegetation and needle litter will ignite far more easily. Spruce trees will burn well enough if ignited, for example when an already developed fire hits a stand, but dense spruce forests are usually humid, and the litter tend to be too compact to be ignited by sparks, embers or lightning. The pine forest will dry much faster and is a far more common starting point for a fire.

Fires in coniferous forests are more troublesome in general. All fires originate on the ground and spread by burning sub story vegetation, but if the conditions are right and the winds strong enough, the flames may reach the canopy. This results in a crown fire, which is a completely different beast altogether. The available fuel is instantly more than doubled, and the fire will start to spread far more quickly. During the fifth day of the famous fire in Västmanland 2014, strong winds propelled the fire forward at 80 meters per minute, or 2,7 meter per second. The winds may also throw burning debris and sparks far ahead of the fire front, allowing it to jump across barriers such as roads and water.

In extreme cases, the fire may also create its own weather system. This increases the winds even further, and may cause thunder and lightning that can cause ignitions several kilometers ahead of the fire. Winds in general is a very important driving factor regarding the spread of a fire. In head winds, the speed is rarely more than one meter per minute and the flames remain small.

Moisture and drought

The second condition is that the available fuel is dry enough – less than 25 % moisture, as a rule of thumb. Obviously, this becomes increasingly more likely with increasing heat and periods without rain, but in general, one week without precipitation is enough to make the mosses of a pine forest susceptible to ignition from a discarded cigarette or sparks from forest machinery. Therefore, extreme heat might not be the most important force determining risk of fire, although it certainly helps.

Weather systems that are maintained for a long time are even more important, such as when a high pressure system continuously prohibits any kind of downpour. This was especially true in the summer of 2018, during which pretty much perfect conditions for forest fires subsisted across large parts of the country for over two weeks. During a normal summer, any given location only sees such conditions for one or two days.


The third requirement is that this sufficiently dry and prevalent fuel somehow catches fire. In Sweden, this happens nine times out of ten due to the dealings of man (such as careless campers, playing children, ill-doers, and sparks from trains). The remaining tenth is caused by lightning. The more human activity that is going on in a forest, the more common is a human involvement in fires. Thus, there is a strong correlation between the number of fires and the population density in a given area. But there is a just as strong reversed correlation between population density and the size of fires.

The more people, the more likely that a fire will be spotted at an early stage, which means that first responders will be alerted more quickly, or even that the fire may be put out by those finding it. But more important still is that higher people density also means more fire stations. For example, Åsele and Lycksele communes put together contains about as much forested land as the province of Kalmar, but only 2 stations, compared to 35 in Kalmar. A fire starting in Åsele commune is far more likely to grow large before being spotted, and larger still before reached by the servicemen. Also, roads are fewer and the continuous forests larger in sparsely populated areas, which also affects the possibility to reach the fire quickly.

Past and present fires

Since these three prerequisites naturally occur all the time, forest fires are natural phenomena in all forests. Historically, around 1 % of the forested area of Sweden has burned every year, and any given location burned once every 50 to 100 years on average. Fires would rage until put out from natural causes, such as weather changes or the arrival of autumn, and could reach enormous sizes. Excavations and dendrochronology have revealed traces of a huge fire in Fredrika in 1514, at least 20 000 ha large but probably far larger still. There must be countless other, even greater fires that have not been found. The fire in Västmanland 2014, commonly called “the largest in modern times”, spanned 14 000 ha, and was far from remarkable from a historical standpoint. From a more modern one, however, it was a very rare event.

In other boreal areas, such as Canada, Russia and in Alaska, a large part of the forested area is natural forests, very distant, and hard to reach. That makes big fires fairly common: approximately 0.5 % of the forested area is in flames every year. But in Sweden and Finland, things are different. Here, almost 90 % of all forests are tended, and linked to enormous financial investments and promises of future benefits. In order to conduct this kind of forestry, it is necessary to put out wildfires as soon as the pop up.

On the other hand, our forestry and network of forest roads make extinguishing fires a lot easier, and partly explain why we have become highly efficient in limiting forest fires. Almost all damage from fires are caused by the small percentage that grows out of control. In Canada, it is said that 3 % of the fires cause 97 % of the losses, and that is probably true for Sweden as well. Before we started to put out fires, some 200 000 ha of Swedish forests burned every year. In the early 21st century, that number had shrunk to 5 000 ha, with no fires exceeding 2 000 ha – and that is in a bad year. During normal years, no single fire grow larger than 100 ha. This means that the part of the Swedish forest that burns every year, has decreased from 1 % to 0.01 %.

The summer of fire

Regarding bad years, the summer of 2018 is – so far – an outstanding exception in modern times. The emergency service in Jämtland described it as “something they would never have imagined in their wildest dreams”. Over the course of two weeks in mid-July, the region suffered 80-90 confirmed forest fires, and in the entire country, about 10 times more than during a mean year. Out of the 7 000 fires that started, only four reached sizes larger than 1 000 ha. These four, along with a couple more, represented ca 20 000 of the 22 000 ha that burned, and they all burned between the 12th and 18th of July. The drought had persisted for several weeks, and three days of thunderstorms (12th-14th) was followed by three days of extremely beneficial weather for fires (16th-18th).

All of these fires occurred in communes with a population density below one person per km2. This is relevant, because fire extinguishing is conducted at the commune level. Consider that Lycksele and Ånge has two fire stations compared to 35 in Kalmar. Sparsely populated and vast communes with a less well developed road network will struggle to keep up when many fires occur simultaneously, which increases the risk of the fires growing large. This problem will increase in the future, because the population in most northern communes in Sweden decreases.

The local fire departments are almost exclusively manned by part-time employed fire fighters, which find it increasingly difficult to keep other employment in the vicinity of the fire station. Emigration, aging population, long distances and large continuous forests all contribute to the increasing challenges.

The dry humus layer of the conifer forests played a crucial and devastating role during 2018. Burning humus is hard to put out completely, and if it is dry, the glowing embers may go deep into the ground. If they are not snuffed out completely, the fire may reignite. The fire fighters are only responsible for putting out the open fire – the crucial final extinguishing is left with the land owner. In 2018, this worked poorly, and fires that had been doused once roared again the next day. In the final tally, such reignitions counted for 14 700 of 22 000 burned ha, or 73 % of all destroyed forests. In four places, reignitions even occurred twice.

Room for improvement

If no reignitions would have occurred, 2018 would not have been much worse than a “normal” year of extremes. Several explanations to the outcome have been proposed. All fires that reignited were very large, and difficult to thoroughly extinguish with limited, already exhausted resources. The land owners argue that the problems arose already at the transition of responsibility between the fire department and the owners.

But it has also been said that the forest owners keep a thin field organization, involving a minimum of personnel. Those put to the task of killing embers have a very varied level of competence. Many are short-term employed or migrant workers, and communication problems due to language barriers are bound to arise. The extinguishing was probably rarely conducted by someone who had ever done such work before.

Having said that, a report over the 2018 fires state that the fire departments generally did well, with good coordination between communes and branches, and with a strong mobilization of foreign aid. However, it also pointed out that the two-year long fire fighter training spend too little time on forest fires – only two days. Among other shortcomings, this led to a lack of knowledge in how to make good use of the very large amount of civilian volunteers that wanted to help. Someone described the 7 000 volunteers as more of a “gathering of people” than a “gathering of force”, and only some 1 000 of them were utilized in a meaningful way.

The report also mentions that forestry personnel also need training in how forest fires are initiated and prohibited, to prevent forestry itself from being a risk factor. Several actions have since been taken to remedy such shortcomings. Early identification of fires is another key to more efficient control. This has so far primarily been done by airplanes, but starting 2023, satellites are being used to spot developing fires early and automatically warn first responders.

A database of forest fires

To better understand how fires arise and develop, and better identify risk factors, a database covering all forest fires in Sweden has been proposed on several occasions. Although a report is handed to the Swedish Contingencies Agency (MSB) for every extinguished fire, no such database currently exists. The gathered information is neither readily available nor synchronized, and lacks a lot of information that could be included.

Since the 2018 fires, the Swedish Forest Agency (Skogsstyrelsen) has mapped all fires larger than 0.5 ha. It is based on information from SOS and satellite images, but only results in a dot map over confirmed fires. It contains no information on when the fire occurred, or over how large area.

Using GPS technology, it would be entirely feasible to log information of the starting point and the outer perimeter of a given fire. Such information could be linked to data from the Forest Agency and provide information on what type of forest burned, and even estimate a cost of the losses. A report made by Anders Granström at SLU in 2023 suggests that such a database should be made, and that the information is based on the reports sent to MSB and complemented with GPS data on starting point and perimeter. The database can also be linked to data from SMHI regarding weather conditions and assessed relative risk of fire at the time in question.

Fires and climate change

Exactly in what way climate change will affect the occurrence of fires in the future, in Sweden or abroad, is not easily predicted. Prognoses do however suggest that there will be more days with better conditions for fires. The higher temperatures will lead to more drought in general and to a longer summer season. A drier climate will also affect vegetation and probably benefit more drought-resistant species. We will also suffer more thunderstorms; models predict that a one-degree increase in temperature will lead to a 12 % increase in lightning strikes. The risk of fire will keep increasing in Sweden, in particular in the south east. Aside from increased summer drought, that region steadily gets less snow and consequently worse conditions for the soil to refill its water supplies.

But most importantly, and most difficult to foresee, is how common long lasting weather system will turn out to be. When a high pressure system remains for many straight days, the risk increases of both ignition and a fast development into large fires. The risk of many simultaneous fires also increases, which erodes the resources of the fire departments.

Forest fires also contribute to climate change by their considerable output of atmospheric greenhouse gases. Fire in vegetation is assumed to generate 1/3 of the carbon dioxide of our total combustion of fossil fuels. In this regard, fires in boreal forests may be extra important since large deposits of carbon are stored in the litter and humus – although it might be argued that such a carbon source must have existed always since the forests have always burned.

Humus has a huge potential to store water. The drier the humus, the more of it will burn, and the more carbon will be released. The forest fire contribution to carbon dioxide production is thus tightly linked to how long drought precedes the fire, and if periods of drought will be more prevalent in the extreme weathers of the future, it is likely that so will the combustion of humus. Thus, there is a strong potential that climate change will increase the contribution from forest fires to carbon emission.

Having said that, forest fires in Nordic countries still are not an important source of carbon emission. Out of the total carbon that every year disappears from the forest, almost everything is due to harvests. The part played by fires is negligible. But that all comes down to the fact that the forests are managed and the fires put out. In Alaska, where very little forest is managed, the fires are responsible for 90 % of the losses of forest carbon. In Canada, 49%, and in Russia, 37 %.

We still tend to refer to such climate changes as something that awaits us in the future. But they are already upon us. In the boreal region, fires are already more common than they have been for 10 000 years, and extreme fire seasons have been more commonly occurring all over the globe. In both Portugal and Greece, the 2017-2018 fires were called “the largest in modern times”. In 2018, the notorious Camp Fire raged in California, almost five times the size of Västmanlandsbranden, and the 2019-2020 fires in Australia have been called “the worst fire season ever”. That season, 20 % of Australia’s forests were on fire, which is the highest percentage ever seen, anywhere.

A summer such as that in Australia has been modelled to possibly occur once every 400 years, if we do not consider greenhouse gases produced by man. However, if we do include those gases, the estimated frequency of such summers are once in eight years. Small wonder that the Australian scientist Tim Flannery has dubbed our days as “the era of the mega fire”.

Consequences for humans…

The fires in Australia caused significant human suffering. 34 people died in the direct fire, and over 400 have since died of long term effects from the smoke. “Camp fire” in California killed 85 people and leveled 18 000 buildings. Several communities were more or less eradicated. The damage have been estimated to exceed 16 billion dollars.

I Sweden, the impact on society has been very different. The Västmanland fire took one life, destroyed 20 or so buildings and forced 1 200 people to evacuate their homes. In the 2018 fires, a fire fighter was killed, and in total some hundred people evacuated. In relation to the size of the fires, this must be considered a fairly slight toll, however tragic for those affected. There may be several reasons for this. We are good at putting out fires, but our efforts are infinitely more effective at their early stages. Once a fire is fully developed and rages in favorable winds, few counter actions make any real difference aside from maybe fire airplanes, and even their role is limited compared to the part played by weather or geography. Just to mention one example; in order to contain the largest fires in 2018, many miles of firebreaks were cut open. Not once did a fire actually reach these barriers.

Perhaps the reason we rarely suffer physically as a result of our forest fires, is that we rarely suffer extreme, long lasting drought, at least in comparison to California, Greece and Australia. Also, fires close to people are observed quickly, and put out quickly. However, by no means does this mean we are out of harm’s way permanently. The 2014 fire did not occur in very thinly populated areas, and the towns of Fagersta and Norberg could both have been reached by the flames if the optimal winds and drought had lasted a couple of more days.

…and for forests

Although the spruce forest is usually not the starting point of a fire, its low branches and thin bark makes the spruce tree very susceptible to fire. Pines are more adapted to sustain fire – probably because fires in pine stands are that much more common. Old trees have thick bark and no low branches, and oftentimes survive less intense fires. However, if the fire develops into a crown fire, or otherwise is particularly difficult or long lasting, even the pines will die. Deciduous forests tend to suffer less damage, partly because the water-rich canopy does not catch fire like needles do. What actually grows in a forest will thus affect both the risk of a fire starting, its subsequent spread and development, and the ecological, cultural and economic costs of the destruction.

But for the actual development of a given situation, it is just as important, or even more so, in what fashion the fire enters the stand. A birch forest in an uphill slope, hit by a fire in strong tail winds during low humidity, will be more severely damaged than a spruce forest in a humid depression on a day of no winds.

To the land owner, a fire can mean ruin. To the forest as a biologic habitat, it is a natural process. The forest in Västmanland 2014 consisted mainly of pines, but is today an 8 000 ha large natural reserve: one of the largest natural forests in southern Sweden. Vegetation quickly returned to the site – a week after the fire the first mosses and grasses appeared, and birch shoots emerged from stumps. Plants needing fires to thrive, such as Geranium bohemicum, is rare in Sweden but thrived during the first summer after the fire. A large variety of insects, drawn to the dead trees, meant glory days for the rare three-toed woodpecker Picoides tridactylus. Bird life is differently affected when the dead trees are allowed to remain (good for robins and flycatchers) compared to when they are removed (beneficial for white wagtails and whinchats).

Since the area is no longer manually tended, early tree colonizers such as birch, aspen and sale are dominating, and it is likely that the reserve will develop into the largest deciduous forest of the country.

Forest fires in the future

The number of large fires in the future will depend on the balance between the efficiency of the fire fighters, and the number of high risk days and events of ignition. This will be driven by the depopulation of the countryside, the land owners’ ability to attract and keep trained personnel, climate change and, importantly, how we decide to tend our forests.

Our focus in our preparations should solely focus on one thing – how do we prevent large fires from occurring. When they are an established fact, our fire trucks, helicopters and even fire planes are similar to a butterfly spitting on a camp fire. The weather remains the only thing that can really stop such a fire.

Writer: Mårten Lind.