Warmer, drier, more flammable: what climate change means for Sweden's forests

Robles, who studies extreme drought and return periods, points to a clear driver behind the pattern.

– Anthropogenic climate change is making extreme climate events like heatwaves, droughts, and wildfires more frequent, intense, and longer-lasting, she says.

A warmer atmosphere holds more water vapor, which can fuel heavier rainfall in some regions while drying out soils and vegetation in others.

– Somewhat paradoxically, the same warming that fuels heavier precipitation events in some places can also make other places drier, Robles explains.

Northern Sweden is warming fastest

Because of Arctic amplification, warming is strongest in northern Sweden. According to SMHI, the whole country will get warmer, with the biggest increases in the north and in winter. Precipitation is also expected to rise, but higher temperatures increase evaporation, so summer drought risk can still grow in parts of the country.

Sweden had a preview of this in 2018, when a persistent high-pressure system brought prolonged heat and drought, leading to the country's worst wildfire season in decades. Whether that event can be attributed to climate change depends on how it is measured. Studies looking at fire weather found only weak evidence, while studies looking at temperature alone found a clear climate signal.

Rare events becoming common

Robles' own research focuses on return periods, a concept used to describe how unusual an extreme event is, and how likely it is to happen again.

– A return period tells us how likely it is that we will experience an event of a certain magnitude, or something even more extreme, in any given year, she explains.

A "100-year drought" is often misunderstood, Robles notes. It does not mean such an event happens like clockwork once a century — it means there is a 1% chance of an event at least that severe occurring in any single year.

The concept becomes especially useful when comparing different climates. Researchers can calculate how likely an event would have been without human-caused emissions, and compare that to how likely it is today, or under future warming scenarios.

– If climate change increases the probability of an event occurring, its return period becomes shorter, she says. A drought once considered a 100-year event may become a 20-year event in a warmer climate, meaning something that used to be rare becomes far more common.

Forests under stress, fires still need a spark

Even without fire, drought slows tree growth and increases mortality, and weakens trees' natural defenses against pests. Research shows Norway spruce is particularly vulnerable to drought in southern Sweden, while Scots pine is more tolerant, though even tolerant species can suffer under repeated extreme drought.

Robles stresses that climate change does not start fires directly as an ignition source is still needed, and in Sweden most fires are linked to human activity. Research shows that areas with higher road and population density see more fires, likely due to more opportunities for accidental ignition.

– Reducing human-caused ignitions becomes especially important under a warming climate, because climate change is increasing the frequency of the conditions that allow fires to spread, she says.