France is on Fire and I am not talking about Mbappé

France is on fire

Not the Mbappé at the World Cup kind. The 40°C kind.

The kind where Paris hits 40°C and your apartment becomes a slow cooker. The kind where hospitals are overwhelmed, trains are suspended, and a three-day music festival gets cancelled because it is simply too dangerous to be outside. The kind where you scroll through the news and every headline is a new country breaking a record it never wanted to break.

France. Spain. Germany. United Kingdom. Italy. Belgium. All of them. Same week. Temperatures running 5 to 12°C above seasonal averages. Tropical nights offering no respite. A continent that has warmed faster than any other on Earth, now confronting the consequences in real time.

The explanations are familiar. A persistent high-pressure system. Decades of fossil fuel emissions. A climate system pushed beyond its historical range. The science is clear, the trajectory well documented for decades, and the urgency beyond dispute.

But there is another dimension to this crisis. One that receives considerably less attention. One that has been deteriorating quietly, invisibly, beneath our cities for decades. This is a story about soil.

The ground has lost its ability to cool us

Before this heatwave arrived, something important had already happened underground.

Following months of record dry conditions, the topsoil across Europe had been stripped of moisture, greatly reducing the soil’s natural ability to cool itself and turning major urban centres into concrete ovens.

That fact may seem peripheral to a discussion about extreme heat. It is not.

Soil in good condition is one of nature’s most effective, and most consistently overlooked, thermal regulation mechanisms. It cools itself through evaporation, releasing water into the atmosphere in a process that absorbs heat rather than radiating it back. It buffers temperature extremes. And critically, it supplies the water that vegetation needs to perform the same function above ground.

When topsoil loses moisture at scale, cities lose that buffer. The ground shifts from absorbing heat to amplifying it. Urban surfaces, already compromised by concrete and asphalt, become more extreme. What was already a dangerous heatwave becomes something harder to manage.

This is not a secondary effect. It is a foundational one.

Urban trees cannot do their job without the soil in good condition

Across Europe, cities have invested significantly in urban greening: more trees, expanded green corridors, nature-based cooling strategies. The logic is sound: trees provide shade and, through transpiration, release water vapour that actively cools the surrounding air.

But that cooling effect is conditional.

Transpiration requires water. That water comes from the soil. When soil moisture is depleted, through drought, compaction, or surface sealing, trees close their stomata, restrict water uptake, and their cooling capacity diminishes substantially. The tree remains. It remains green. But its function is compromised. Soil moisture availability is a primary driver of urban tree cooling performance. In dense built environments without sufficient soil moisture, even significant tree planting yields limited thermal relief on extreme heat days.

We are planting trees in depleted soil and wondering why our cities are still burning.

Soil security is not just a rural problem

And here is where the conversation needs to go further.

When most people hear “soil security” they think of farmland. Rural landscapes. Fields and food production. That association is understandable and it is also part of the problem.

Because soil does not disappear in cities. It lies beneath our streets, our buildings, our parks, our green corridors. With more than half of the global population now living in cities, this matters enormously. Urban soil is the hidden infrastructure beneath every park, street tree, and green corridor. It stores water, regulates temperature, filters stormwater, supports biodiversity, and provides the foundation for the cooling services cities increasingly depend upon.

Yet it is also among the most threatened soil on Earth. Contamination from industry, traffic, and urban waste leaves behind heavy metals and pollutants. Sealing with asphalt and concrete stops soil function entirely. Compaction from decades of construction restricts the movement of air, water, and roots. In most European cities, soil has been treated as a surface to build on, not a living system to protect.

The results of that neglect are not hypothetical. They are visible right now, on a 40°C day in Paris, in the shade of a tree that can no longer cool the street beneath it.

We cannot manage what we do not measure. And we cannot secure what we do not value.

Urban soil security is not a niche concern for soil scientists. It is a heat resilience issue. A public health issue. And this heatwave is making that argument more powerfully than any policy paper could.

We paved over the very thing that could protect us

The drought compounds a problem that predates it. Across European cities, soil sealing rates range from 31.5% to 72.6% of urban surface area, meaning in the most affected cities, nearly three quarters of the ground beneath our feet has been rendered largely incapable of performing many of its natural functions. Roads, pavements, car parks, and buildings restrict water infiltration, reduce rooting volume, and limit the biological activity that keeps soil functional. Trees planted in this environment are under physiological stress from the outset, constrained in their root development, limited in water access, and reduced in their capacity to provide the ecosystem services cities are counting on.

Research published in Urban Ecosystems found that reducing soil sealing by just 20% could enhance tree biomass by 21.5% and canopy cover by 25%, with a measurable increase in transpiration-based cooling.

Twenty percent. That is a policy-relevant threshold. It is also one that most European cities are nowhere near meeting.