How Unprecedented UK Heatwaves Are Igniting a New Era of Extreme Wildfire Risk
- Rob Beeson
- Sep 2
- 7 min read
This blog post provides a detailed summary of the paper: Unprecedented UK heatwave harmonised drivers of fuel moisture creating extreme temperate wildfire risk.
Remember the sweltering summer of 2022? The UK experienced an unprecedented heatwave, with temperatures soaring above 40 °C for the first time. What many might not realize is that this isn't just about uncomfortable weather; it marked a critical turning point for wildfire risk across the country.
While we often associate devastating wildfires with hot, dry Mediterranean climates, our traditionally humid temperate landscapes, like those found in the UK, are now facing a surge in high-intensity blazes.
This isn't a fluke; it's a stark warning. Climate change is making these extreme heatwaves more frequent and severe, with predictions for an increase of up to 2 heatwave days every decade in the UK.
Recent research reveals a startling truth: during these record-breaking events, the natural defenses that typically protect our landscapes from severe fires break down.
In this post, we'll summarise these research findings and look at how these extreme heatwaves are fundamentally changing the drivers of fuel moisture, creating unprecedented wildfire risk in regions previously considered non-fire-prone.
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The Unseen Threat: Why UK Wildfires Are Different Now
For years, the UK, with its generally mild and moist climate, wasn't seen as a hotspot for wildfires, especially compared to the established fire-prone regions of the Mediterranean. However, the reality on the ground is rapidly shifting.
In the last two decades, Europe has seen a growing prevalence of extreme heatwaves, driven by high-pressure systems. The July 2022 heatwave, caused by a "heat dome," shattered temperature records across the UK and was ranked as the highest intensity July heatwave since records began in 1878.
This extreme weather created conditions ripe for severe fire weather, leading to dried fuels and a high risk of both ignitions and extreme fire behaviour. The 2022 heatwave coincided with an unprecedented number of wildfires across the UK, overwhelming emergency services and causing significant property damage.
This presents a major challenge for firefighters and land managers who lack the fundamental understanding of fuel moisture dynamics in these traditionally non-fire-prone, humid temperate regions, leaving them ill-equipped to handle the escalating threats.
Decoding Fuel: What Ignites a Blaze?
To understand how wildfires are changing, we first need to understand the 'fuel' - the vegetation and organic matter that burns. The study focused on three critical fuel constituents found in UK heathlands and peatlands, which are often dominated by the dwarf shrub Calluna vulgaris (heather) and account for a substantially higher number of fire detections than forests or arable land.
Normally, each type of fuel has different factors influencing its moisture content, which traditionally helped prevent widespread severe fires.
Live Calluna: The Green That Usually Doesn't Burn Wildly
What it is: The living heather plants, which provide the primary fuel for fire spread in these ecosystems.
Normal Behaviour: The moisture content of live Calluna is usually complex, influenced strongly by phenological variables (seasonal shifts like plant growth and green-up) and landscape factors. Its moisture content is typically lowest in spring before the summer "green-up". As a result, large wildfires are more common in spring than in the warmer, drier summer, when live Calluna is generally moist enough to limit fire spread.
Dead Calluna: The Spark of Danger
What it is: This includes dead branches on living plants or dead plants that haven't yet decomposed.
Normal Behaviour: The moisture content of dead Calluna is primarily controlled by short-term weather variables like rainfall and Vapour Pressure Deficit (VPD). It's crucial because it directly affects the probability of fire ignition.
Organic Ground Fuels: The Hidden Inferno Beneath Our Feet
What it is: Carbon-rich materials like moss, litter, and organic soils (including peat) found beneath the surface vegetation.
Normal Behaviour: When the moisture content of these fuels is low, high-severity fires can trigger dangerous smouldering groundfires, leading to significant carbon emissions. Landscape factors, such as soil type and elevation, play a major role in regulating their moisture content by influencing water retention and drainage. Typically, peaty soils at higher elevations, which hold the largest carbon stocks, retain higher moisture, limiting smouldering combustion and showing low sensitivity to short-term weather.
The Game Changer: UK's Unprecedented 2022 Heatwave
The July 2022 heatwave was a wake-up call as the UK experienced a "heat dome" event where high pressure trapped warm air, pushing temperatures past 40 °C for the first time. The Met Office issued a red warning for extreme heat, and this event broke temperature records across the country.
The researchers’ intensive field sampling campaign, conducted across the UK during the most extreme three days of the heatwave (July 17th-19th, 2022), provided crucial insights into how fuel moisture responded to such unprecedented conditions.
When Everything Aligns: The "Harmonization" of Dryness
The research uncovered a critical finding: during the record 2022 heatwave, the controls on fuel moisture across all three fuel constituents harmonized, meaning they were driven by weather alone. This led to synchronised extreme dryness, far outside normal seasonal patterns, at the same time and place.
Let's break down what this unprecedented alignment meant for each fuel type:
Live Calluna: Summer Green-Up Fails
Normally, the high moisture content of live Calluna in summer acts as a natural fire break. But during the 2022 heatwave, this protective mechanism was severely disrupted.
Unexpected Drying: The phenological cycle for live Calluna was strongly disrupted by the extreme weather. In regions like East Anglia and the South East, live Calluna moisture content was substantially lower (averaging 84% and 79% respectively) compared to 128% in July 2021.
Spring-like Dryness in Summer: These summer moisture levels were comparable to the spring "pre-green-up" conditions of 74% in 2021 and 69% in 2022. This unexpected drying in summer, particularly in lowland heaths, allowed fires to spread more easily than usual.
Regional Impact: The disruption was most severe in East Anglia and the South East, areas that experienced the highest temperatures (e.g., 33 °C–43 °C in East Anglia) and lowest humidity readings (16–26% in East Anglia) during the heatwave. Given that the South East is predicted to see the greatest increase in heatwave events, this finding has significant implications for future risk.
Dead Calluna: A Tinderbox Ready to Ignite
The dead Calluna canopy became exceptionally dry, creating ideal conditions for ignition.
Extreme Dryness: Dead Calluna moisture during the heatwave was significantly below anything measured in previous spring or July periods. It averaged a critically low 4.2%, ranging from 3.2% to 6.0%, a stark contrast to the 17.7% average in July 2021.
High Ignition Probability: This extremely low moisture content created a highly ignitable fuel load. Our simulations showed that the probability of ignition rose to a staggering 87% during the heatwave, compared to less than 11% during typical spring or non-heatwave July conditions. This highlights how extreme weather overrides normal controls, making ignition highly likely.
Organic Ground Fuels: Smouldering Threat Unleashed
Even the deep organic layers, typically resistant to rapid drying due to landscape factors like soil type and elevation, succumbed to the extreme heat.
Lowest on Record: The organic moisture content was the lowest observed over our three-year measurement period in every region.
Significant Decline: In East Anglia and the South East, organic layer moisture declined by up to two-thirds of their July average, dropping to just 12% and 38% respectively. Even in more northern regions with carbon-rich peat deposits, organic moisture declined to 85%.
Smouldering Risk: These low moisture levels are extremely concerning. Organic rootmat soils have a 61% chance of sustained smouldering below 93% moisture, while smouldering fires can spread horizontally in peats below 150% moisture content. A 50% probability of ignition has been estimated for peat moisture contents between 110% and 125%. This implies a considerably increased ignition risk for regionally important carbon stocks in organic layers across all study sites during the heatwave, with significant potential for smouldering wildfires and higher carbon emissions.
The Dire Consequences: What This Means for Wildfire Behavior
The harmonization of dryness across all fuel layers has dramatic implications for how fires behave in these typically resistant landscapes.
Explosive Fire Spread: Our simulations demonstrated that heatwave fuel conditions produced a surface fire rate of spread more than double that of spring conditions and over four times faster than typical July fuel conditions.
Uncontrollable Flames: Flame length was also approximately doubled compared to spring and regular July conditions. Critically, under heatwave conditions, the flame length exceeded 1.5 meters. This is a crucial threshold, as flames over 1.5 meters high are the limit above which direct firefighting attack using hand tools is not permitted. This means fires can quickly become too dangerous for conventional ground-based efforts.
Ecosystem Adaptation Challenged: The extreme fire behaviour and risk of smouldering combustion challenge the existing fire adaptations of plant communities like Calluna. While Calluna can typically regenerate after light to moderate fires, severe fires, such as those caused by heatwave conditions, can destroy extensive seed banks required for its sexual reproduction, potentially leading to long-term ecosystem changes.
Looking Ahead: Preparing for the Future
The findings paint a clear picture: future intense summer heatwaves can be expected to align the most severe conditions for fire ignition, spread, and impact in traditionally non-fire-prone regions like the UK. This transforms humid temperate landscapes into areas highly susceptible to extreme wildfire events.
We've already witnessed the devastating impact of such events, with the 2022 heatwave leading to unprecedented wildfires, property evacuations, and damage. These events are predicted to become more frequent and severe in the future due to climate change.
This emerging threat demands urgent and strategic action:
Adaptive Landscape Management: We need effective fuel management strategies including the use of controlled burning to build resilient communities, maintain biodiversity, and reduce carbon loss. This includes understanding how different management techniques can alter fuel loads and their response to extreme drying.
Enhanced Firefighting Capabilities: With flame lengths and rates of spread potentially exceeding traditional direct attack strategies, firefighting services will need to consider indirect attack through control lines or advanced aerial direct attack options. This requires significant investment in resources and training for firefighters and land managers.
Proactive Preparedness: It is essential to develop fire preparedness strategies that evolve alongside these changing fire regimes, ensuring the allocation of sufficient resources for fire services and the land management community during such high-risk events.
The "harmonisation" of fuel moisture controls under extreme heatwaves is a critical new understanding for fire management in humid temperate regions. It highlights how climate change is not just increasing temperatures, but fundamentally altering the very mechanics of fire risk.
By understanding these shifts, we can better prepare and protect our unique landscapes, communities, and vital carbon stores from the escalating threat of extreme wildfires. The time to acknowledge this new reality and invest in robust, adaptive strategies is now. Let's work together to build a more fire-resilient future for the UK.
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