Microclimates can amplify and mitigate extreme heat as climate warms

When the United Kingdom recorded its first-ever 40 °C day in July 2022, weather stations told only part of the story. On the ground, in the grasslands and scrub of a Bedfordshire nature reserve, temperatures told a far more alarming tale. A new study published in Frontiers of Biogeography reveals that the microclimates small organisms actually experience can differ from official air temperatures by as much as 20 °C, and that these local conditions can either shield species from extreme heat or push them perilously close to their physiological limits. The findings challenge a widely held assumption that microclimatic refugia will reliably buffer biodiversity against climate change.

The research, led by scientists studying Totternhoe Quarry, an 8.9-hectare Site of Special Scientific Interest managed by the Wildlife Trust for Bedfordshire, Cambridgeshire and Northamptonshire, deployed a network of 131 temperature data loggers across the reserve’s remarkably heterogeneous landscape. Medieval quarrying left behind steep slopes of every aspect, patches of bare chalk, grasslands of varying heights, and encroaching scrub, all within a small area. By recording temperatures at 1 cm, 5 cm and 50 cm above the soil from May 2018 to August 2019 and again from May to August 2022, the team captured six heatwave events, including the summer that produced the highest air temperatures ever recorded in the UK.

The scale of local thermal variation was striking. Across more than 629,000 hourly recordings, microclimate temperatures ranged from −16.0 °C to 63.0 °C, while the nearby Met Office weather station at Woburn, roughly 16 kilometers away, recorded macroclimate temperatures between −7.1 °C and 39.3 °C. During heatwaves, microclimate temperatures spanned from 17.2 °C below ambient to a staggering 36.0 °C above ambient. Perhaps most tellingly, more than 3,000 recordings exceeded 40 °C, higher than the maximum air temperature the weather station ever logged, and over half of those extreme readings occurred outside officially defined heatwave events. Species in these habitats, the study suggests, may be encountering lethal temperatures far more frequently than heatwave statistics imply.

Using random forest classification models trained on 500 decision trees, the researchers ranked environmental variables by their ability to predict whether a microhabitat would become a heat trap, with temperatures above ambient, or maintain cool refugia, with temperatures below ambient. Macroclimate temperature itself proved the most influential variable, followed by vegetation type, topographic aspect, and degree of shelter, with slope steepness ranking last. When the 2022 data were analysed separately with two additional variables, vegetation height emerged as the strongest environmental predictor while percentage of bare ground ranked lowest. Every environmental variable tested improved the model’s predictive accuracy, confirming that landscape structure genuinely shapes the temperatures organisms experience at fine scales.

The strongest buffering capacity belonged to encroaching scrub on north-facing slopes. Ranked combinations of environmental structures showed that north-facing slopes with scrub produced a coefficient of just 0.017, meaning microhabitat temperatures rose by only 0.017 °C for every 1 °C increase in air temperature, keeping local conditions remarkably close to ambient. At the opposite end of the spectrum, south-facing slopes with short grass showed the strongest heat amplification, with a coefficient of 0.328. Intermediate slopes, low percentages of bare ground, and tall vegetation also increased the probability of maintaining cool refugia. Notably, the degree of shelter did not alter the scrub effect, implying that even low-density scrub can provide sufficient shade and shelter to buffer high temperatures, a finding with significant implications for grassland management.

Height above the ground emerged as a critical axis of thermal experience. A linear mixed effects model revealed a significant change in microclimate offsetting with distance from the soil (χ² = 11.082, p < 0.001). At 50 cm above the ground, temperatures closely tracked the macroclimate recorded by the weather station, whose thermometer sits at 1.25 m, and remained relatively stable even as air temperatures climbed. Near the ground, at 1 cm and 5 cm, temperatures deviated increasingly from ambient conditions and became progressively warmer as air temperatures rose. Several processes likely drive this ground-level amplification: surfaces absorb solar radiation and heat the air in contact with them by conduction, reflected radiation warms near-ground air pockets, and complex topography and vegetation reduce air mixing, trapping air that varies more in temperature than the well-mixed air higher up.

The segmented analysis uncovered a thermal breakpoint with profound consequences. At a macroclimate temperature of 7.24 °C ± 0.07, confirmed by the Davies test (p < 0.001), the relationship between air temperature and microclimate offsetting changed direction. Below this threshold, microclimates became marginally cooler relative to the macroclimate as air temperatures increased, possibly because cold air sinks and lingers in complex topography and vegetation, creating a lag behind the exposed weather station. Above 7 °C, microclimate temperatures became warmer and more variable relative to ambient conditions as air temperatures climbed. This breakdown implies that cool refugia become increasingly rare and unpredictable precisely when species need them most, during hot weather. The authors caution that no combination of environmental structures they measured could consistently maintain temperatures below ambient during heatwaves.

For the reserve’s celebrated butterfly fauna, including the regionally rare Duke of Burgundy (Hamearis lucina) and the chalkhill blue (Polyommatus coridon), these findings carry weight. Small ectothermic organisms are acutely sensitive to temperature, which alters metabolism, oxygen demand, immune functioning, and hormone production, effects that cascade to behaviour, phenology, distribution, and survival. Species living close to the ground, such as caterpillars and many ground-dwelling insects, face the most amplified temperatures, while flying adults or insects that climb into tall vegetation can escape to cooler strata. The study suggests patches of tall herbaceous vegetation, maintained through grazer exclusion or reduced mowing, could provide vertical escape routes, though what would benefit strictly surface-dwelling organisms remains unclear.

The evolutionary dimension adds urgency. Extreme temperature events impose truncation selection, where only individuals with thermal tolerance above a threshold survive and reproduce, a form of strong selection that can raise extinction risk. Yet the innate unpredictability and scarcity of such events erodes adaptation, particularly for short-lived species, returning populations to phenotypes suited to averages rather than extremes. Microclimatic refugia may therefore be critical for persistence irrespective of adaptation. The study’s authors note that species may have been experiencing extreme near-ground temperatures for longer than previously suspected, potentially allowing more time to adapt, but that increasingly rare and unpredictable refugia may force species assemblages to reorganise according to species’ capacity to locate and compete for suitable microclimates.

The practical recommendations are actionable. Because topography is difficult to manipulate, landscapes containing north-facing slopes should be highly valued for future conservation, and the authors point to artificial topographies already being constructed by the RSPB, Butterfly Conservation, and the Wildlife Trust as emerging mitigation methods on flat land. Vegetation is more easily managed: retaining some scrub, allowing grass to grow long, protecting areas from grazers, or pausing mowing during the hottest months could all support refugia. Yet the study delivers a sobering caveat. Macroclimate temperature had a substantially weaker impact on microhabitat offsetting than any environmental variable, meaning there is only so much local structure can achieve. As ambient temperatures rise, many microhabitats will amplify rather than mitigate the effects of climate change, and weather stations will increasingly underestimate the temperatures species actually endure. Understanding when and where microclimates provide genuine refugia, and when they become heat traps, will be essential for predicting biodiversity’s fate under a warming climate.

Subject of Research: Local microclimates can both amplify and mitigate extreme temperatures associated with climate change

Article Title: Local microclimates can both amplify and mitigate extreme temperatures associated with climate change

Article References: Ashe-Jepson, E., Turner, E. C., & Bladon, A. J. (2025). Local microclimates can both amplify and mitigate extreme temperatures associated with climate change. Frontiers of Biogeography, 18, Article e164843. https://doi.org/10.21425/fob.18.164843

Image Credits: AI Generated

DOI: 10.21425/fob.18.164843

Keywords: microclimate, climate change, heatwaves, biodiversity, ecology, insects, conservation, Frontiers of Biogeography, Local, microclimates, both, amplify