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

Climate change is a threat to global biodiversity, with changes to mean temperatures and increasing frequency and intensity of extreme weather events. Heatwaves in particular pose a threat to species’ persistence, as temperatures may rise above physiological tolerance. However, individuals rarely experience temperatures measured at the macroclimatic scale: topographic or vegetation differences result in microclimates that provide cool refugia (local temperatures below ambient) or even result in heat traps (local temperatures above ambient) during heatwaves. However, little is known about the stability of microclimates through a period of regional warming. In this study, we recorded microclimate temperatures across different microhabitats within a calcareous grassland nature reserve in Bedfordshire, UK, in 2018, 2019 and 2022. During this time, six heatwave events occurred, including the highest air temperatures ever recorded in the UK. We found that the ability of microhabitats to offset air temperatures varied with topographic aspect, slope, amount of bare ground, shelter, vegetation height, and vegetation type, with encroaching scrub and north-facing slopes showing the strongest abilities to maintain relatively stable microclimate temperatures with increasing air temperatures, in contrast to short vegetation on south-facing slopes which became heat traps. However, no combinations of environmental structures consistently maintained cool refugia during heatwaves. Microclimate temperatures were amplified close to the ground, whereas at 50 cm height temperatures were more stable and similar to the macroclimate temperature, therefore surface-dwelling species, such as many insects, may be particularly vulnerable to extreme heat. We identified a breakdown in the ability of microhabitats to maintain cool refugia above 7 °C, implying cool refugia become increasing rare and unpredictable with increasing temperatures. Our results indicate that many microhabitats will amplify the effects of climate change rather than mitigate them. Highlights The innate unpredictability and scarcity of extreme temperature events makes them an evolutionary challenge; instead, microclimatic refugia are often suggested as a way for species to cope under climate change. By comparing microclimate temperatures to macroclimate temperatures (from a nearby weather station) in central England, we highlight how variable local temperatures can be at fine scales relevant to small surface-dwelling organisms, with microclimate temperatures differing by as much as 20 °C. Microclimate performance changed with increasing temperature, with more heat traps (microclimate temperatures above ambient) and fewer cool refugia (microclimate temperatures below ambient). No environmental variables tested reliably maintained cool refugia at high temperatures. Ground-level microclimate temperatures were amplified compared to temperatures at 50 cm height, implying that surface-dwelling organisms in grasslands, including many insects, may be particularly vulnerable compared to species that fly, climb, or live in tall vegetation.