Temperature variation in caves and its significance for subterranean ecosystems

Temperature variation in caves vs. on the surface

The temperature in the caves was always more stable than on the surface. We found this across climate zones, lithologies, altitudes and latitudes and cave morphologies. The annual average temperature in caves is highly correlated with the average surface temperature. This agrees with other comparative studies that measured, simultaneously, temperature in caves and at their surface in tropical caves⁸, in cave air temperature in Slovenia²², and in cave-floor temperature in the Czech Republic²³. This implies that temperature variations—such as those imposed by climate change—at surface will be reflected underground. Thermal stability in caves exercises strong selective pressure on all organisms thriving in the underground¹³, and the temperature increase due to climate change is known to threaten the ecological sustainability of subterranean ecosystems^5,20,21,24.

Annual temperature variation patterns in caves compared to surface

Cave temperatures are known to react “to long term temperature drifts with some delay”¹⁸, due to the inertia of the rock and fluids infiltration. Therefore, cave temperature is directly influenced by the temperature of the outer atmosphere²⁵, and the surface heat transmission through the Earth’s upper crust is mainly via conductivity^26,27. The signal delay has putatively been related to the depth of the cave zone¹³, i.e., in deep zones of caves forced ventilation seems to be the primary influence on temperature²⁷.

Measuring temperature at soil level in the deep zone of each cave, we found three main correlation patterns in the annual temperature variation of the caves compared to the surface: (1) caves with high positive correlation with the surface temperature and identical thermal signal to the surface, but smaller amplitude, (2) caves with low correlation with the surface temperature and slight thermal delay of the signal from the surface, and (3) caves with high negative correlation to the surface temperature and extreme delay from the surface. This indicates that the thermal regime of the caves is influenced by the surface, but also by the individual characteristics of each cave. Rock properties, where igneous rocks have higher thermal conductivity than sedimentary rocks^26,28, may lead to lower thermal delay in volcanic caves, which could explain the patterns found in Talofofo Cave (Guam) and Balcões Cave (Azores), but the opposite was found for Viento Cave (Tenerife), which was the most thermally stable of all caves measured. Cave morphology may affect the thermal regime as well, with deep parts of large caves usually being more stable^11,29, which was observed for Planinska and Viento caves; however, Vale Telheiro (Portugal) is a very shallow cave (< 20m depth) and was also very stable. Latitude and altitude, which controls temperature at the surface and consequently in depth²⁶. Moreover, air and water circulation contribute significantly to the regulation of cave temperatures^30,31,32,33.

Caves are semi-closed complex systems and must therefore be understood as dynamic environments where the interaction of factors acting in the past plays a role in controlling the actual cave temperature. Air circulation is particularly relevant for caves with large and multiple entrances where air can be rapidly renewed, as in Guam, the Azores and Honda de Güímar Cave in Tenerife. Also, the geometry of cave passages may be a predictor of air circulation and consequently of temperature^34,35. The surface air is denser in winter than in summer, resulting in cooler air entering the cave towards the lower points. The cold air pushes the warm air deeper into the most stable parts of the cave, forming an atmospheric “cul-de-sac”, where air renewal is more limited³⁶, which might explain the pattern observed in caves with negative correlation to surface temperature³⁷. These include some of the most biodiverse caves in the world^38,39. On the other hand, the permanently undercooled caves, with often permanent presence of ice due to the downward gravitational movement of cold air (a so-called ice traps) are also known for high subterranean biodiversity, especially from the Dinarides⁴.

At higher latitudes and altitudes, seasonal ice layers formed during the cold season act as a thermal buffer, resulting in a lower thermal amplitude inside caves during winter, due to the lack of percolating water during that season³⁰. This happened in Setergrotta Cave in northern Norway where we recorded a plateau in winter and early spring.

Other factors may be influencing the cave’s temperatures such as surface vegetation, by providing shadow¹³, humidity^7,40,41, and potentially the geothermal gradient where temperature increases with depth^36,42, although in karst regions this effect seems to be buffered by the advection of groundwater³¹.

Thermal cyclicity in caves

Despite the general thermal stability in caves compared to the surface, caves with temperatures highly correlated with the surface showed daily thermal cycles. We expected this cycle to occur in caves that are most influenced by their respective surface, such as those with multiple entrances, where airflow plays an important role in controlling the cave microclimate. This pattern has already been observed in tropical caves, but limited to the shallowest parts of the cave⁸. We found daily cycles in the deep zones of caves located in different climatic areas, suggesting that daily temperature cycles in deep parts of caves may be frequent.

Circadian rhythms are intimately related to environmental cues such as light, and regulate different processes in organisms^43,44. Previously, it was assumed that there were no daily variations in caves that could exert control over organisms⁴³. However, the observed daily thermal cycles could play an important role to mark the circadian rhythms in cave-adapted organisms. Cave-adapted biodiversity is controlled by ecological, climatological, temporal and geological conditions⁴⁵, but interestingly, we can observe that some of the most biodiverse caves (Planinska, Vale Telheiro and Cerâmica) have no daily cycles, suggesting that thermal stability could be a factor promoting high species richness below ground.

Our findings on thermal patterns and cyclicity in caves are particularly relevant for studying the impact of climate change in subterranean ecosystems and niche partitioning, but also for speleothems genesis⁴⁶, and paleoclimatic reconstructions⁴⁷, with potential implications on our capacity to interpret historical data from cave records. Further studies are needed to disentangle the role of the different drivers influencing cave microclimates.

We studied the variation at soil level in the deepest parts of different caves in many parts of the world, but thermal stratification may also occur in caves^{11,29,33,40,48}. Further studies should include the effect of thermal stratification within individual caves, because the variation in temperature across the cave zones and between the floor and the roof of a gallery may have a major impact on speleogenesis, the formation and maintenance of ice in caves and, consequently in the creation of distinct ecological niches³⁰.

Subterranean ecosystems harbors 95% of the world’s freshwater resources available for direct human consumption and the largest water reservoir for plants and agriculture^5,46. Consequently, it is pivotal to understand the factors that influence cave temperatures and how they may affect cave species and ecosystem services^{18,20,21,24,46}. The Intergovernmental Panel on Climate Change (IPCC) synthesis report from 2014 confirms that climate change has and will continue to impact ecosystems and geographical species’ distribution at surface, which will be reflected underground, by mean annual surface temperatures increase but also by the decrease of rainfall and extreme climatic events². While epigean/surface dwelling species may have the ability to disperse to other altitudes and latitudes, cave-adapted communities are isolated in caves and with none or very limited survival capacity at surface⁴⁹. This is even more evident in terrestrial communities that may be doomed without the ability to disperse⁵⁰. We observed that the average temperature in the deep zones of the caves reflects the average surface temperature for each cave, therefore, we expect the increase in surface temperature to be reflected in the underground. In addition to global climate change, other human activities are also known to increase temperature in subterranean ecosystems, such as proximity to cities^51,52,53. Moreover, in caves where the temperature is highly dependent on the surface, such as Balcões in the Azores and Talofofo in Guam, climate extremes may even be detected inside the cave. However, this work also shows how difficult it is to explain the factors that control cave climate. This is still very understudied and should be explored further to better understand the degree to which caves depend on the surface, and how vulnerable they are to anthropogenic threats such as climate change.