Evidence of climate warming in Iberian caves

Most people imagine caves as places where time stands still, shielded from the shifting weather above. It is a reasonable assumption; deep underground, the air feels constant, and the rock acts as a massive insulator, keeping the temperature steady regardless of whether it is a scorching summer day or a freezing winter night. For a long time, scientists treated these underground spaces as natural laboratories that simply recorded the average temperature of the surface over many years, acting as a calm, unchanging baseline. However, this view is shifting. As the planet warms, the question has become whether these deep, quiet sanctuaries are truly immune to the changes happening on the surface, or if the heat is slowly seeping down, altering the delicate balance of the subterranean world. This is particularly urgent because many creatures living in caves have evolved to survive in very narrow temperature ranges, making them incredibly sensitive to even small shifts in their environment.

A team of researchers in Portugal has now provided clear evidence that these underground worlds are indeed warming, but not in the simple, direct way one might expect. By placing sensitive temperature sensors deep inside two very different caves in the Iberian Peninsula, the scientists tracked the air temperature over several years and compared it to the weather on the surface. They found that both caves were getting warmer, but the speed of that warming and the way the caves reacted to the seasons were surprisingly different from each other and from the world above. In one cave, the temperature rose faster than the surface air did, while in the other, the surface warmed much more quickly than the cave. This discovery challenges the idea that caves are passive mirrors of the surface climate; instead, it shows that each cave has its own unique way of handling heat, influenced by its shape, how air moves through it, and how water flows through the rock.

The study focused on two specific locations in Portugal, each with its own character. The first is Algar do Pena, a massive underground chamber in the central part of the country, known as the largest subterranean room in the nation. It sits deep within a limestone mountain and has a single entrance. The second is Vale Telheiro, a smaller but biologically rich cave in the southern region, famous for hosting a unique community of cave-dwelling species, including a giant pseudoscorpion. To understand what was happening underground, the researchers installed digital recorders in the sediment deep inside both caves. These devices measured the temperature every two hours for several years, creating a detailed, continuous record of the cave's climate. They then compared these underground readings with data from the surface, using satellite-based weather records that tracked the air and ground temperatures right above the caves.

The results revealed a clear trend of warming in both locations, but the story of how that heat arrived was different for each. In Algar do Pena, the cave temperature rose by about 0.053 degrees Celsius per year. During the same period, the surface temperature above it was rising much faster, at a rate of nearly 0.2 degrees Celsius per year. This suggests that the deep, massive rock of Algar do Pena is acting as a buffer, slowing down the heat from the surface and dampening the intensity of the warming. The cave still followed the seasons, getting warmer in the autumn and cooler in the spring, but it did so with a delay of about two months compared to the surface.

In contrast, the Vale Telheiro cave told a different story. Here, the underground temperature was rising at a rate of 0.111 degrees Celsius per year, which was actually faster than the warming observed on the surface, where the increase was so small it was not statistically significant. Even more striking was the relationship between the seasons. In this cave, the temperature cycle was almost the opposite of the surface. When the air outside was at its hottest in the summer, the cave was cooling down, and when the surface was coldest in the winter, the cave was warming up. This inverse pattern, with a delay of about five months, indicates that the heat reaching this cave is not just a simple reflection of the current weather. Instead, it suggests a complex process where the cave is slowly absorbing and storing heat over a long period, perhaps through the movement of water or air currents, creating a warming trend that is independent of the immediate surface conditions.

These findings are significant because they show that there is no single rule for how caves respond to climate change. The way a cave warms depends on its specific geography, how deep it is, how air circulates through its tunnels, and how water interacts with the rock. The researchers noted that caves like these are often home to species that have evolved to live in very stable conditions, with bodies that cannot easily adjust to new temperatures. If the temperature inside a cave shifts even slightly, it could threaten the survival of these specialized creatures, potentially causing a chain reaction that disrupts the entire underground ecosystem. The fact that one cave warmed faster than the surface while another warmed slower highlights the complexity of the situation; it means that protecting these environments requires understanding the unique thermal personality of each cave, rather than assuming they all react the same way.

The study also looked at historical data from Vale Telheiro, comparing recent years with measurements taken over a decade ago. While the older data did not show a significant warming trend, the recent years showed a clear acceleration in temperature. This suggests that the changes happening underground might be catching up to the surface changes, or perhaps reflecting a longer-term buildup of heat that is only now becoming visible. The researchers emphasize that while the warming is real, the exact mechanisms driving these differences are still being unraveled. Factors such as the shape of the cave entrance, the presence of cold air traps that trap cool air in the winter, and the flow of groundwater all play a role in how heat is transferred from the surface to the deep underground.

Ultimately, this research extends the evidence of climate change beyond the surface and into the hidden world of the Iberian Peninsula's karst landscapes. It proves that caves are not immune to the warming planet, but they do not simply copy the surface changes either. Instead, they filter and transform the climate signal in their own unique ways. For the scientists, this means that long-term monitoring is essential to understand the future of these subterranean ecosystems. By continuing to watch the temperature in these deep, dark places, researchers can better anticipate how the delicate life within them will be affected, ensuring that these hidden biodiversity hotspots are not lost to a warming world. The work serves as a reminder that even in the most isolated and stable environments, the pulse of the changing climate is beginning to be felt.