Although the temperatures outside climb into the high sixties, the ice caves found just a few feet below ground remain frozen solid.
Ice caves, found across Europe, Russia, Central Asia and North America, are dazzling natural wonders of nature that are often found hidden underneath glaciers. However, some are found under unfrozen mountain ranges where temperatures at the surface can reach into the high sixties. But how can the caves remain frozen if the weather outside is so warm?
That is a question scientists have been trying to answer for 150 years.
One explanation involves a local reversal of geothermal heat — warming from Earth’s mantle. This “cold source” theory suggests that the warming currents in the mantle might miss a region of the ground, leading to icy deposits if a cave in that area is the right shape.
However, scientists from the Chinese Academy of Geological Sciences (CAGS) studying the Ningwu cave, located in the Shanxi Province in China, said that this cold source theory doesn’t stand up to scrutiny.
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For example, even though geothermal heat does vary in places, these differences often influence surface temperatures too. Summer temperatures above the cave can reach 63 degrees Fahrenheit (17 degrees Celsius) while the caves below remain frosty.
It turns out, staying cool may be a combination of the cave’s particular shape and position, the seasonal flows of air through the caves, and the heat exchange with the rock walls that creates this micro-environment below the surface.
Shape and Position
Ningwu is an 279-foot deep, bowling-pin-shaped cave found in the side of a mountain more than 6,500 feet above sea level. It acts as a cold air trap, where a little cool air sinks down the neck of the cave in the spring, summer and fall. This air is warmed very slightly by geothermal heat from the limestone cave walls, but it is such an inefficient process that any warming takes a very long time.
Seasonal Air Flow
Some ice caves have more than one entrance, which affects the seasonal air flow and the extent the ice melts and re-grows each year. If the entrances also open at different heights, it encourages even more flow of cold air through the cave.
Heat Exchange
Once ice has formed in the cave, it acts like a buffer to stabilize the cave’s temperature. If warm air passes through, some of the ice starts to melt. However, melting ice takes a lot of energy, and the meltwater absorbs a lot of the incoming heat, preventing the rest of the cave from warming up.
The reverse is also true — when cold air gets in, liquid water in the cave will freeze, releasing energy and stopping the cave from being as cold as outside. The caves tend to keep quite a constant temperature throughout the year.
Unfortunately, ice caves are very sensitive to changes in surrounding melting conditions, and CAGS member Yaolin Shi’s model of the ice caves suggests that some caves may already be threatened.
Many ice caves are starting to be lost to climate change. “Here in New Mexico where I live, there has been significant ice loss over the past decade at the Bandera ice cave, which is a volcanic lavatube cave with permanent ice,” said Penny Boston to BBC, a speleologist at the New Mexico Institute of Mining and Technology in Albuquerque.
This has led to efforts to try to preserve the caves. However, “[m]anagement of an ice cave has to be based on sound science,” said Boston. “Sometimes in an attempt to preserve the ice, people have inadvertently harmed the ice masses.”
Ice caves provide both important biological and climate data, and Shi thinks that the caves may also be able to lead to the development of a new form of air conditioning for buildings — basements that act as cold air traps, just like the Ningwu cave.
Ice caves are beautiful — definitely something we want to keep around!
