The accuracy of stalagmites and stalactites as measures of past climates has been tested for the first time. A study of caves worldwide shows this is an accurate measure of past temperatures, but only under certain conditions. At least now we know what those conditions are, revealing when and where this can be an important tool to understand our climatic past, and predict the future.
Global temperatures are rising at a rate not seen for tens of millions of years. Nevertheless, if we are to have any hope of anticipating, and preparing for, the climate crisis we need to understand what happened in the most similar eras we can find. However, climate scientists have been frustrated by the shortage of proxies that reveal local conditions more than a few thousand years ago.
Potentially the most valuable proxies are limestone stalagmites and stalactites, collectively known as speleothems, which incorporate the oxygen isotopes of the water from which they formed. The reliability of these as a measure has been unclear, so Professor Andy Baker of the University of New South Wales set out to test them.
The oxygen isotopes in rainwater can provide a good indication of rainfall amount and moisture source. Paleoclimatologists would love it if speleothems simply incorporated the average year-round oxygen-18 concentrations into their stone, preserving records that stretch back hundreds of thousands of years. But do they? If most rain evaporates before it can trickle down to make underground limestone wonderlands then the portion that does make it through may be unrepresentative.
This is the question Baker and co-authors have answered in Nature Communications. They collected dripping water at 163 sites in 39 caves around the world, and found that in places where the average temperature (day and night) is below 10ºC (50ºF), oxygen-18 samples provide a good indication of the year-round temperature. Today this includes most of the world above 35 degrees latitude or from high altitude.
On the other hand, in warmer conditions, most dry season rainfall evaporates before it can reach the caves. Oxygen isotopes can still be revealing but in a different way.
“In regions like mainland Australia, with extreme weather events like drought and flooding rains, it’s a tool to see how often both occurred in the past,” Baker said in a statement. He told IFLScience allowances will need to be made by anyone trying to calculate annual averages, however, particularly if they are studying an area whose temperature has been above and below the crucial 10-degree mark depending on whether the world was in an ice age or not.