In an attempt understand climate trends from thousands of years in the past, researchers have turned from looking up into the sky to looking deep into the earth.
Researchers from Vanderbilt University, Cambridge University California's Berkeley Geochronology Center and other institutions are pioneering the examination of mineral deposits in caves – known as speleothems – to uncover a record of prehistoric climate.
A significant amount of data on cycles of heat and cold, and on rainfall and drought in the atmosphere can be found in the deposits created by the steady drip of water from the surface into subterranean caverns. Water seeping through the ground absorbs minerals – chiefly calcium carbonate – then deposits those minerals in layers on cave floors.
Those layers are laid down thickly during periods of wet climate above ground, and in thinner dusty skins during periods of drought, the researchers explained.
Those differences in the thickness of mineral layers yield a surprisingly precise measure on the levels of rainfall occurring over the cave in past centuries, the researchers reported in the journal Geophysical Research Letters.
To test the technique, a research team led by Vanderbilt's Jessica Oster, an assistant professor of earth and environmental sciences, analyzed the past 50 years of development in a stalagmite at the Mawmluh Cave in the northeastern Indian state of Meghalaya. The region is considered one of the rainiest places on Earth.
Their analysis of the layers in the stalagmite predicted precipitation levels that closely matched historical records, the team reported.
Those records showed – and the analysis confirmed – that reduced monsoon precipitation in the central part of India occurred when ocean-surface temperatures in parts of the Pacific Ocean were warmer than usual, in phenomena known as El Niño Modoki events, which occur in the central Pacific.
"Now that we have shown that the Mawmluh cave record agrees with the instrumental record for the last 50 years, we hope to use it to investigate relationships between the Indian monsoon and El Niño during prehistoric times such as the Holocene," said Oster.
The Holocene Climate Optimum, a period of global climate warming between six and nine thousand years ago, is of great interest to climate scientists because it matches the range of warming climatologists are predicting for our future, due to the buildup of greenhouse gases in the atmosphere from the burning of fossil fuels.
Global average temperatures during the Holocene era were between 4 to 6 degrees Celsius (or 7 to 10 degrees Fahrenheit) higher than they are today.
Understanding the behavior of the Indian monsoon during that time could yield clues to how it may behave in the future — a matter of vital importance to the 600 million people on the Indian subcontinent who depend on the monsoon season to bring 75 percent of the country's rainfall, the researchers added.
