Stalagmites

Over the past century, the Earth’s average temperature has swiftly increased by about 1 degree Celsius (1.8 degrees Fahrenheit). The evidence is hard to dispute. It comes from thermometers and other sensors around the world. But what about the thousands of years before the Industrial Revolution, before thermometers, and before humans warmed the climate by releasing heat-trapping carbon dioxide from fossil fuels? Back then, was Earth’s temperature warming or cooling? Even though scientists know more about the most recent 6,000 years than any other multimillennial interval, studies on this long-term global temperature trend have come to contrasting conclusions. To try to resolve the difference, we conducted a comprehensive, global-scale assessment of the existing evidence, including both natural archives, like tree rings and seafloor sediments, and climate models. Our results, published Feb. 15, 2023, suggest ways to improve climate forecasting to avoid missing some...

In a remote cave in northeast India, rainwater has slowly dripped from the ceiling in the same spots for over 1,000 years. With each drop, minerals in the water accumulate on the floor below, slowly growing into calcium carbonate towers known as stalagmites. These stalagmites are more than geological wonders – like tree rings, their layers record the region’s rainfall history. They also carry a warning about the potential for catastrophic multiyear droughts in the future. By analyzing the geochemistry of these stalagmites in a new study published Sept. 19, 2022, in the Proceedings of the National Academy of Sciences, we were able to create the most precise chronology yet of the summer Indian monsoon over the past millennium. It documents how the Indian subcontinent frequently experienced long, severe droughts unlike any observed in the last 150 years of reliable monsoon rainfall measurements. The drought periods we detected are in striking synchrony with historic...