Every year, particles eroding from the continents are transported to the oceans by the wind as dust and by rivers as sands and gravels. Once they get to the ocean, they mix with billions of tons of dead plankton shells, sink, and settle on the seafloor. There, they accumulate vertically in layers on top of previous years’ material. Similarly, this year’s snow accumulates on top of the previous years’ snow at the polar regions in places like Greenland and Antarctica. Over time, this process forms new layers of ice. Trees, much the same way, add yearly layers of new cells in concentric circles just below their bark – called tree rings. And, in many caves around the world, the strength of the seasonal cycle of a wet monsoon followed by a dry season is recorded in the chemistry of stalagmites rising up from the cave floor, formed by drips of mineral-rich water from the roof of the cave.
Paleoclimatologists have learned that all of these yearly processes are sensitive to changes in regional climate, allowing them to be used like “natural thermometers,” or what scientists call “paleoclimate proxies.” For the last several decades, Paleoclimatologists have been collecting data from these natural archives of environmental change and using them to explore how climate change occurs naturally. Together with our observations of present climate, the data collected from these natural thermometers tells us that the climate changes we are observing today are unique with respect to much of Earth’s recent history.
For example, the bubbles filled with ancient atmosphere and trapped in the layers in ice cores show us that heat-trapping gases haven’t been at the levels they are today in nearly 800,000 years, and likely for millions of years. Similarly, tree rings show us that recent human-caused warming trends far outpace any natural warming the Earth did by itself over much of the last few thousands of years. These paleoclimate proxies can also tell us about nutrient cycling in the ancient ocean, the amount and pattern of past precipitation events throughout the tropics, the extent and magnitude of recent volcanic eruptions, and many other things – in fact, scientists are constantly discovering new and imaginative ways to leverage these natural archives of environmental change to help us understand present and future changes to our global climate.