Science News Desk – Over the last century, Earth’s average temperature has risen by about one degree Celsius (1.8 °F). It is difficult to dispute the evidence. It comes from thermometers and other sensors around the world. But what happened thousands of years before the Industrial Revolution, before thermometers, and before humans warmed the climate by releasing heat-trapping carbon dioxide from fossil fuels? At that time the temperature of the earth was warming or cooling? Even though scientists know more about the most recent 6,000 years than about any other multimillennial interval, studies on this long-term global temperature trend draw contrary conclusions.
To try to resolve the gap, we conducted a comprehensive, global-scale assessment of the existing evidence, including both natural archives, such as tree rings and seafloor sediments, and climate models. Our results, which were published on February 15, 2023, suggest ways to improve climate forecasting while avoiding missing some important slow-moving, natural climate feedbacks. Scientists like us who study past climate, or palaeoclimate, look for temperature data from thermometers and satellites from a long time ago. We have two options: we can obtain information about past climate stored in natural archives, or we can simulate the past using climate models.
There are many natural records that record changes in climate over time. Annual growth rings in trees, stalagmites and corals can be used to reconstruct past temperatures. Similar data can be found in small circles found in glacier ice and in sediments formed over time at the bottom of oceans or lakes. These serve as substitutes or proxies for thermometer-based measurements. For example, changes in the width of tree rings can record fluctuations in temperature. If temperatures are too cold during the growing season, the tree ring formed that year is thinner than in a year with warmer temperatures.
Another temperature estimate is found in ocean floor sediments, in the remains of tiny sea-dwelling organisms called foraminifera. When a foraminifer is alive, the chemical composition of its shell changes depending on the temperature of the ocean. When it dies, the shell sinks and over time is buried by other debris, forming layers of sediment on the ocean floor. Paleoclimatologists can then extract sediment cores and chemically analyze seeps in those layers to determine their composition and age, sometimes for millennia.