“The next ice age isn’t going to materialize”

The Earth is more than four and a half billion years old, and its climate has changed dramatically over time. Our current climate is therefore only a snapshot, but what began around 150 years ago with industrialization is unparalleled in Earth’s history. Ever since humans first began making a significant impact on their environment, there has been a development that never occurred before in millions of years of our climate’s history—a fact that various scientific methods are able to demonstrate.

Our present climatic conditions originated some 65 million years ago, in the Cenozoic era. Since then, it has frequently been much warmer on Earth than it is today, and the level of CO₂ in the atmosphere has sometimes been considerably higher, too. There are many influencing factors, which change at regular intervals: The elliptical orbit of our planet around the sun (over the course of 100,000 years) for example, or the tilt of the Earth’s axis (over the course of about 41,000 years). All of these factors caused a permanent change in the global climate. However, it was a change that occurred over very long periods of time.

Due to these constant, very long-term fluctuations, environmental factors affecting the climate also changed. The expansion of ice sheets, for example, influenced atmospheric circulation, as ice sheets had an effect not only on atmospheric temperatures, but on the oceans as well. Ice reflects more sunlight, causing temperatures to drop. On the other hand, the oceans absorb sunlight and transport heat to high latitudes. At the end of the last ice age 14,000 years ago, the onset of this transport caused the North Atlantic to warm and ice sheets to melt. The considerable quantities of meltwater altered oceanic currents in turn.

The last major ice age period began approximately 35 million years ago. Since then, Antarctica has been a separate continent at the South Pole. An annular mode of wind and water circulation shielded it from an exchange with warmer, lower latitudes, allowing the water around Antarctica to cool and sink into the depths of the oceans. As a result, the world’s oceans are about 10 degrees Celsius colder today than they were 70 million years ago. Land ice has also existed in the north polar region for about three million years—permanently on Greenland, and during ice ages in North America and Eurasia.

“In sync with the Earth’s changing orbit around the sun, the global climate has gone through long-term cycles over the past millions of years,” Lohmann says. “During cold periods, parts of the northern hemisphere were covered extensively with ice sheets up to several kilometers thick, which largely disappeared again during warm periods. This repeated itself about every 80,000 to 120,000 years.” Reconstructions from ice cores reveal that between the cold and warm periods, CO₂ levels varied, ranging from about 180 to 290 parts per million (ppm).

With the onset of industrialization in the mid-19th century, however, our climate began experiencing a development unparalleled in Earth’s history. Within approximately 150 years, atmospheric concentration of CO₂ surged faster than ever before—and with it, global warming. In 2020, annual CO₂ levels in the atmosphere at the monitoring stations on the Zugspitze and on Mauna Loa in Hawaii averaged 414 ppm. The Mauna Loa monitoring station, for example, shows a rapid increase in the concentration of CO₂ of 100 ppm over the last 60 years. These figures therefore represent an important indicator of human-induced alterations, in other words, anthropogenic climate change. The carbon that was sequestered between 200 million and 65 million years ago, chiefly in coal, oil, and natural gas, is now being released by humans within a very short period.

In order to understand the highly complex climate system, researchers are taking samples all across the globe: on land, in the oceans, on the ocean floor, and in the ice. These efforts are joined by remote sensing via satellites and aircraft. “In addition to the readings and monitoring results from the past two centuries, we are drawing on information that tells us something about climate and environmental conditions in the distant past,” Lohmann explains. Such proxy data (or simply proxies) can be obtained from organisms, for instance. Tree growth rings are one example that can be used to identify extremes, such as hot summers. Remains of amphibians and reptiles can be found where tropical to subtropical climates once prevailed. Sediment also provides information about bygone climatic conditions.

Carbon isotopes also play an important role as climate indicators. “With the help of the various isotopes, we can reconstruct changes in the carbon cycle, among other things,” says Peter Köhler from AWI’s Glaciology section. To this end, an international team of scientists that included Köhler analyzed nearly 15,000 samples over the past seven years. The team is able to determine for as far back as 57,000 years how high the share of radiocarbon was in the atmosphere at certain times.

“The last 10,000 years have been very stable,” says Grosse. “But now we’ve ushered in a phase unlike almost any other in Earth’s history, in which warming has progressed nowhere near as rapidly as it is occurring now.” The three scientists agree: We can still reach the 1.5-degree target and avoid tipping points, but time is running out.