The Last Glacial Maximum (LGM) is defined as the time period when the continental ice sheets reached their maximum total mass during the last ice age, a long period of reduction in the temperature of the Earth’s surface and atmosphere, resulting in the presence or expansion of continental and polar ice sheets and alpine glaciers.
There is vast information available about this time period as it has been studied for so long. But one question science has long wanted answers to is simple: How cold was the ice age?
A research team at the University of Arizona found the answer. The Last Glacial Maximum of 20,000 years ago—to about 46 degrees Fahrenheit (7.8 C).
Jessica Tierney, associate professor in the UArizona Department of Geosciences, said, “The average global temperature of the ice age was 6 degrees Celsius (11 F) cooler than today. For context, the average global temperature of the 20th century was 14 C (57 F).”
“In your own personal experience, that might not sound like a big difference, but it’s a huge change.”
This study is expected to help climate scientists better understand today’s rising levels of atmospheric carbon dioxide and the average global temperature.
Scientists also created maps to illustrate how temperature differences varied in specific regions across the globe.
Tierney said, “In North America and Europe, the most northern parts were covered in ice and were extremely cold. Even here in Arizona, there was big cooling. But the biggest cooling was in high latitudes, such as the Arctic, where it was about 14 C (25 F) colder than today.”
Knowing the ice age temperature matters since it is utilized to ascertain climate sensitivity, which means how much the global temperature shifts because of atmospheric carbon.
Scientists determined that for every doubling of atmospheric carbon, the global temperature should increase by 3.4 C (6.1 F), which is in the middle of the range predicted by the latest generation of climate models (1.8 to 5.6 C).
Atmospheric carbon dioxide levels during the ice age were about 180 parts per million, which is very low. Before the Industrial Revolution, levels rose to about 280 parts per million, and today they’ve reached 415 parts per million.
Thermometers were not available during the ice age. Thus, scientists developed models to translate data collected from ocean plankton fossils into sea-surface temperatures. They then combined the fossil data with climate model simulations of the LGM using data assimilation, which is used in weather forecasting.
Tierney said, “What happens in a weather office is they measure the temperature, pressure, humidity and use these measurements to update a forecasting model and predict the weather. Here, we use the Boulder, Colorado-based National Center for Atmospheric Research climate model to produce a hindcast of the LGM. Then we update this hindcast with the actual data to predict what the climate was like.”
Scientists are further planning to use the same technique to recreate warm periods in Earth’s past.
- Tierney, J.E., Zhu, J., King, J. et al. Glacial cooling and climate sensitivity revisited. Nature 584, 569–573 (2020). DOI: 10.1038/s41586-020-2617-x