How cold was the ice age? Researchers now know

[Aug. 26, 2020: University of Michigan]

A team of scientists has nailed down the temperature at the peak of the last ice age, a time known as the Last Glacial Maximum, to about 46 degrees Fahrenheit.

Their findings allow climate scientists to better understand the relationship between today’s rising levels of atmospheric carbon dioxide—a major greenhouse gas—and average global temperature.

The Last Glacial Maximum, or LGM, occurred about 20,000 years ago and was a frigid period when glaciers covered about half of North America, Europe and South America and many parts of Asia, while flora and fauna that were adapted to the cold thrived.

“We have a lot of data about this time period because it has been studied for so long. But one question science has long wanted answers to is simple: How cold was the ice age?” said Jessica Tierney of the University of Arizona, who led the team that also includes scientists from the University of Michigan, the National Center for Atmospheric Research and the University of Washington.

Tierney is lead author of a paper published today in Nature that found that 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).

Co-authors of the Nature paper include Christopher Poulsen, a professor in the U-M Department of Earth and Environmental Sciences, and former U-M postdoctoral researcher Jiang Zhu, who is now at the National Center for Atmospheric Research.

For the project, the researchers used a technique that combines fossil data of past temperatures with climate model output to create maps that show how temperature differences varied in specific regions around the globe. Zhu and Poulsen were responsible for developing the climate model simulations of the LGM.

“Six degrees of global average cooling is enormous. The world would have looked much different during the last glacial maximum,” said Poulsen, who is also the associate dean for natural sciences at the U-M College of Literature, Science, and the Arts.

“The northern portions of North America, including here in Ann Arbor, Michigan, were covered by kilometers of ice,” he said. “The biggest cooling was in the high latitudes.”

The researchers said their findings fit with scientific understanding of how Earth’s poles react to temperature changes.

“Climate models predict that the high latitudes will get warmer faster than low latitudes,” Tierney said. “When you look at future projections, it gets really warm over the Arctic. That’s referred to as polar amplification. Similarly, during the LGM, we find the reverse pattern. Higher latitudes are just more sensitive to climate change and will remain so going forward.”

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Knowing the temperature of the ice age matters because it is used to calculate climate sensitivity, meaning how much the global temperature shifts in response to atmospheric carbon. The researchers determined that for every doubling of atmospheric carbon, 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).

“Without having an accurate estimate of the LGM temperature, we couldn’t be confident in how temperature responded to changes in atmospheric carbon,” Zhu said. “Our results provide that confidence.”

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.

“The Paris Agreement wanted to keep global warming to no larger than 2.7 F (1.5 C) over pre-industrial levels, but with carbon dioxide levels increasing the way they are, it would be extremely difficult to avoid more than 3.6 F (2 C) of warming,” Tierney said. “We already have about 2 F (1.1 C) under our belt, but the less warm we get the better, because the Earth system really does respond to changes in carbon dioxide.”

Since there were no thermometers in the ice age, the researchers 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 a technique called data assimilation, which is used in weather forecasting.

In the future, the researchers plan to use the same technique to recreate warm periods in Earth’s past.

“If we can reconstruct past warm climates,” Tierney said, “then we can start to answer important questions about how the Earth reacts to really high carbon dioxide levels, and improve our understanding of what future climate change might hold.”

The research was supported by the Heising-Simons Foundation and the National Science Foundation.