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By poring over the data, scientists claim to have found the most likely temperature during the last ice age, which could help us predict future temperatures.
The ‘last glacial maximum’ – otherwise called the last ice age – occurred 20,000 years ago and had a profound impact on our planet. Now, researchers from the University of Arizona believe they have solved the mystery of how cold this time period actually was.
Writing in Nature, they said the average global temperature was 6 degrees Celsius cooler than it is today. For context, the average global temperature of the 20th century was 14 degrees Celsius.
Jessica Tierney, associate professor in the University of Arizona Department of Geosciences, said that while this might not sound like a big difference, it is actually a “huge change”.
“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,” she said. “But the biggest cooling was in high latitudes, such as the Arctic, where it was about 14 degrees Celsius colder than today.”
The breakthrough came after the development of climate models that could translate data from collected ocean plankton fossils into sea-surface temperatures. Using a technique called data assimilation, researchers were able to combine the fossil data with climate model simulations from the ice age.
Aside from helping solve an age-old question, finding out the temperature of the planet at this time can help researchers calculate climate sensitivity, meaning how much the global temperature shifts in response to atmospheric carbon.
A global map showing temperature differences compared to pre-industrial times, with dark blue showing cooler temperatures. The ice sheets of the past are superimposed on the continents. Image: Jessica Tierney/University of Arizona
Atmospheric carbon levels
Tierney and her team were able to determine that for every doubling of atmospheric carbon, the planet’s average temperature should increase by 3.4 degrees Celsius. This is right in the middle of the range predicted by previous climate models, which ranged between 1.8 and 5.6 degrees Celsius.
During the ice age, atmospheric carbon levels were very low, at approximately 180 parts per million. Prior to the industrial revolution, levels rose to 280 parts per million, and today that number is 415 parts per million.
“The Paris Agreement wanted to keep global warming to no larger than 1.5 degrees Celsius over pre-industrial levels, but with CO2 levels increasing the way they are, it would be extremely difficult to avoid more than 2 degrees Celsius of warming,” Tierney said.
“We already have about 1.1 degrees Celsius under our belt, but the less warm we get the better, because the Earth system really does respond to changes in CO2.”
Looking to the future, Tierney said she and her team want to use these newly developed models to recreate warm periods in Earth’s past.
“If we can reconstruct past warm climates, then we can start to answer important questions about how the Earth reacts to really high CO2 levels, and improve our understanding of what future climate change might hold,” she said.
