How Cold Was Earth in the Last Ice Age? The Answer May Surprise You

Artist’s impression of ice age Earth at glacial maximum. (By Wikipedia user Ittiz, CC BY-SA 3.0, Link)

(CN) — Climate scientists recently revised their prediction for the global temperature drop that occurred during the last Ice Age, finding the Earth was likely colder than previously suspected.

The Last Glacial Maximum, also known as an ice age, occurred around 20,000 years ago when ice sheets were at their greatest volume, stretching across much of North America, Northern Europe and Asia. Following that period, the planet slowly began to warm, melting much of the glacial regions outside the poles, and causing sea levels to rise in turn.

A lingering question remains for scientists: exactly how cold was the last ice age?

A team of researchers sought to answer that question in a new study published Wednesday in the journal Nature. Led by Dr. Jessica Tierney, associate professor in the Department of Geosciences at the University of Arizona, the team attempted to reconstruct the average global temperature during the Last Glacial Maximum to better understand the relationship between greenhouse gas levels and rising temperatures today.

According to the study, the average global temperature has risen 11 degrees in the intervening period, to 57 degrees in the 20th century — far above the global average of 46 degrees in the Last Glacial Maximum.

“In your own personal experience that might not sound like a big difference, but in fact, it’s a huge change,” Tierney said in a statement.

Tierney’s team mapped out temperature changes across the globe to get a better idea of which areas have been most impacted by climate change in the post-industrial era. She said the greatest cooling occurred in high latitudes, such as the Arctic, where it was 25 degrees cooler than it is today.

“We’ve come up with a new estimate for how cold the last ice age (called the Last Glacial Maximum) was using both geological information and climate model simulations,” Tierney said in an email interview. “Our estimate for global cooling is a bit cooler than previous work. Our method also allows us to recreate maps of what temperatures were like during the LGM across the world.”

Ice ages provide researchers a baseline by which to compare later temperature shifts. Atmospheric carbon levels during the previous ice age are thought to have reached 180 parts per million. Those levels rose to 280 parts per million leading up to the Industrial Revolution, and now sit at a comparatively eye-watering 415 parts per million.

Climate scientists expect Earth to continue warming around its poles due to excessive greenhouse gas emissions. Warming poles lead to melting ice, resulting in a negative feedback loop where warming begets more warming. However, that wasn’t always the case.

“Climate models predict that the high latitudes will get warmer faster than low latitudes,” Tierney said in a statement. “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.”

Equilibrium climate sensitivity is an important measure used to predict how much Earth’s atmosphere would warm were carbon dioxide emissions to be doubled. Tierney pegged the sensitivity during the Last Glacial Maximum at 6 degrees, which is slightly higher than previous estimates, but still in line with the traditionally given range of 3.6 to 8 degrees.

The researchers compared proxy values derived from plankton fossils to model temperature changes over time using a method called data assimilation. These models are comprised of 50-year intervals taken from simulations of glacial activity and can help climate scientists track future atmospheric changes.

“The technique we used to combine geological evidence with climate model simulations is called ‘data assimilation,’ and it’s used every day for weather prediction. However, it hasn’t been used very much for ‘hindcasts’ of how Earth’s climate changed in the past,” Tierney said by email.

Tierney and her team plan to flip this research on its head next time to study Earth’s past warming periods, which can help researchers assess the trajectory of climate change going forward.

“The LGM was our first test of whether this method can be used to reconstruct ancient climates,” she added, “and now we’d like to apply the same technique to other past climates, especially warm climates that are a guide for what to expect in the future.”

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