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Study of temperatures over last 24,000 years shows modern-day warming is unprecedented

The findings illustrate just how far out of bounds human-driven global warming is pushing the climate system.

(CN) — The Earth is warming at rates not seen in at least the last 24,000 years, according to a new study, settling a longstanding debate over whether temperatures are trending warmer or cooler over time and something scientists say should be cause for alarm.

Using temperature data from marine sediments and computerized climate simulations, researchers at the University of Arizona led an effort to reconstruct the Earth’s climate since the last ice age, or about 24,000 years ago.

The findings illustrate just how far out of bounds human-driven global warming is pushing the climate system.

"This reconstruction suggests that current temperatures are unprecedented in 24,000 years, and also suggests that the speed of human-caused global warming is faster than anything we've seen in that same time," Jessica Tierney, study co-author and geosciences associate professor at the University of Arizona, said in a statement.

Tierney has contributed to the Intergovernmental Panel on Climate Change reports and climate briefings for the U.S. Congress.

“The fact that we're today so far out of bounds of what we might consider normal is cause for alarm and should be surprising to everybody,” lead study author Matthew Osman, geosciences postdoctoral researcher at the University of Arizona, said in a statement.

The study, published Wednesday in the journal Nature, confirmed that the main drivers of climate change — at least since the last ice age — are rising greenhouse gas concentrations and the retreat of ice sheets.

It also shows that the global mean temperature has slightly but steadily warmed by about half-a-degree Celsius in the past 9,000 years, and that the magnitude and rate of warming over the past 150 years has far surpassed that of the last 24,000 years.

To illustrate their findings, researchers created maps of global temperature changes in 200-year intervals.

"These maps are really powerful," Osman said. "With them, it's possible for anyone to explore how temperatures have changed across Earth on a very personal level. For me, being able to visualize the 24,000-year evolution of temperatures at the exact location I'm sitting today, or where I grew up, really helped ingrain a sense of just how severe climate change is today."

To determine what temperatures used to be like, the team used two methods.

First, they looked at the chemical signatures of marine sediments. Temperature changes over time can affect the chemistry of a long-dead animal's shell, so paleoclimatologists can use those measurements to estimate temperature in an area.

Then, they used computer-simulated climate models, which are used in current weather forecasting to provide temperature information based on scientists' best understanding of the physics of the climate system.

The team then combined those two datasets to capture a more complete picture.

"With this method, we are able to leverage the relative merits of each of these unique datasets to generate observationally constrained, dynamically consistent and spatially complete reconstructions of past climate change," Osman said.

Now, researchers are working on using this method to investigate climate changes even further in the past.

"We're excited to apply this approach to ancient climates that were warmer than today because these [ancient] times are essentially windows into our future as greenhouse gas emissions rise,” Tierney said.

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