New method shows today’s warming is “unprecedented” in 24,000 years

Environment | Search | Sciences | UW News Blog

November 10, 2021

The blue line shows the global average surface air temperature since the last Ice Age 24,000 years ago, created by equating paleoclimatic records with a computer model of the climate system. Time is stretched over the past 1000 years to visualize recent changes. Warming begins at the end of the last ice age, around 18,000 years ago, and then temperatures stabilize. While previous studies showed a slight cooling over the past 10,000 years, the new analysis shows a slight warming trend. The curve is steepening in recent decades with the accumulation of atmospheric greenhouse gases.Osman et al./Nature

A new effort to reconstruct Earth’s climate since the last Ice Age around 24,000 years ago highlights the main drivers of climate change and how human activity has pushed the climate system out of bounds.

The study led by the University of Arizona uses a technique for reconstructing past temperatures developed by coauthors at the University of Washington. The study, published Nov. 10 in Nature, has three main findings:

  • He verifies that the main drivers of climate change since the last ice age are increasing greenhouse gas concentrations and retreating ice caps.
  • It suggests a general warming trend over the past 10,000 years – settling a decade-long debate in the paleoclimatic community as to whether this period tended to warm or cool.
  • The magnitude and rate of warming over the past 150 years far exceeds the magnitude and rate of change at any time in the past 24,000 years

“This reconstruction suggests that current temperatures are unprecedented for 24,000 years, and also suggests that the rate of man-made global warming is faster than anything we have seen at the same time,” said lead author Jessica Tierney, associate professor at the University of Arizona.

The UW team developed the method that allowed researchers to use computers to make sense of paleoclimatic data in marine sediments, allowing for more regional detail and precision in temperature history.

“The fact that we are now so far from the limits of what we might consider normal is a cause for concern and should surprise everyone,” said lead author of the study, Matthew Osman, researcher. postdoctoral fellow at the University of Arizona.

An online search for “change in global temperature since the last ice age” would produce a graph of how global temperature has changed over time that was created eight years ago.

“Our team’s reconstruction improves this curve by adding a spatial dimension,” Tierney said.

These maps show the global average surface temperature at various times in Earth’s history, stretching back 24,000 years. The darker the shade of blue, the cooler the temperature compared to today.Osman et al./Nature

Different methods exist to reconstruct past temperatures. The team combined two independent datasets – temperature data from marine sediments and computer simulations of the climate – to create a more complete picture of the past.

“Paleoclimatic records provide the only record we have of these past climates, but these records are imperfect and have gaps in space and time. Climate models provide simulations based on the laws of physics, but do not have the observational data, ”said co-author Gregory Hakim, professor of atmospheric sciences at UW. “The combination of paleoclimatic models and proxies – using the technique we developed for reanalysis of the last millennium – provides the best spatially complete estimate of the actual past climate, constrained by physics. “

The researchers looked at the chemical signatures of marine sediments to gain information about past temperatures. Because changes in temperature over time can affect the shell chemistry of a long-dead animal, paleoclimatologists can use these measurements to estimate the temperature in an area. It’s not a perfect thermometer, but it’s a place to start.

On the other hand, computer-simulated climate models provide temperature information based on scientists’ best understanding of the physics of the climate system, which is also not perfect.

The team decided to combine methods to harness each other’s strengths. This is called data assimilation and is also commonly used in weather forecasting.

“To forecast the weather, meteorologists start with a model that reflects the current weather, then add observations such as temperature, pressure, humidity, wind direction, etc. to create an updated forecast,” Tierney said.

The team applied this same idea to the past climate.

“We found remarkable agreement between the result of our assimilation method and the values ​​of independent paleoclimatic records,” Hakim said. “This independent validation of our estimates for past climate variability is an indication of the reliability of the results.”

The other co-authors are Robert Tardif at UW; Jiang Zhu at the National Center for Atmospheric Research; Jonathan King at the University of Arizona; and Christopher Poulsen at the University of Michigan. The research was funded by the National Science Foundation, the Heising-Simons Foundation, and the National Center for Atmospheric Research.

For more information, contact Hakim at, Osman at and Tierney at

Note: This article was adapted from a press release from the UArizona.

NSF Grants: AGS-1602301, AGS-1602223; FSS Grants: 2016-012, 2016-014, 2016-015

Tag (s): climate change • College of the Environment • Department of Atmospheric Sciences • Gregory Hakim

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