Global temperatures over last 24,000 years show today's warming 'unprecedented'

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The University of Arizona's effort to reconstruct Earth’s climate from the last ice age (24,000 years ago) highlights the major drivers of climate change as well as how far human activity has pushed it.

Three main findings were found in the Nature study published this week.

It was found that climate change has been driven mainly by rising greenhouse gas concentrations, and retreating ice sheets.

It indicates a general warming trend in the past 10,000 years, which settles a decade-long dispute within the paleoclimatology community about whether this period was warmer or colder.

The rate and magnitude of warming in the past 150 years is far greater than the rate and magnitude of changes over 24,000 years.

"This reconstruction suggests the unprecedented temperature rises in the last 24,000 years and suggests that human-caused global warming has been occurring at a faster pace than any other time in the past," Jessica Tierney, a UArizona geosciences associate professor, and coauthor of the study.

Tierney is the head of the lab where this research was done. She is also well-known for her contributions to reports from the Intergovernmental Panel on Climate Change and briefings on climate for Congress.

Matthew Osman (a geosciences postdoctoral researcher from UArizona) said that the fact that we are today so far outside of what we might consider normal should alarm everyone.

A search online for "global temperature changes since the last Ice Age" will produce a graph showing global temperature changes over time, which was created eight years ago.

Tierney stated, "Our team's reconstruction enhances that curve by adding an spatial dimension."

The team created maps showing global temperature changes over 200 years, going back 24,000 year.

Osman stated that these maps were "really powerful." It's easy for anyone to see how the temperature has changed over time, and it's very personal. It was a great way to get a feel for how serious climate change is.

There are many ways to reconstruct past temperatures. The team combined two independent datasets--temperature data from marine sediments and computer simulations of climate--to create a more complete picture of the past.

To determine past temperatures, the researchers examined chemical signatures in marine sediments. Paleoclimatologists are able to use these measurements to determine the temperature of an area because temperature fluctuations over time can alter the chemistry in a long-dead animal’s shell. Although it is not an exact thermometer, it can be a good starting point.

Computer-simulated climate models on the other side provide temperature information based upon scientists' best understanding of climate system physics, which is also not perfect.

They decided to combine both methods in order to maximize the strengths of each. This is known as data assimilation, and it is often used in weather forecasting.

Tierney stated that meteorologists use a model to reflect current weather and then add observations like temperature, pressure, humidity, wind direction to create a forecast.

This idea was also applied to the past climate.

Osman stated that this method allows us to use the relative strengths of each dataset to produce observationally constrained, consistent, and spatially complete reconstructions past climate change.

The team is now working to improve their methods for studying climate change.

Tierney stated, "We're thrilled to apply this approach ancient climates which were warmer than now," "because these times are essentially window into our future as greenhouse gases emissions rise."

Continue reading How cold was the Ice Age? Now researchers know

Additional information: Matthew Osman. Globally resolved surface temperature since the Last Glacial Maximum. Nature (2021). Journal information: Nature Matthew Osman, Globally resolved surface temperatures since the Last Glacial Maximum,(2021). DOI: 10.1038/s41586-021-03984-4