Newswise — A team of climate scientists at Northern Arizona University, along with their international collaborators, has completed new reconstructions of Earth’s temperature over the past 2,000 years, placing the exceptional 20th-century warming into a longer-term context, as chronicled in a study published today in Nature Geoscience.

This international collaboration, part of the PAGES 2k Consortium, used a new, highly curated set of paleoclimate data and compared seven different statistical methods for reconstructing past global average temperature. Put simply, they found these different ways of measuring climate change in the last two millennia all came back with similar results, especially when focusing on decade-to-decade changes. This allowed the researchers to explore important questions about how much of the past temperature changes can be ascribed to different natural causes, including the randomness of the climate system, which is difficult to quantify without a long-term perspective. 

Results showed the most rapid, multi-decade-long global warming of the past 2,000 years occurred during the second half of the 20th century, highlighting the extraordinary pace of current climate change, due mostly to human emissions of heat-trapping gases. The study also quantified changes in global temperature due to volcanic eruptions and natural variability before the industrial revolution. These are important benchmarks for evaluating whether climate models accurately simulate changing global climate.

“Reconstructions of the past help us place recent climate changes into an appropriate context,” said Michael Erb, co-author and a postdoctoral scholar at NAU’s School of Earth and Sustainability. “This study allowed us to estimate climate variability over the past 2,000 years, showing just how large recent man-made warming is compared to natural and volcano-forced climate variations over the recent past. It also showed that climate models are doing a good job capturing these sort of changes, which is helpful for model validation.”

The PAGES 2k Consortium includes Regents’ Professor Darrell Kaufman and assistant professor Nicholas McKay, also from NAU’s School of Earth and Sustainability, and a host of collaborators from around the nation and world, such as Costa Rica, China, Switzerland, Germany and Australia. To generate the new temperature reconstructions, they first needed to gather together the PAGES 2k database, which consists of biological and geological sources that provide information on historical temperatures and climate. These data sources include tree rings, corals, glacier ice and sediments, providing information from all of the world’s continents and oceans. The collaboration is coordinated through the Past Global Changes (PAGES) project, which makes this valuable data available to other scientists for use in diverse paleoclimate research as well.

McKay and Kaufman led the development of this database and were delighted to see that effort paying off.

“Pulling together such a diverse dataset is extremely challenging, but it’s exciting to see that work help us better understand the climate system,” McKay said. “This is only the tip of the iceberg of what scientists can learn from these data.”

“This study used the recently developed PAGES 2k database to improve on previous temperature reconstructions at the global scale,” Kaufman said. “This is a major accomplishment that was completed in time for the results to be considered in the upcoming climate assessment report from the Intergovernmental Panel on Climate Change. 

A companion study, which used the same database to map how temperature over the past 2,000 years varied from place to place around the Earth, was published concurrently in Nature.