Ice Age trees helped stabilize Earth's atmosphere by suffocating

UNIVERSITY PARK, Pa. — Ancient trees may have played a key role in regulating Earth’s climate during the last ice age — by breathing less efficiently.  

A new study, led by a researcher at Penn State and published today (Nov. 5) in the journal Nature Geoscience, examined chemical fingerprints in subfossil wood, or preserved trees, from across North America to understand how plants responded to the low carbon dioxide (CO2) levels and cooler temperatures of the last glacial period, about 20,000 years ago.   

The researchers found that as temperature and CO2 levels dropped, trees in many locations increased their photorespiration, a process akin to labored breathing for plants and a sign that they are potentially wasting energy and releasing carbon dioxide back into the atmosphere.  

The increased release of CO2 may have inadvertently kept the climate just warm enough with enough carbon in the atmosphere for plants to survive — acting as a kind of natural handbrake helping to keep Earth's environment habitable.  

“When we're thinking ahead about what's going to happen as the climate changes, one big question is: If we continue to increase atmospheric CO2, how will the plant world respond?” said Max Lloyd, assistant professor of geosciences at Penn State and lead author on the paper. “We found a clear link between changes in climate and responses in the biosphere. As atmospheric CO2 levels and temperatures dropped, many plants became less efficient at fixing carbon, which in turn slowed further drawdown of CO2 from the atmosphere. There is a natural feedback loop we’re just starting to understand.” 

To study how plants fared during the last ice age, which lasted from about 115,000 to 12,000 years ago, Lloyd and his colleagues used a new technique to reconstruct photorespiration rates in ancient trees. Photorespiration is the process in which plants take in oxygen and release carbon dioxide, essentially undoing some of the work of photosynthesis, the process through which plants make energy from sunlight and carbon dioxide.  

The novel technique used a chemical process that measures molecules with multiple rare varieties, called clumped isotopes, in wood. Isotopes have similar chemical properties but different physical properties, and clumped isotopes act like a fingerprint for photorespiration, Lloyd explained. Comparing isotope analyses from trees in the glacial period with modern trees, the team found that trees from warmer regions during the ice age had higher photorespiration rates than their modern counterparts, suggesting that low CO2 levels during the last ice age significantly hampered plant productivity, reduced the amount of carbon they could store in wood and soils — and forced plants into distress.  

Some of the key samples in the study came from the La Brea Tar Pits in Southern California, where researchers analyzed ancient juniper wood preserved in tar. The team found clear signs of elevated photorespiration in the samples, meaning that the trees were releasing CO2 back into the atmosphere nearly as fast they removed it. 

“The relatively understudied plant fossils at La Brea Tar Pits are an excellent resource for understanding the responses of plants to climate change, not just in the past, but in the future,” said Regan Dunn, assistant deputy director of the La Brea Tar Pits & Museum and co-author on the paper. “We’re only scratching the surface on what these ancient plants can tell us.”  

The findings help explain why previous studies have found that atmospheric carbon dioxide levels never fell below the threshold of around 185 to 210 parts per million during glacial periods, Lloyd said.  

“To our knowledge, this the first time that we could test the long-held hypothesis that elevated photorespiration helped keep atmospheric carbon dioxide at these levels tens of thousands of years ago,” Lloyd said. “Testing this required making measurements of trees that were actually growing at the time.”  

He added that photorespiration is a key control on how much carbon is in the atmosphere. In a time when there is a sense of urgency around modeling climate scenarios, Lloyd said it’s vital to understand and account for the effect of plants on the atmosphere. One way to look forward is to turn to the past and study how Earth’s biosphere may have self-regulated in previous times of climate stress.  

“We're trying to understand how plants respond to dramatic changes in their world by looking at a time when the climate was changing relatively quickly,” Lloyd said.  

The other authors on this paper are Daniel A. Stolper and Todd E. Dawson of the University of California, Berkeley; Daniel E. Ibarra of Brown University; and Rebekah S. Sprengel (née Stein) and Barbara E. Wortham, at the University of California, Berkeley, at the time of the research. 

The research was funded by the Aguoron Institute, the National Oceanic and Atmospheric Administration, the U.S. National Science Foundation and the Heising-Simons Foundation.