A relatively new way of analyzing tree rings has allowed researchers to reconstruct temperatures in Mongolia since 1269 CE. The new reconstruction confirms that since the 1990s, summer temperatures have been the hottest the region has seen in the past eight centuries.
Posted in Geophysical research letters, the study is led by Nicole Davi, Associate Principal Investigator at Columbia universityLamont-Doherty terrestrial observatory.
Central Asia is one of the fastest warming places on the planet. In the past 15 years alone, summer temperatures have warmed by 1.59 degrees C, or nearly 3 degrees F, almost three times the global average rate. During the same period, the region suffered from extreme and prolonged droughts.
To date, there are only a handful of long-term climate records in Central Asia that can help put these trends into context. Tree ring analysis can tell scientists about temperature and precipitation patterns hundreds or thousands of years ago, but suitably old trees and logs in this region can be difficult to sample, in part. because of their remoteness.
The paucity of data on tree rings in the region makes the new reconstruction all the more important. To create it, Davi and his colleagues analyzed tree ring cores initially collected in 1998 and 2005 for a project led by his mentor, Gordon Jacoby, co-founder of the Tree Ring Lab in Lamont. Jacoby had tried to reconstruct the region’s temperature history using ring widths, but the data was not strong enough, so he put it aside. Before Jacoby’s death in 2014, Davi requested permission to take over the project.
Samples come from several high altitude forests in western Mongolia. “People think of it as a vast steppe system, but there are remarkable old growth forests all over the country and it is very pristine,” Davi said. The locations were very far apart, she added. “It’s quite an expedition to arrive in these forests. “
Video footage recorded by Gordon Jacoby showing one of the study sites and the surrounding vegetation and landscape.
Carrots come from a combination of living Siberian larches dating back 400 to 500 years and relict woods – ancient trees that had fallen but had not rotted due to the cold, dry conditions. “When we find relic wood it’s super exciting because we know we can go back further in time,” Davi said.
She wanted to make good use of the samples, and since the ring width model hadn’t worked, the team decided to try another way of analyzing them: measuring the density of the wood. This is done by taking an extremely fine shard of the tree core – thinner than a human hair – and making it glow with light. More light will penetrate through less dense rings, and less dense rings indicate cooler growing conditions. Davi and her team tried this method, but unfortunately she said, “It’s expensive, it takes a long time, and it’s quite destructive. It would break the core and we couldn’t get what we needed. “
Finally, the team turned to a more recent method that entered service a few years ago and has shown promising results. Called Delta Blue Intensity, the method examines how well each ring reflects blue light in its latewood (the darker band that forms later in the growing season) compared to the lighter original wood. Less dense wood resulting from cooler conditions absorbs less blue light.
The best results of the delta blue light technique allowed the team to build a model of summer temperatures in the region from 1269 to 2004 CE. The reconstruction matches well with data from regional weather stations dating back to the 1950s, as well as cooling events associated with several large-scale volcanic eruptions.
For Davi, publishing these results is personally significant. “Gordon Jacoby was my thesis supervisor, my mentor and my friend,” she said. “We had a lot of adventures working together in the field. Putting an end to some of the research he’s started is really good.
The results support the growing potential of the delta blue intensity method to improve our understanding of past climates, Davi said. They also put the warming of Central Asia into context and improve projections, that the region is expected to warm by 3 to 6 degrees C (5.4 to 10.8 F) by the end of the century. Rapid warming is already hurting fragile ecosystems and causing devastating livestock losses for pastoralists, who have traditionally been the backbone of Mongolian economy.
“What does this mean for livelihoods in Mongolia? Davi asks. “It’s a largely agrarian culture. Some people live in cities, but there are also nomadic pastoralists who have lived the same way for thousands of years. This reconstruction certainly adds context to the warming of recent decades and to global climate models showing what it might look like in the future. “
The document recommends continued investments in infrastructure and climate resilience programs such as index livestock insurance to help communities cope with changing conditions.
Reference: “Accelerated Recent Warming and Temperature Variability Over the Past Eight Centuries in the Central Asian Altai From Blue Intensity in Tree Rings” by NK Davi, MP Rao, R. Wilson, L. Andreu-Hayles, R. Oelkers, R. D ‘Arrigo, B. Nachin, B. Buckley, N. Pederson and C. Leland, B. Suran, July 26, 2021, Geophysical research letters.
DOI: 10.1029 / 2021GL092933
Co-authors of the new study include several members of the Tree Ring Lab at Columbia University’s Lamont-Doherty Earth Observatory – Mukund Rao, Robert Wilson, Laia Andreu-Hayles, Rose Oelkers, Rosanne D’Arrigo, Baatarbileg Nachin, Brendan Buckley and Caroline Leland – as well as Neil Pederson from Harvard University and Byambagerel Suran from National University of Mongolia.