Construction of the Great Wall of China spanned millennia. The most notable section, the snaking fortifications constructed during the Ming dynasty, was built during the 14th to the 17th centuries. It draws 10 million tourists a year. The 2,000-year-old Han dynasty wall—which today looks a bit like dusty crackers stacked in the desert—draws a much smaller crowd.
Lately, climate scientists have been taking an interest in the more ancient sections of the wall and other fortifications. Grasses and reeds in these fascine and rammed earth structures reveal that China’s hottest, driest corner lost much of its water relatively recently.
The finding highlights the utility of reeds as a paleoclimate tool and demonstrates the region’s past hydrology. The research was published in Scientific Reports and presented at AGU’s Annual Meeting 2023 in San Francisco.
Reading the Reeds
“The grasses are so pristine—if you look at them, it’s just like last year.”
Western China wasn’t always a desert. It once boasted lakes and oases that formed the spine of the Silk Road. Some research suggests that humans drained lakes for irrigation as early as 2,000 years ago.
With permission from local archaeological authorities, researchers collected both modern reeds and those preserved from 14 ancient fortification sites, split into two clusters. The eastern cluster comprised seven spots located in Gansu province, while the western cluster included sites in Xinjiang province, roughly 320 kilometers (200 miles) to the west. Between the two regions sits Lop Nur, an ancient dried-up lake.
When constructing a fortification, ancient engineers and masons would have grabbed nearby plants to mix in amid layers of gravel, said Robert Patalano, an archaeologist and paleoecologist at Bryant University and lead author of the study. Though embedded in earthen structures, those plants retain a signal of local environmental conditions at the time they grew, he explained.
“The grasses are so pristine—if you look at them, it’s just like last year,” said coauthor Hong Yang, an Earth scientist also at Bryant University.
Radiocarbon dates from the plants in Gansu largely match historical and archaeological evidence for when the wall was constructed. The Han dynasty stretched roughly from 200 BCE to 200 CE; the grasses in Gansu were gathered between 132 and 116 BCE. The towers in Xinjiang—which are independent outposts and not part of the Great Wall—were built from 677 to 1160 CE, according to the radiocarbon dates.
To Yang’s knowledge, the study is the first to analyze ancient isotopes from the historic buildings’ materials. This methodology is valuable, Patalano said, because paleoclimate studies often rely on off-site proxies such as coastal sediment cores or limestone from caves. Those findings are then applied across a broad region. These grasses provide a hyperlocal window.
The researchers looked at carbon signatures for climate indicators. Plants growing in hot, arid climates close their stomata to reduce water loss. That reduces the flow of carbon dioxide and creates a larger share of heavier carbon isotopes. The ancient reeds show little variation in carbon signatures between the earlier eastern and later western clusters, meaning both sides of Lop Nur experienced a similar climate through at least 1160 CE.
In addition to carbon signatures, the team also explored nitrogen (N) isotopes, which point to ancient agriculture. Grasses near two sites in Xinjiang—a castle dated to 770 CE and a tower from around 1160 CE—showed elevated 15N isotopes, indicating they intercepted the runoff from manure, guano, or other intentional fertilizer, likely from population centers surrounding the garrisons.
“It’s modern human activity that is causing this desertification.”
Whereas others have suggested that irrigation was the chief contributor to regional desertification, the new research indicates it is modern anthropogenic warming—including a temperature increase of more than 1°C since 1955—that reduced surface water. Because the reeds from 132 BCE mirror those from 1160 CE, they argue, the region likely remained consistently wetter and cooler and maintained uniform surface water over that 1,000-year stretch, even with demonstrated increases in agriculture.
“It’s modern human activity that is causing this desertification,” Patalano said.
A Call for Collaboration
The study has preliminary value but would benefit from more interdisciplinary collaboration, said Brian Lander, an environmental historian at Brown University who was not involved in the research.
The authors showed that their methods are useful, but overlooked directly relevant work from historians and archaeologists, Lander explained. General dates for each structure were already known through artifacts such as coins, silks, and bamboo slips. The authors acknowledge this, but “there’s so much archaeological and textual evidence that could be combined with this type of research,” Lander said.
Lander said he hopes more climate scientists collaborate with historians and archaeologists when their research areas overlap. But he said he’s happy to see more work in northwestern China.
“It’s a place where the long-term environmental history could be reconstructed very well,” he explained.
—J. Besl (@J_Besl), Science Writer