A Continental Climate Archive
The Altay Mountains, straddling the borders of China, Russia, Mongolia, and Kazakhstan, rise from the arid heart of Central Asia. Far from any oceanic influence, their glaciers respond primarily to shifts in atmospheric systems like the Siberian High and the mid-latitude Westerlies. This location makes the glacial landforms—especially the moraines from advancing and retreating ice—a high-fidelity archive of past climate. Scientists use these landforms to reconstruct how temperature and precipitation have changed over millennia.
To read this archive, researchers employ techniques like cosmogenic nuclide dating. Cosmic rays constantly bombard Earth's surface, creating rare isotopes like Beryllium-10 (¹⁰Be) in the quartz crystals of boulders. When a glacier deposits a boulder on a moraine and then retreats, that boulder's surface begins to accumulate ¹⁰Be at a known rate. By measuring the concentration of this isotope, scientists can calculate how long the rock has been exposed and, therefore, when the glacier abandoned that position.
These methods reveal a complex history. The Altay experienced multiple major glaciations during the Quaternary Period. Studies in the Kanas River valley show evidence for at least three distinct glacial advances during the last major Ice Age alone. During the Last Glacial Maximum (globally dated between 26,500 and 19,000 years ago), valley glaciers here expanded significantly. The equilibrium line altitude (ELA)—the point where annual snow accumulation equals ablation—was depressed by 800 to 1,100 meters in the more humid parts of the Altay.
Reading the Moraines
The ridges of rock and sediment called moraines are the most direct evidence of past glacial extents. In the Altay, these landforms map out a dynamic history of ice advances that were different from a simple retreat from the last Ice Age. Radiocarbon and dendrochronological (tree-ring) dating have identified several distinct Holocene glacial advances.
After a significant warming period in the early Holocene (starting around 11,400 years ago) that caused many glaciers to shrink or disappear, the ice advanced again during several important periods. These include a Middle Holocene stage (around 4,900–4,200 years ago), a "Historical" stage (2,300–1,700 years ago), and the "Aktru" stage, which corresponds to the Little Ice Age (LIA) of the 13th to 19th centuries. During the LIA maximum around 1850, the total glacierized area in the Altay was estimated at 2,288 square kilometers; since then, it has shrunk by nearly half.
Sediment cores from Kanas Lake, a glacial ribbon lake formed around 200,000 years ago, provide another line of evidence. The layers of sediment, pollen, and diatoms (microscopic algae) in the lake bed create a continuous environmental record and the more episodic record of the moraines. For example, pollen records from the region show an increase in effective humidity between 550 and 1050 CE. Tree-ring data from old trees in the Altai also identified a severe cold phase in the 6th century known as the Late Antique Little Ice Age. This multi-proxy approach allows for a detailed reconstruction of the region's response to both long-term glacial cycles and shorter-term climate shifts.