How can you work out what the climate was like in the past, without having had someone take measurements? One solution is to look at tree rings. The pattern of tree rings can give you an indication of what climate was like overall for a year. That works, so long as you have trees.
But not everywhere has trees. They’re missing in the polar regions and from any mountain range – once you get above the tree-line.
In some places, you don’t need trees and Liang and colleagues found you can use juniper dwarf shrubs on the Tibetan Plateau. In a new paper Jakob Wernicke and colleagues follow up on the earlier work by examining Wilson juniper (Juniperus pingii var. wilsonii) from the shore of lake Nam Co.
They took five shrub discs and examined the δ18O signal in them. Water in the atmosphere can have ordinary oxygen in its H2O or heavy Oxygen, Oxygen-18. The mix of the two forms varies due to how water evaporates from the oceans. If you can track the ratio of the two isotopes, then you can make inferences about the moisture of the atmosphere in a given year.
The team found the δ18O signal from the shrubs correlated with the same signal found in a tree of a nearby genus. This same signal worked as a reliable climate proxy. It means that in parts of the Tibetan Plateau where there are no trees, you can expect these shrubs to work as an adequate stand-in for climate records.
Once you have these records, you can start looking at the broader effect of the Asian summer monsoons. You can refine models to see how changing conditions affect the billions of people living on the Indian subcontinent.
Liang, E., Lu, X., Ren, P., Li, X., Zhu, L., & Eckstein, D. (2011). Annual increments of juniper dwarf shrubs above the tree line on the central Tibetan Plateau: a useful climatic proxy. Annals of Botany, 109(4), 721–728. https://doi.org/10.1093/aob/mcr315
Wernicke, J., Stark, G., Wang, L., Grießinger, J., & Bräuning, A. (2019). Air moisture signals in a stable oxygen isotope chronology of dwarf shrubs from the central Tibetan Plateau. Annals of Botany. https://doi.org/10.1093/aob/mcz030