Understanding the dynamics and variability of the paleo Asian Summer Monsoon (ASM) system is crucial to discuss spatiotemporal monsoon rainfall patterns in Asia and future climate projections.

However, ambiguities exist because paleoclimate records currently lack sufficient geographic details in ASM influenced regions, especially the eastern Tibetan Plateau (TP).

Recently, a research team led by Prof. ZHOU Weijian from the Institute of Earth Environment of the Chinese Academy of Sciences (IEECAS) investigated a multi-parameter, absolute-dated lacustrine record from the eastern Tibetan Plateau (RG-2018), which provided a continuous history of ASM over the past ~6000 years. Their study was published in Climate Dynamics.

They found that sediment properties (magnetic susceptibility, organic matter content, δ¹³C_org and grain size) showed that climate was relatively warmer and wetter before ~4.2 ka BP. Climate conditions became gradually drier during ~4.2-1.3 ka BP.

RG-2018 record broadly followed the trend in northern hemisphere summer insolation over the past ~6000 years. However, it diverged during a period of increased moisture and thermal conditions during the past ~1300 years, broadly corresponding to the ASM "2-kyr shift".

This shift as referred by CHENG et al. (Nature, 2016, 534:640–646) might be related to Atlantic Meridional Overturning Circulation variations, and potentially to increased forcing by greenhouse gases coincidented with reduced ENSO activity.

They observed several sub-millennial-scale weak monsoon intervals in the RG-2018 record, including the Little Ice Age centered at ~0.5 ka BP. Cross-correlation of the sub-millennial-scale monsoon precipitation record with the solar activity and ENSO records showed that most of the monsoon variability potentially resulted from changes in solar irradiation and was correlated with ENSO activity.

The RG-2018 record confirmed the theory that ASM responded rapidly to changes in solar irradiation through atmospheric processes. Internal feedbacks like the meridional migration of the ITCZ and the east-west displacement of the Walker circulation might be pivotal in enlarging these minor external fluctuations.