Multi-centennial climate variability: Observational, theoretical and modeling studies

Research using long-term proxy data suggests the existence of multi-centennial climate variability in the Earth's climate system. Its origin and mechanism have been bewildering for climatologists and archaeologists for a long time. Considering that the variation of external forcing of the Earth' climate was relatively small from the mid-Holocene to the pre-industrial time, it is important to investigate the role of internal natural oscillation of the climate system during this period. The Atlantic meridional overturning circulation (AMOC) is thought to be the first candidate for such multi-centennial timescale variability. It is thus critical to investigate systematically the connection between the AMOC and the Earth climate system at this timescale. From the mid-Holocene to the pre-industrial time, the human civilization experienced rapid development. Historical documents in China suggest that in the past 2000 years, the historical climate in China had a low-frequency variation with 200-300 year period, which might have affected the ancient Chinese civilization. The multi-centennial variability of the AMOC may have played a role in the vax and vane of the human civilization. Currently, there is a lack of in-depth studies on the multi-centennial variability of the AMOC. This is mainly due to the following factors. First, the time period of modern instrumental observations is less than 200 years, which is not long enough to confirm the existence of the multi-centennial climate variability. Second, there is a lack of a well-recognized theory that can account for the multi-centennial variability of the AMOC. Third, it is much easier for a researcher to study the Earth climate change due to external forcing than to study the internal variability of the Earth climate system under a stable external forcing, particularly at this long timescale. Several coupled Earth climate system models have simulated the multi-centennial variability of the AMOC. However, the results from both coupled models and proxy data include comprehensive factors. To fundamentally understand the multi-centennial climate variability, a simple theoretical model is needed. Unfortunately, there is a lack of theoretical studies on the internal multi-centennial variability under the background of stable climate. In this work, we systematically review the current studies on multi-centennial climate variability from observational, theoretical and coupled modeling aspects. We hope that by proposing innovative theory and creative climate modeling approach, we can identify the intrinsic mode of multi-centennial climate variability, picture its spatial pattern, decipher the origin of its timescale, and reveal its internal mechanism. The outcome of these studies will help us understand deeply the wax and wane of the human civilization during the past several thousand years. It will also be of great significance for a better prediction of the long-term trend of future climate change.