Climate Mode Covariability and Climate shifts

It has previously been shown from the collective behavior of a network of observed climate indices that this network synchronized several times in the period 1900–2000. Further, it has been found that in those cases where the synchronous state was followed by a steady increase in the coupling strength between the indices, the synchronous state was destroyed, after which a new climate state emerged. These shifts are associated with significant changes in global temperature trend and in El Niño/Southern Oscillation variability. Subsequently, the evidence for such type of behavior has been found to occur in three climate simulations using stateof-the-art models as well as in the observed data in the 21st century. This was the first time that this mechanism, which appears consistent with the theory of synchronized chaos, has been discovered in a physical system of the size and complexity of the climate system. Here we extend this approach to consider proxy data for climate modes going back several centuries. While noise in the proxy data in some cases masks the mechanism, we find significant coherence between both synchronization and coupling and global temperature. These results provide further support that the above mechanism for climate shifts is a robust feature of the climate system.