The Little Ice Age was 1.0–1.5 °C cooler than current warm period according to LOD and NAO

We study the yearly values of the length of day (LOD, 1623–2016) and its link to the zonal index (ZI, 1873–2003), the Northern Atlantic oscillation index (NAO, 1659–2000) and the global sea surface temperature (SST, 1850–2016). LOD is herein assumed to be mostly the result of the overall circulations occurring within the ocean-atmospheric system. We find that LOD is negatively correlated with the global SST and with both the integral function of ZI and NAO, which are labeled as IZI and INAO. A first result is that LOD must be driven by a climatic change induced by an external (e.g. solar/astronomical) forcing since internal variability alone would have likely induced a positive correlation among the same variables because of the conservation of the Earth’s angular momentum. A second result is that the high correlation among the variables implies that the LOD and INAO records can be adopted as global proxies to reconstruct past climate change. Tentative global SST reconstructions since the seventeenth century suggest that around 1700, that is during the coolest period of the Little Ice Age (LIA), SST could have been about 1.0–1.5 °C cooler than the 1950–1980 period. This estimated LIA cooling is greater than what some multiproxy global climate reconstructions suggested, but it is in good agreement with other more recent climate reconstructions including those based on borehole temperature data.