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We study the yearly values of the length of day (LOD, 1623-2016) and its linkto the zonal index (ZI, 1873-2003), the Northern Atlantic oscillation index(NAO, 1659-2000) and the global sea surface temperature (SST, 1850-2016). LODis herein assumed to be mostly the result of the overall circulations occurringwithin the ocean-atmospheric system. We find that LOD is negatively correlatedwith the global SST and with both the integral function of ZI and NAO, whichare labeled as IZI and INAO. A first result is that LOD must be driven by aclimatic change induced by an external (e.g. solar/astronomical) forcing sinceinternal variability alone would have likely induced a positive correlationamong the same variables because of the conservation of the Earth's angularmomentum. A second result is that the high correlation among the variablesimplies that the LOD and INAO records can be adopted as global proxies toreconstruct past climate change. Tentative global SST reconstructions since the17th century suggest that around 1700, that is during the coolest period of theLittle Ice Age (LIA), SST could have been about 1.0-1.5 {\deg}C cooler than the1950-1980 period. This estimated LIA cooling is greater than what somemultiproxy global climate reconstructions suggested, but it is in goodagreement with other more recent climate reconstructions including those basedon borehole temperature data.

The Little Ice Age was 1.0-1.5 {\deg}C cooler than current warm period according to LOD and NAO