Superimposed on this interannual to multicentennial variability, N-scan reveals a long-term cooling trend of −0.31 °C per 1,000 years (±0.03 °C) over the 138 BC–AD 1900 period (the dashed red curve in Fig. 2) in line with evidence from low-resolution Holocene proxies^{2, 3}. The cooling trend, representing a −0.34 °C temperature difference between the first and second millennium AD