Orbital Forcing Strongly Influences Seasonal Temperature Trends During the Last Millennium

Insolation changes caused by the axial precession induce millennial trends in last millennium temperature, varying with season and latitude. A characteristic seasonal trend pattern can be detected in both insolation and modeled surface temperature response. In the extratropical Northern Hemisphere, the maximum insolation trend occurs around April/May, while the minimum trend occurs between July and September. The temperature trend lags behind insolation trend by around a month. Hence orbital forcing potentially affects long-term trends in proxy data, which are often sensitive to a distinct seasonal window. We find that tree-ring reconstructions based on early growing season dominated records show different millennial trends from those for late summer dominated proxies. The differential response is similar to that seen in pseudo proxy reconstructions when considering proxy seasonality. This suggests that orbital forcing has influenced long-term trends in climate proxies. It is therefore vital to use seasonally homogeneous data for reconstructing multicentennial variability.