Abstract
AbstractInsolation 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.
Highlights
It is a unique property of orbital forcing that it can be reconstructed over very long timescales with almost complete certainty, as the variation of the orbital parameters is mathematically well understood
Analyzing temperature data produced by global climate models, we found that orbital forcing causes a distinct seasonal trend pattern over the last millennium, which should influence global and regional climate
We investigate the effect of the seasonality of insolation trends due to orbital forcing on simulated and reconstructed temperature over the preindustrial last millennium (AD 850–1850)
Summary
It is a unique property of orbital forcing that it can be reconstructed over very long timescales with almost complete certainty, as the variation of the orbital parameters is mathematically well understood. Phipps et al (2013) found that simulations which are only subjected to orbital forcing could not replicate the long-term trends previously observed by Esper et al (2012) and Kaufman et al (2009) for global annual mean temperature They suggest that seasonal or geographical biases in proxy reconstructions could potentially be responsible for the larger trends, as proxy reconstructions tend to be more representative of higher latitudes and some records are biased toward summer. Klippel et al (2020) investigated the magnitude of the trend displayed by proxy records and their inherent large-scale properties including latitude (mid vs high) or seasonal sensitivity (summer vs annual temperature), but found no significant relationship They found that millennial trends in ice cores, marine, and lake sediments were stronger than in tree-rings. Our results could resolve certain model-proxy discrepancies, and provide an explanation for discrepancies between different proxy types, such as ring width and density data
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