Orbital Forcing Strongly Influences Seasonal Temperature Trends During the Last Millennium. Issue 4 (22nd February 2021)

• Record Type: Journal Article
• Authors: Lücke, Lucie J.; Schurer, Andrew P.; Wilson, Rob; Hegerl, Gabriele C.
• Is Part Of: Geophysical research letters. Volume 48:Issue 4(2021)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2021-02-22
• Subjects: climate variability -- insolation -- last millennium -- long‐term trends -- orbital forcing -- proxy reconstructions; Geophysics -- Periodicals; Planets -- Periodicals; Lunar geology -- Periodicals; 550
• DOI: 10.1029/2020GL088776
• Languages: English
• ISSNs: 0094-8276

Abstract

Abstract: 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. Plain Language Summary: The Earth's axis spins like a slightly wobbly gyroscope, with one spin taking around 24, 000 years. Depending on the phase of the spin, more or less sunlight comes in during the different seasons, impacting the Earth's climate. This is orbital forcing. 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. These patterns are very different from other natural climate forcings‐like volcanic eruptions or changes of the Sun's energy output. We have also found evidence of orbital forcing in reconstructions of past climate using natural climate archives such as tree rings and other climate proxies. These findings can potentially resolve previous inconsistencies in long‐term climate change found in proxy reconstructions and climate model output. Key Points: Orbital forcing induces a strong seasonal fingerprint in climate model simulations, which is distinct from any other forcings Over the last millennium, surface temperature trends simulated by climate models lag asymmetrically behind the insolation by around a month Differences between long‐term trends of proxy records could be caused by seasonal orbital forcing, which influences climate reconstructions