Insights into Low-frequency Climate Dynamics from a Surface Temperature Reconstruction Spanning the Last 2,000 Years

Abstract:

Reconstructions of surface temperature over the past 2000 years extend our knowledge of temperature changes beyond the instrumental era, and thus allows for the characterization of climate variability on multidecadal to centennial timescales. This lends insight into our understanding and quantification of the influence of exogenous and endogenous global climate variability. In this study, we do so via a set of global temperature reconstructions based on the latest incarnation of the PAGES 2k global multi-proxy database (http://www.pages-igbp.org/ini/wg/2k-network/data/phase-2-data-status). Two climate field reconstruction (CFR) methods are employed: Gaussian graphical models embedded within the regularized EM algorithm (GraphEM, Guillot et al., 2015) and Canonical Correlation Analysis (CCA, Smerdon et al., 2010).

We find a globally warm Medieval period, which was colder than the late twentieth-century by 0.5 C. With a probability of 87%, the 1961 – 1990 period was the warmest 40-year period in the past 2000 years in most regions, especially in the high latitudes of the Northern Hemisphere. We show that surface temperature has a robust large-scale cooling pattern shortly after a volcanic eruption; in particular, over the North Atlantic Ocean, the cooling can persist up to 3 years after an eruption. An El Niño-like response (~0.2 C) is also found in 2 and 3 years after an eruption. Solar irradiance forcing is found to be an important modulator of multidecadal climate variability, with the strongest solar response (0.25 C) in high latitude North America. These key features are echoed in multiple GCM simulations of the last millennium, though we find notable differences, in particular regarding the timing of the post-volcanic ENSO response, and the magnitude of the temperature response to solar irradiance forcing.

The results suggest that there is no fundamental discrepancy between simulated and reconstructed climates of the last millennium, and thus lend credibility to both.