A document by Luke A Parsons. Click on the document to view its contents.

Explosive volcanism imposes impulse-like radiative forcing on the climate system, providing a natural experiment to study the climate response to perturbation. Previous studies have identified disagreements between paleoclimate reconstructions and climate model simulations (GCMs) with respect to the magnitude and recovery from volcanic cooling, questioning the fidelity of GCMs, reconstructions, or both. Using the paleoenvironmental data assimilation framework of the Last Millennium Reanalysis, this study investigates the causes of the disagreements, using both real and simulated data. We demonstrate that discrepancies since 1600 CE can be largely resolved by assimilating tree-ring density records only, targeting growing-season temperature instead of annual temperature, and performing the comparison at proxy locales. Simulations of eruptions earlier in the last millennium may also reflect uncertainties in forcing and modeled aerosol microphysics.

Arctic sea-ice extent (SIE) has declined drastically in recent decades, yet its evolution prior to the satellite era is highly uncertain. Studies using SIE observations find little variability prior to the 1970s; however, these reconstructions are based on limited data, especially prior to the 1950s. We use ensemble Kalman filter data assimilation of surface air temperature observations with Last Millennium climate model simulations to create a fully gridded Arctic sea-ice concentration reconstruction from 1850–2018, and investigate the evolution of Arctic SIE during this period. We find a decline of ~1.25 x 106 km2 during the early 20th-century warming (1910-1940). The 25-year trends during this period are ~33-38% smaller than the satellite era (1979-2018) but almost twice as large as previous estimates. Additionally we find that variability of SIE on decadal timescales prior to satellite era is ~40% greater than previously estimated.

We use online data assimilation to combine information from a linear inverse model of coupled atmosphere-ocean dynamics with proxy records to create a new annual-resolution reconstruction of atmosphere and ocean fields over the last millennium. Instrumental validation of reconstructed sea-surface temperature and 0-700 m ocean heat content shows broad regions of positive spatial correlations, and high correlations (~0.6-0.9) for global averages and indices of large-scale modes of atmospheric variability. Compared to previous reconstructions, the online reconstructions show global and hemispheric averages with little-to-no millennial-scale trend and global-mean temperatures ~0.25-0.5 K cooler during early periods (1000-1400 C.E.). The spatial anomaly differences of average temperature between an early (1000-1250 C.E.) and later (1400-1700 C.E.) period show warm anomalies over high-latitude Europe and cool tropical conditions in partial agreement with previous assessments. The addition of online data assimilation, which provides dynamical memory to climate proxy information, is shown to be crucial for adequately characterizing decadal-to-centennial-scale variability of 0–700 m ocean heat content. Furthermore, the climate forecasts provide model-based physical constraints for atmosphere-ocean interaction, which become increasingly important during early periods when less proxy information is available for assimilation.