HOLOCENE GLOBAL WARMING DRIVEN BY RETREATING ICE-SHEETS AND RISING GREENHOUSE GAS CONCENTRATIONS (Invited Presentation)

Establishing a natural baseline for modern climate change is a fundamental goal for the global scientific community. As such, massive efforts have been undertaken to compile databases of records from across the globe spanning the past 10,000 years, starting with the Marcott et al. (2013) synthesis. Subsequent syntheses as recent as Kaufman et al., (2020), added hundreds of temperature sensitive proxy records, and refined the precision of these estimates, but generally agreed with the idea of warmer mean annual temperature at the early-mid Holocene followed by cooling to the present. These proxy-based reconstructions, however, are at odds with climate models that simulate robust warming throughout the Holocene in response to retreating ice sheets and rising greenhouse gas concentrations across the Holocene period. This model-data discrepancy, a. k. a. the Holocene Conundrum, suggests fundamental problems in proxy reconstructions, current climate models, or both, and inhibits an understanding of the driving forces behind recent climate changes. Here, we suggest a fundamental assumption that underlies global proxy temperature reconstructions, that most proxies record mean annual temperature rather than a particular season, is flawed. Using a new method that (1) identifies seasonal biases in individual proxy records and (2) converts seasonal to mean annual temperatures, we present a revised history of Holocene climate. We show that global mean annual temperatures have been rising since the early Holocene, first in response to retreating ice sheets and then in response to rising greenhouse gas concentrations. These findings therefore suggest the Holocene Thermal Maximum is a seasonal feature, driven by the early Holocene maximum in boreal summer insolation.