An Absence of Early Interglacial Warmth in Global Mean Annual Temperature Reconstructions

Abstract:

Reconstructions of global surface temperature showing early interglacial warmth followed by cooling are inconsistent with reconstructions of sea level from the last and current interglacials. We suggest that the apparent discrepancy is because most proxy temperature reconstructions reflect the evolution of seasonal temperatures and do not truly record the annual state of global climate. To resolve this, we propose a method for converting seasonally biased SST to mean annual temperatures. Our new method indicates a lack of a thermal maximum in mean annual temperature during the first half of the last and current interglacials. We show that global mean annual temperatures have been rising since the early Holocene, first in response to retreating ice sheets (12-6 ka) and then in response to rising greenhouse gas concentrations (6-0 ka). Mean annual temperatures during the last interglacial (LIG), in contrast, were stable but warmer; LIG mean annual temperatures were on average 0.43 ± 0.36°C warmer than the highest temperatures observed during the Holocene. As mean annual insolation and GHG forcing are not sufficient to explain the observed offset, we attribute LIG warmth to differences in ice extent and thus the strength of the ice-albedo feedback at the start of the last and current interglacial periods. Remnant glacial ice sheets persisted through the first half of the Holocene, whereas ice sheets had already reached their modern extent prior to the start of the LIG. We propose that the faster rate of sea-level rise during the penultimate deglaciation could be related to the initial oceanic and cryospheric conditions in the preceding glaciations and the greater rate of change of northern hemisphere summer insolation due to the high eccentricity state. Thus, a seasonal origin for interglacial thermal maximum does not negate the role of northern hemisphere summer insolation as the pacemaker of the ice ages.