Increased Northern Hemisphere vegetation cover as a potential solution to the Holocene Temperature Conundrum

Abstract:

While global temperature proxies suggest warmer global mean surface temperatures during the early and middle Holocene relative to the Pre-industrial era (PI), transient climate model simulations predict cooler relative temperatures. Whether this model-proxy discrepancy, known as the Holocene Temperature Conundrum, stems from problems with the proxy record or climate models is open to debate. One line of evidence suggests that the problem may exist in climate models as they have thus far failed to account for the increase in early and middle Holocene Northern Hemisphere vegetation, specifically in the African Sahara, mid-latitudes, and Arctic, suggested by pollen records. Here, we investigate the extent to which increases in Northern Hemisphere vegetation resolve the Holocene Temperature Conundrum by using the fully coupled iCESM1 to perform time-slice simulations at 3,000-year intervals from 12 ka BP to the PI with various prescribed vegetation schemes. We then compare our simulated surface temperature with the up-to-date Temperature12k proxy dataset. In control simulations with PI vegetation, we find that disagreement between models and proxies remains, as simulated temperatures for the early and middle Holocene fall outside of the 95^th percentile range of proxy-inferred temperatures. However, when Northern Hemisphere vegetation increases in the early and middle Holocene, providing a better match with pollen estimates, simulated temperature warms. This simulated warming improves model-proxy agreement to within the 95^th percentile range of proxy temperatures, both globally and at 30° latitudinal bands. This research has broad implications for the impact of Northern Hemispheric vegetational change on regional and global temperature within climate models and in resolving the Holocene Temperature Conundrum.