Last Glacial Maximum and Mid-Holocene Thermal Growing Season Simulations

The growing season-the number of days per year during which the climate favors plant growth -is an important factor for describing climate. Based on daily data of all available numerical experiments from the Paleoclimate Modeling Intercomparison Project phases two and three, we examine the growing season over global ice-free land for the last glacial maximum and mid-Holocene to improve our understanding of glacial and interglacial climates. Compared to the preindustrial period, the last glacial maximum growing season was shortened at midlatitude and high latitude and by 26.1 days globally in terms of nine climate models due to a later start and an earlier end. Growing degree days above 5 degrees C (GDD5) decreased by 24% due to both cooling and shortening of the growing season, agreeing with pollen-based reconstructions of globally decreased signal and spatial pattern of GDD5 changes at 88 grid cells. Twenty-four models indicate that the mid-Holocene growing season start and end dates and length were similar to preindustrial states globally but featured a latitude-varying difference. The length increased north of similar to 50 degrees N and decreased in much of the area from 20-50 degrees N owing to variations in both the start and end dates. In response to orbital forcing, both the growing season temperature and GDD5 increased at northern midlatitude and high latitude but decreased elsewhere. The GDD5 simulations agreed in sign with the reconstructions at 516 grid cells more often than not, though their spatial patterns were different to some extent.