Climatic Warming Increases Spatial Synchrony in Spring Vegetation Phenology Across the Northern Hemisphere

Climatic warming has advanced spring phenology across the Northern Hemisphere, but the spatial variability in temperature sensitivity of spring phenology is substantial. Whether spring phenology will continue to advance uniformly at latitudes has not yet been investigated. We used Bayesian model averaging and four spring phenology models, and demonstrated that the start of vegetation growing season across the Northern Hemisphere will become substantially more synchronous (up to 11.3%) under future climatic warming conditions. Larger start of growing season advances are expected at higher than lower latitudes (3.7-10.9 days earlier) due to both larger rate in spring warming at higher latitudes and larger decreases in the temperature sensitivity of start of growing season at low latitudes. The consequent impacts on the northern ecosystems due to this increased synchrony may be considerable and thus worth investigating. Plain Language Summary Recent climatic warming has not only triggered a notable advance of spring phenology over the past decades but also changed the spatial pattern of its temperature sensitivity. Whether the shifts in spring phenology would continue to follow the latitude gradient remains unclear. Based on simulations over the end of this century, we concluded that the advance of spring phenology at high latitudes is larger than the low latitudes, consequently resulting in more synchronous spring phenology. Further analysis suggests that besides greater spring warming at high latitudes, the larger decrease in temperature sensitivity of spring phenology possibly due to shorter day length and chilling loss at low latitudes also contributes to this finding. Our study, therefore, reports the possibility of improving phenological modules in dynamic vegetation models and thus promoting our understanding of the response of northern ecosystem to ongoing climate change.