Identifying Global‐Scale Patterns of Vegetation Change During the Last Deglaciation From Paleoclimate Networks. Issue 12 (20th December 2021)

• Record Type: Journal Article
• Authors: Adam, Moritz; Weitzel, Nils; Rehfeld, Kira
• Is Part Of: Paleoceanography and paleoclimatology. Volume 36:Issue 12(2021)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2021-12-20
• Subjects: arboreal pollen -- deglaciation -- networks -- vegetation -- hydroclimate -- model‐data comparison; Paleoceanography -- Periodicals; Paleoclimatology -- Periodicals; 551.46
• DOI: 10.1029/2021PA004265
• Languages: English
• ISSNs: 2572-4517

Abstract

Abstract: During the last deglaciation (∼19–11 ka before present), the global mean temperature increased by 3–8 K. The concurrent hydroclimate and land cover changes are not well constrained. Here, we use a pollen database to quantify global‐scale vegetation changes during this transitional period at orbital (∼10 4 years) and millennial timescales (∼10 3 years). We focus on the proportion of tree and shrub pollen, the arboreal pollen (AP) fraction. Temporal similarities over long distances are identified by a paleoclimate network approach. At the orbital scale, we find coherent AP variations in the low and mid‐latitudes which we attribute to the global climate forcing. While AP fractions predominantly increased through the deglaciation, we identify regions where AP fractions decreased. For millennial timescales, we do not observe spatially coherent similarity structures. We compare our results with networks computed from three deglacial climate simulations with the CCSM3, HadCM3, and LOVECLIM models. Networks based on simulated precipitation patterns reproduce the characteristics of the AP network. Sensitivity experiments with statistical emulators indicate that, indeed, precipitation variations explain the diagnosed patterns of vegetation change better than temperature and CO2 variations. Our findings support previous interpretations of deglacial forest evolution in the mid‐latitudes being the result of atmospheric circulation changes. The network analysis identifies differences in the vegetation‐climate‐CO2 relationship simulated by CCSM3 and HadCM3 . We conclude that network analyses are a promising tool to benchmark transient climate simulations with dynamical vegetation changes. This may result in stronger constraints of future hydroclimate and land cover changes. Plain Language Summary: We do not understand changes in rainfall and plant cover since the last ice age as good as temperature changes. Pollen is widely used to study which plants grew under which climate in the past. We check how many tree and shrub pollen, versus how many from herbs and grasses can be found in many locations. This shows how similar plant cover changes were in different regions. We find that plant cover changed similarly across all continents from the last ice age to the current warm period. During this transition, tree and shrub pollen increased while herbs and grasses decreased. However, we identify distinct regions where the change is the other way around. To understand this better, we use data from three climate models. The vegetation components of the climate models calculate how Earth's plant cover changed. By comparing the model results to pollen data, we find that the tree and shrub cover changes since the last ice age are better explained by rainfall than by temperature and carbon dioxide in the low and mid‐latitudes. Comparing the pollen data and model results in this way can help us to understand how well climate models simulate plant cover and rainfall changes. Key Points: An analysis of arboreal pollen networks shows largely coherent vegetation changes in the low and mid‐latitudes during the last deglaciation A comparison with climate simulations suggests that hydroclimate changes explain regionally anti‐correlated vegetation variations best Our work is a promising step toward process‐based benchmarking of vegetation and hydroclimate in transient simulations of the deglaciation