CMIP5 models predict rapid and deep soil warming over the 21st century

Authors: SOONG, JENNY - Lawrence Berkeley National Laboratory; Phillips, Claire; LEDNA, CATHERINE - University Of California; KOVEN, CHARLIE - Lawrence Berkeley National Laboratory; TOM, MARGARET - Lawrence Berkeley National Laboratory

Submitted to: Journal of Geophysical Research-Biogeosciences
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/27/2019
Publication Date: 1/5/2020
Citation: Soong, J., Phillips, C.L., Ledna, C., Koven, C.D., Tom, M. 2020. CMIP5 models predict rapid and deep soil warming over the 21st century. Journal of Geophysical Research-Biogeosciences. 125(2). https://doi.org/10.1029/2019JG005266.
DOI: https://doi.org/10.1029/2019JG005266

Interpretive Summary: This study summarized predictions of 21st century soil warming from 14 global models participating in the Coupled Model Intercomparison Project Phase 5 (CMIP5). The goal of the project was to evaluate if air warming is a good proxy for soil warming, and to determine if warming of deep soil (at 1 m depth) will lag behind warming of near-surface soil (at approximately ~1 cm depth). The models predicted that deep and near-surface soils will warm at almost exactly the same rate everywhere. The models also predicted that soil warming will keep pace with air warming in tropical and dry regions, but lag air warming in colder regions where snow and ice occur. Although freezing conditions impeded soil warming, high latitude soils were still projected to warm faster than elsewhere, albeit at slower rates than the surface air above them. When compared with observations, the models were able to capture soil thermal dynamics in most biomes, but some failed to recreate thermal properties in permafrost regions. These results indicate that in cold regions, using soil rather than air warming projections may improve predictions of temperature- sensitive soil processes related to agriculture and ecology.

Technical Abstract: Despite the fundamental importance of soil temperature for Earth’s carbon and energy budgets, ecosystem functioning, and agricultural production, studies of climate change impacts on soil processes have mainly relied on air temperatures, assuming they are an accurate proxy for soil temperatures. We evaluated changes in soil temperature, air temperature, and soil moisture predicted over the 21st century for 14 models in the Coupled Model Intercomparison Project Phase 5 (CMIP5). The model ensemble predicted global average soil warming of 2.3 ± 0.7 and 4.5 ± 1.1 ºC by the end of the 21st century for RCPs 4.5 and 8.5, respectively. Deep (100 cm) and near-surface (~1 cm) soils warmed at almost exactly the same rate everywhere. Globally-averaged soil warming was slightly slower than air warming above it, and this difference increased over the 21st century. Regionally, soil warming was projected to keep pace with air warming in tropical and dry regions but lag air warming in colder regions. Thus, air warming is not necessarily a good proxy for soil warming in cold regions where snow and ice impede the direct transfer of sensible heat from the atmosphere to soil. Despite this effect, high latitude soils were still projected to warm faster than elsewhere, albeit at slower rates than the surface air above them. When compared with observations, models were able to capture soil thermal dynamics in most biomes, but some failed to recreate thermal properties in permafrost regions. Particularly in cold regions, soil warming rather than air warming projections may improve predictions of temperature sensitive processes related to agriculture, ecosystem processes, and soil feedbacks to climate change.