Cai et al. (2025) Boreal forest cover change since 2000 contributes to cold winters in Eurasia

Identification

• Journal: npj Climate and Atmospheric Science
• Year: 2025
• Date: 2025-10-06
• Authors: Lei Cai, Ji Ma, Y. Zhang, Wenwen Shi, Ruowen Yang
• DOI: 10.1038/s41612-025-01219-1

Research Groups

• State Key Laboratory of Vegetation Structure, Function and Construction (VegLab), Yunnan University, Kunming, China
• Yunnan Key Laboratory of Meteorological Disasters and Climate Resources in the Greater Mekong Subregion, Yunnan University, Kunming, China
• Lincang Meteorological Observatory, Lincang, China

Short Summary

This study investigates the impact of observed boreal forest cover change (2000-2020) on winter climate using an Earth System Model, revealing that afforestation in western Europe drives Eurasian winter cooling by enhancing snow-albedo feedback, stimulating planetary waves, and weakening the stratospheric polar vortex.

Objective

• To explore the biophysical effect of observed boreal forest cover change (2000-2020) on modulating winter climate over the arctic-boreal area and the potential mechanism behind, particularly its relationship with surface air temperature distribution and the Warm Arctic-Cold Eurasia (WACE) pattern.

Study Configuration

• Spatial Scale: Global, with a specific focus on boreal regions (40°N poleward), Eurasia, western Europe, central/eastern Siberia, Arctic, and North America. Model resolution: 2 degrees (144 × 96 points).
• Temporal Scale: Forest Cover Change (FCC) data from 2000 to 2020. Simulation period from 2000 to 2020 (after a 100-year spin-up). Focus on winter (December, January, February - DJF), with critical processes identified in November.

Methodology and Data

• Models used:
  • Community Earth System Model (CESM2) with Community Land Model (CLM5) and Community Atmospheric Model (CAM6).
  • Land Use Model Intercomparison Project (LUMIP) models for comparison: BCC-CSM2-MR, CESM2, CMCC-ESM2, IPSL-CM6A-LR, NorESM2-LR, MPI-ESM1-2-LR.
• Data sources:
  • Satellite: Global Forest Watch (GFW) forest cover (1 km resolution, 2000 & 2020), MODIS Leaf Area Index (LAI), MODIS snow cover.
  • Observation/Reanalysis: ERA5 reanalysis (snowfall, forcing for CLM5), Copernicus Climate Change Service (C3S) surface albedo (SPOT-VGT and PROBA-VGT), Hadley Centre (climatological Sea Surface Temperature and sea ice data for CESM2 boundary conditions).

Main Results

• Observed boreal Forest Cover Change (FCC) from 2000 to 2020 shows deforestation in Central Siberia, Alaska, and northern Canada, contrasted by pronounced afforestation in western Europe.
• Offline simulations (land model only) show minimal winter surface air temperature (SAT) responses (<0.1 °C), primarily in deforestation regions.
• Online simulations (coupled land-atmosphere) reveal significantly amplified winter SAT responses: a dipole-like pattern with pronounced warming over the central Arctic and Greenland, and cooling across Eurasia and parts of North America.
• The average cooling over mid- and high-latitudes (40°N poleward) of Eurasia is 0.83 ± 0.37 °C, with the coldest anomalies in central/eastern Siberia, driven by boreal FCC alone.
• Atmospheric circulation responses exhibit vertically coherent anomalies characteristic of stratosphere-troposphere coupling, resembling a modified negative Arctic Oscillation phase at the surface and a positive geopotential height anomaly in the central Arctic at 200 hPa.
• The stratospheric polar vortex weakens, evidenced by positive geopotential height anomalies in high latitudes, negative in low latitudes (upper troposphere/lower stratosphere), and significant deceleration of zonal westerly winds in the stratosphere.
• Upward propagation of stationary planetary waves (Eliassen-Palm flux) is strong in mid-latitudes (30-60°N) in November and January, with significant convergence in the Arctic stratosphere, contributing to polar vortex weakening.
• Afforestation in western Europe leads to dramatic snowfall reductions in November due to positive temperature anomalies, causing precipitation to fall as rain, resulting in 13.6 ± 3.4% decreases in ground snow cover in northwest Europe.
• Afforested areas show snow cover persistence shortened by 4.9 ± 1.83 days, while deforested regions exhibit comparable lengthening.
• Surface albedo decreases by 3.1 ± 1.1% in afforested zones and increases by 5.3 ± 1.8% in deforested areas in Eurasia.
• Afforestation intensifies the snow-albedo feedback (SAF) by up to 1.15%/°C in western Europe and Scandinavia, leading to substantial warming of 1.99 ± 0.92 °C in November in northwestern Europe.
• Increased forest cover leads to greater snow interception by canopy (0.75% rise in canopy-intercepted snow for every 1% increase in forest cover) and increased snowmelt (0.29% increase for every 1% increase in forest cover) in November, even without atmospheric temperature feedback.
• LUMIP models with idealized global deforestation scenarios fail to reproduce a similar polar vortex weakening or consistent Eurasian cooling, highlighting the importance of realistic FCC patterns (including afforestation) and land-atmosphere coupling.

Contributions

• Proposes a novel mechanism for boreal FCC's impact on winter climate: afforestation in western Europe inhibits ground snow accumulation and reduces surface albedo, strengthening snow-albedo feedback, which then stimulates upward planetary waves, weakening the stratospheric polar vortex, and leading to Eurasian winter cooling via stratosphere-troposphere coupling.
• Emphasizes the critical role of land-atmosphere coupling and dynamic atmospheric circulation effects in transmitting FCC impacts during cold seasons, when radiative and evaporative effects are minimized.
• Highlights the importance of realistic spatial patterns of both forest loss and gain (afforestation vs. deforestation) in Earth System Models, demonstrating their asymmetric influences on winter atmospheric dynamics, contrasting with idealized deforestation scenarios in LUMIP models.
• Expands understanding of land cover-climate interactions by showing how regional FCC can generate hemispheric-scale winter climate impacts through a specific chain of events initiated in the shoulder season (November).

Funding

• National Science Foundation of China (42375042, 42305061)
• Fundamental Research Program of Yunnan (202401AT070429, 202301AV070001, 202302AN360006, 202201AU070042)

Citation

@article{Cai2025Boreal,
  author = {Cai, Lei and Ma, Ji and Zhang, Y. and Shi, Wenwen and Yang, Ruowen},
  title = {Boreal forest cover change since 2000 contributes to cold winters in Eurasia},
  journal = {npj Climate and Atmospheric Science},
  year = {2025},
  doi = {10.1038/s41612-025-01219-1},
  url = {https://doi.org/10.1038/s41612-025-01219-1}
}