Li et al. (2026) Spectral albedo, vegetation greenness, and radiative forcing responses of the Amazon to drought and wet conditions from 2005 to 2016

Identification

• Journal: Agricultural and Forest Meteorology
• Year: 2026
• Date: 2026-03-09
• Authors: Shaopeng Li, Isabel F. Trigo, Xiongxin Xiao, S. Wunderle
• DOI: 10.1016/j.agrformet.2026.111113

Research Groups

• Institute of Geography, University of Bern, Switzerland
• Oeschger Centre for Climate Change Research, University of Bern, Switzerland
• Instituto Portuguˆes do Mar e da Atmosfera (IPMA), Lisbon, Portugal
• Instituto Dom Luiz (IDL), Faculdade de Ciˆencias, Universidade de Lisboa, Portugal

Short Summary

This study investigates the responses of spectral albedo, vegetation greenness, and albedo-driven radiative forcing to drought and wet conditions in the Amazon (2005-2016) across evergreen broadleaf forest, grassland, and savannas. It finds that visible and shortwave albedo negatively correlate with wetness over grasslands and savannas, while evergreen broadleaf forests show less pronounced and more complex responses, with significant implications for surface radiative forcing.

Objective

• To investigate the effects of hydroclimatic variability (drought and wet conditions) on spectral albedo (visible, near-infrared, shortwave), vegetation greenness dynamics, and albedo-driven radiative forcing across three major Amazonian land surface types: evergreen broadleaf forest (EBF), grassland (GRA), and savannas (SAV).

Study Configuration

• Spatial Scale: Amazon basin, extending from 10° N to 20° S and 80° W to 45° W. Analysis focused on three predominant land surface types: evergreen broadleaf forest (~80%), savannas (~8%), and grassland (~7%).
• Temporal Scale: 2005 to 2016 (12 years).

Methodology and Data

• Models used:
  • FAO-56 Penman-Monteith formulation (for potential evapotranspiration in SPEI calculation).
  • Quantile regression (for analyzing relationships between SPEI and albedo anomalies).
• Data sources:
  • Land Cover: MCD12C1 IGBP classification (MODIS, 0.05° spatial resolution, yearly).
  • Hydroclimatic: Standardized Precipitation Evapotranspiration Index (SPEI, 6-month accumulation period, 0.5° spatial resolution, monthly).
  • Radiation: Clouds and the Earth’s Radiant Energy System (CERES) Energy Balanced And Filled (EBAF) Surface product (version 4.2.1, 1° spatial resolution, monthly mean downwelling shortwave radiation).
  • Albedo:
    • MODIS MCD43C3 (0.05° spatial resolution, daily, aggregated to monthly).
    • C3S (SPOT/VEGETATION and PROBA-V, 1 km spatial resolution, 10-day, resampled to 0.05° and aggregated to monthly).
    • GAC43 (AVHRR, 0.05° spatial resolution, 8-day, aggregated to monthly).
  • Vegetation Indices:
    • GIMMS3g NDVI (Global Inventory Modeling and Mapping Studies-3rd v1.2, 0.0833° spatial resolution, twice per month, aggregated to monthly).
    • MOD13C2 NDVI (MODIS, 0.05° spatial resolution, monthly).
    • MOD13C2 EVI (MODIS, 0.05° spatial resolution, monthly).
  • Ancillary Data (for context/comparison):
    • CHIRPS v2.0 (Climate Hazards Group InfraRed Precipitation with Station data, for precipitation).
    • ERA5 (ECMWF Reanalysis v5, for 2 m air temperature).
    • GOSIF (Solar-Induced Chlorophyll Fluorescence, monthly, for vegetation functional status).
    • GRACE (Gravity Recovery And Climate Experiment, for terrestrial water storage comparison).

Main Results

• Hydroclimatic Conditions: Major drought years were 2005, 2010, 2015, and 2016, with 2015-2016 being the most severe and widespread. Extensive wet conditions occurred in 2006, 2008, and 2009.
• Spectral Albedo Responses:
  • Grassland (GRA) and Savannas (SAV): Visible (VIS) and shortwave albedo exhibited strong, significant negative correlations with SPEI, meaning drier conditions led to increased albedo. Near-infrared (NIR) albedo responses were more complex and product-dependent, with some products showing elevated NIR albedo under drought, potentially due to increased exposed soil.
  • Evergreen Broadleaf Forest (EBF): VIS and shortwave albedo responses were much less pronounced. NIR albedo consistently showed negative correlations with SPEI across all products, suggesting wetter conditions are associated with reduced NIR reflectance.
  • Regional Differences (EBF): In the northern Amazon, NIR albedo anomalies predominantly showed negative correlations with SPEI, while VIS albedo anomalies showed positive correlations. In the southern Amazon, almost all three albedo products exhibited consistently negative anomalies across NIR, VIS, and shortwave bands under both drought and humid conditions.
  • Quantile Regression: Confirmed significant negative relationships between VIS/shortwave albedo and SPEI over GRA and SAV. EBF surfaces showed the weakest albedo sensitivity to SPEI but the most robust responses. Shortwave albedo responses at EBF were primarily driven by NIR albedo, while at GRA and SAV, VIS albedo had a greater influence.
• Vegetation Greenness Responses:
  • GRA and SAV: All three vegetation greenness indices (GIMMS3g NDVI, MOD13C2 NDVI, MOD13C2 EVI) displayed significant positive correlations with SPEI, indicating increased greenness with wetter conditions. GRA showed the highest sensitivity, followed by SAV.
  • EBF: Responses were weaker and more complex. MODIS vegetation indices (MOD13C2 NDVI and EVI) showed reductions under extremely wet conditions (SPEI > 2.0) in the northern Amazon, suggesting radiation limitation rather than water availability as a growth factor. GOSIF Solar-Induced Chlorophyll Fluorescence (SIF) corroborated these patterns.
• Albedo-driven Radiative Forcing:
  • Humid Conditions: All three land-cover types exhibited consistently positive radiative forcing (warming effect): SAV (0.3–0.6 W/m²), GRA (0.1–0.3 W/m²), and EBF (0.03–0.2 W/m²).
  • Drought Conditions: Pronounced negative radiative forcing (cooling effect) was observed over GRA and SAV due to drought-induced albedo increases. Over EBF, GAC43 and C3S indicated positive radiative forcing during drought, while MCD43C3 showed slightly negative values.
  • Radiative forcing associated with albedo changes was relatively limited over EBF but significantly stronger over GRA and SAV.

Contributions

• Provides the first comprehensive assessment of the impacts of diverse hydroclimatic conditions (drought and wet) on spectral albedo across its visible, near-infrared, and shortwave domains in the Amazon.
• Quantifies the coupled responses of spectral albedo, vegetation greenness, and albedo-driven radiative forcing across major Amazonian land surface types (EBF, GRA, SAV).
• Offers an intercomparison of multiple satellite-derived albedo products (MODIS, C3S, AVHRR GAC43), highlighting product-specific differences and their potential causes (e.g., sensor spectral configurations, cloud/aerosol algorithms).
• Elucidates the complex, surface- and context-dependent nature of spectral albedo responses, including counter-intuitive NIR albedo increases in grasslands under drought (attributed to exposed soil) and distinct regional patterns in savannas and EBF.
• Advances the understanding of land-atmosphere interactions in the Amazon under hydroclimatic extremes and provides valuable insights for improving land surface models.

Funding

• China Scholarship Council

Citation

@article{Li2026Spectral,
  author = {Li, Shaopeng and Trigo, Isabel F. and Xiao, Xiongxin and Wunderle, S.},
  title = {Spectral albedo, vegetation greenness, and radiative forcing responses of the Amazon to drought and wet conditions from 2005 to 2016},
  journal = {Agricultural and Forest Meteorology},
  year = {2026},
  doi = {10.1016/j.agrformet.2026.111113},
  url = {https://doi.org/10.1016/j.agrformet.2026.111113}
}