Large-scale afforestation impact on precipitation seasonality in global monsoonregions

Afforestation and reforestation (A/R) are widely promoted as cost-effective climate change mitigation strategies. In policy discussions, the focus often lies primarily on the local carbon sequestration potential, while potential side effects relevant for mitigation and adaptation are often overlooked. Forests, however, also exert strong biogeophysical impacts on the climate system, affecting the land-atmosphere coupling through changes in surface albedo, surface roughness, and evapotranspiration. Such processes may alter the local and remote hydrological cycle. This is particularly critical for monsoon regions, which are home to nearly one third of the global population and strongly depend on seasonal rainfall for agriculture.

Here, we assess whether and how large-scale afforestation affects precipitation seasonality and wet-season characteristics in global monsoon systems using fully coupled, emission-driven Earth system model (CESM2.1.5) simulations. We compare a large-scale A/R scenario initiated in 2025 with a reference scenario without land-use and land-cover change, both conducted under an SSP2-4.5 forcing pathway and evaluated for late-century conditions (2071 - 2100). Attribution of the simulated precipitation responses is explored using global warming level comparisons and optimal fingerprinting. These approaches indicate that precipitation responses cannot be fully explained by greenhouse-gas forcing alone, suggesting a contribution from biogeophysical-driven processes and associated feedback within the emission-driven framework.

To robustly quantify changes in monsoon timing, we apply an onset and cessation detection method developed by Dunning et al. (2016) that allows explicit identification of wet-season start, end, and duration, rather than relying on fixed seasonal averages over multiple months. This enables an assessment of agriculturally relevant precipitation metrics, including wet-season length, shifts in onset and cessation, and changes in rainfall distribution within the rainy season. An analysis of general precipitation regimes (wet-year-round, dry-year-round, annual, and biannual regimes) indicates no major spatial shifts in their global distribution. However, our results reveal regionally heterogeneous responses to afforestation within the annual regimes. The onset of the rainy season advances over South America and the Sahel but is delayed over Australia. Cessation shifts toward earlier dates over India and later dates over Australia, indicating both wet-season shortening and lengthening across monsoon domains. These findings suggest that afforestation can modify precipitation seasonality beyond greenhouse-gas–driven changes, with important implications for water availability and food security in monsoon-dependent regions.