Decadal sink-source shifts of forest aboveground carbon since 1988

TL;DR

This study reconstructs a global forest aboveground carbon record from 1988 to 2021, revealing a shift from carbon sinks to sources in tropical and boreal forests, with implications for climate change feedbacks.

Contribution

It introduces a probabilistic deep learning approach to create an uncertainty-aware, harmonized global forest carbon dataset spanning over three decades.

Findings

Moist tropical and boreal forests are becoming carbon sources since early 2000s.

Tropical AGC variability increasingly modulates global carbon cycle, with stronger negative correlation to CO2 growth.

Intact forests in the Brazilian Amazon contribute more to AGC variation over time, surpassing deforested areas.

Abstract

Forest ecosystems are vital to the global carbon cycle, yet their long-term aboveground carbon (AGC) dynamics remain uncertain. Here, we integrate multi-source satellite observations with probabilistic deep learning models to reconstruct a harmonized, uncertainty-aware global forest AGC record from 1988 to 2021 at 0.25-deg. We find that, although global forests sequestered 6.2 PgC, moist tropical and boreal forests have progressively transitioned toward carbon sources since the early 2000s. This shift coincides with a strengthening negative correlation between tropical AGC variability and atmospheric CO2 growth rates (r = -0.63 in 2011-2021), suggesting tropical forests increasingly modulate the global carbon cycle. Notably, in the Brazilian Amazon, the contribution of intact forests to the year-to-year variations in AGC losses increased from 33% in the 1990s to 76% in the 2010s, surpassing that of deforested areas (from 60% to 13%). Our findings highlight the vulnerability of carbon stocks in key biomes and provide a benchmark to track emerging sink-source shifts under anthropogenic climate change.