Biomass burning increase in Southeast Asia is dominated by char black carbon

Biomass burning emits black carbon, a climate-active aerosol, with poorly constrained climate effects —models capture only 30% of observed black carbon in Southeast Asia. Here we use thermal-optical analysis and radiocarbon source attribution to separate black carbon into char and soot. We find model underestimation arises from missing char-rich emissions, which drive over 90% of regional biomass burning-related black carbon increases. Char has lower mass absorption efficiency than soot, yetmost models assign soot-like optical properties to all black carbon. Constraining simulations revises regional direct radiative forcing from 1.95Wm−2 (baseline) to 4.51Wm−2 (mass correction) and 3.71Wm−2 (mass and optical correction). Global analysis reveals nonlinearity: rising biomass burning boosts black carbon loading but reduces its absorption efficiency via char dominance, partially offsetting radiative forcing. This underscores the need to represent black carbon subtypes and their optical properties for accurate climate impact assessments.