Black carbon (BC) strongly absorbs solar radiation, but its climate warming effect is not well quantified. A major challenge is accurately assessing BC light absorption after it mixes with non-BC components. There is a significant gap between observations and models in estimating BC light absorption, indicating a lack of understanding of BC complexity. This study presents comprehensive in situ measurements of BC single-particle microphysics, including size, coating amounts, density, and shape, along with optical closure calculations. The observed heterogeneities in size and coating explain only 20% and 30% of the lower observed BC absorption, respectively. The remaining gap is attributed to the off-center position of BC cores in fully aged spherical BC-containing particles. Global climate model assessments show that fully accounting for the observed BC complexity in aerosol microphysics reduces global BC direct radiative forcing by up to 23%. This highlights the importance of considering BC microphysical complexity in climate models to improve the accuracy of radiative forcing estimates. The study underscores the need for more detailed characterization of BC particles to better understand their climate impact.Black carbon (BC) strongly absorbs solar radiation, but its climate warming effect is not well quantified. A major challenge is accurately assessing BC light absorption after it mixes with non-BC components. There is a significant gap between observations and models in estimating BC light absorption, indicating a lack of understanding of BC complexity. This study presents comprehensive in situ measurements of BC single-particle microphysics, including size, coating amounts, density, and shape, along with optical closure calculations. The observed heterogeneities in size and coating explain only 20% and 30% of the lower observed BC absorption, respectively. The remaining gap is attributed to the off-center position of BC cores in fully aged spherical BC-containing particles. Global climate model assessments show that fully accounting for the observed BC complexity in aerosol microphysics reduces global BC direct radiative forcing by up to 23%. This highlights the importance of considering BC microphysical complexity in climate models to improve the accuracy of radiative forcing estimates. The study underscores the need for more detailed characterization of BC particles to better understand their climate impact.