Wildfires release enormous quantities of carbonaceous aerosols into the atmosphere. These tiny particles can perturb cloud microphysics, resulting in changes in cloud radiative effects. However, the impact of biomass burning aerosols on cirrus clouds, which play an important role in the Earth's energy budget, remains poorly understood. Here we present in situ measurements of cirrus clouds affected by biomass burning aerosols during the 2018 FIREX-AQ campaign. We find that biomass burning aerosols can modify ice crystal habits, leading to an increase in the effective radius and ice water content of cirrus clouds. These modified cirrus clouds show enhanced absorption of solar radiation and reduced longwave cooling to space, indicating a potential warming effect. Our results highlight the importance of considering biomass burning aerosols in climate models to better predict their impact on the Earth's energy budget.