Revised direct radiative forcing of airborne microplastics suggests warming

Microplastics are a common airborne pollutant, capable of being transported over large distances and with potential to alter the radiative properties of the atmosphere in large enough quantities, with implications for global climate. Since the first estimates of microplastic radiative forcing were published in 2021, new constraints on many aspects of the spatial and temporal distribution of airborne plastics and their physical and optical properties have emerged. Using these updated findings concerning the size distribution of airborne microplastics, new spectral absorption data for coloured plastics, and a new microplastic emissions dataset, we investigate the sensitivity of the direct radiative effect of airborne microplastics. We find clear-sky effective radiative forcings ranging from −3.28 to +46.7 mW m −2 depending on assumptions concerning colour, size distribution, vertical profile and spatial distribution. Assuming likely real-world conditions—a mix of coloured microplastics, a power law size distribution and distributing microplastics non-uniformly through the troposphere based on a new emissions dataset—we calculate an updated estimate of +42.1 ± 4.62 mW m −2 for a global-average surface concentration of 3.99 microplastics m −3 . This revised radiative forcing is at least as large in forcing magnitude as the contributions of a number of other species routinely evaluated by the Intergovernmental Panel on Climate Change. Our findings suggest that microplastics are likely contributing to atmospheric warming and, given that plastic pollution is projected to increase, should be included in climate change projections.