This study presents a new parameterization for predicting global atmospheric ice nuclei (IN) distributions and their impacts on climate. The research combines observations from 14 years of field studies worldwide to show that IN concentrations in mixed-phase clouds are related to temperature and the number concentrations of particles larger than 0.5 μm in diameter. This relationship reduces the unexplained variability in IN concentrations at a given temperature from ~10³ to less than a factor of 10. When implemented in a global climate model, this parameterization significantly alters cloud liquid and ice water distributions compared to existing temperature-only parameterizations. The revised treatment indicates a global net cloud radiative forcing increase of ~1 W m⁻² for each order of magnitude increase in IN concentrations, demonstrating the strong sensitivity of climate simulations to assumptions regarding the initiation of cloud glaciation. Ice nucleation in clouds occurs via two primary pathways: homogeneous freezing of liquid particles below about -36°C and heterogeneous ice nucleation, triggered by "ice nuclei" that possess surface properties favorable to lowering the energy barrier to crystallization. The sensitivity of precipitation initiation from, and the climate forcing of, tropospheric clouds that include regions with temperatures below the freezing level to the abundance of IN has led to the proposal of an "ice indirect effect" of aerosols on climate. This effect is the impact of any scenario of altered formation and emission of ice nucleating aerosols from combustion, mechanical or biological processes following changes triggered by human activities, or due to atmospheric temperature changes. The study shows that IN number concentrations are strongly correlated with the number concentrations of particles larger than 0.5 μm in diameter. This relationship improves the accuracy of IN predictions, reducing the spread of potential errors in predicting IN concentrations at a given temperature from a factor of ~1,000 to ~10. This improvement leads to significantly more realistic and well-constrained descriptions of aerosol-ice formation relationships on relatively small time and spatial scales. The study also highlights the importance of accurately representing IN in climate models, as errors in representing IN can propagate into predicted differences in cloud microphysical properties and cloud forcing of climate. The new parameterization is a significant improvement in the representation of aerosol impacts on cold clouds, especially mixed-phase clouds at mid to high latitudes. The parameterization requires specified or prognosed fields of aerosol number concentrations for particles with diameters larger than 0.5 μm, which are now included in many current models. The study concludes that the new parameterization is a simplified version of a more complex scheme that can be readily applied in many existing cloud and climate models. The parameterization results in a global net cloud forcing change (decrease) of 1.3 W m⁻² compared to the Meyers et al. scheme. The strong sensitivity of climate forcing to IN suggests that long-range import of IN from dust storms, boreal biomass burning, and anthropogenic pollution could lead to feedbacksThis study presents a new parameterization for predicting global atmospheric ice nuclei (IN) distributions and their impacts on climate. The research combines observations from 14 years of field studies worldwide to show that IN concentrations in mixed-phase clouds are related to temperature and the number concentrations of particles larger than 0.5 μm in diameter. This relationship reduces the unexplained variability in IN concentrations at a given temperature from ~10³ to less than a factor of 10. When implemented in a global climate model, this parameterization significantly alters cloud liquid and ice water distributions compared to existing temperature-only parameterizations. The revised treatment indicates a global net cloud radiative forcing increase of ~1 W m⁻² for each order of magnitude increase in IN concentrations, demonstrating the strong sensitivity of climate simulations to assumptions regarding the initiation of cloud glaciation. Ice nucleation in clouds occurs via two primary pathways: homogeneous freezing of liquid particles below about -36°C and heterogeneous ice nucleation, triggered by "ice nuclei" that possess surface properties favorable to lowering the energy barrier to crystallization. The sensitivity of precipitation initiation from, and the climate forcing of, tropospheric clouds that include regions with temperatures below the freezing level to the abundance of IN has led to the proposal of an "ice indirect effect" of aerosols on climate. This effect is the impact of any scenario of altered formation and emission of ice nucleating aerosols from combustion, mechanical or biological processes following changes triggered by human activities, or due to atmospheric temperature changes. The study shows that IN number concentrations are strongly correlated with the number concentrations of particles larger than 0.5 μm in diameter. This relationship improves the accuracy of IN predictions, reducing the spread of potential errors in predicting IN concentrations at a given temperature from a factor of ~1,000 to ~10. This improvement leads to significantly more realistic and well-constrained descriptions of aerosol-ice formation relationships on relatively small time and spatial scales. The study also highlights the importance of accurately representing IN in climate models, as errors in representing IN can propagate into predicted differences in cloud microphysical properties and cloud forcing of climate. The new parameterization is a significant improvement in the representation of aerosol impacts on cold clouds, especially mixed-phase clouds at mid to high latitudes. The parameterization requires specified or prognosed fields of aerosol number concentrations for particles with diameters larger than 0.5 μm, which are now included in many current models. The study concludes that the new parameterization is a simplified version of a more complex scheme that can be readily applied in many existing cloud and climate models. The parameterization results in a global net cloud forcing change (decrease) of 1.3 W m⁻² compared to the Meyers et al. scheme. The strong sensitivity of climate forcing to IN suggests that long-range import of IN from dust storms, boreal biomass burning, and anthropogenic pollution could lead to feedbacks