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Ice clouds are crucial to the Earth's radiation balance. They cool the Earth-atmosphere system by reflecting solar radiation back to space and warm it by blocking outgoing thermal radiation. However, there is a lack of an observation-based climatology of ice cloud properties and their radiative effects. Two active sensors, the CloudSat radar and the CALIPSO lidar, for the first time provide vertically resolved ice cloud data on a global scale. Using synergistic signals of these two sensors, it is possible to obtain both optically thin and thick ice clouds as the radar excels in probing thick clouds while the lidar is better to detect the thin ones. First, based on the CloudSat radar and CALIPSO lidar measurements, we have derived a climatology of ice cloud properties. Ice clouds cover around 50% of the Earth surface, and their global-mean optical depth, ice water path, and effective radius are approximately 2 (unitless), 109 g m⁻² and 48 μm, respectively. Ice cloud occurrence frequency not only depends on regions and seasons, but also on the types of ice clouds as defined by optical depth (τ) values. Optically thin ice clouds (τ < 3) are most frequently observed in the tropics around 15 km and in the midlatitudes below 5 km, while the thicker clouds (τ > 3) occur frequently in the tropical convective areas and along the midlatitude storm tracks. Using ice retrievals derived from combined radar-lidar measurements, we conducted radiative transfer modeling to study ice cloud radiative effects. The combined effects of ice clouds warm the earth-atmosphere system by approximately 5 W m⁻², contributed by a longwave warming effect of about 21.8 W m⁻² and a shortwave cooling effect of approximately -16.7 W m⁻². Seasonal variations of ice cloud radiative effects are evident in the midlatitudes where the net effect changes from warming during winter to cooling during summer, and the net warming effect occurs year-round in the tropics (∼ 10 W m⁻² ). Ice cloud optical depth is shown to be an important factor in determining the sign and magnitude of the net radiative effect. On a global average, ice clouds with τ < 4.6 display a warming effect with the largest contributions from those with τ ~ 1.0. Optically thin and high ice clouds cause strong heating in the tropical upper troposphere, while outside the tropics, mixed-phase clouds cause strong cooling at lower altitudes (> 5 km). In addition, ice clouds occurring with liquid clouds in the same profile account for about 30% of all observations. These liquid clouds reduce longwave heating rates in ice cloud layers by 0-1 K/day depending on the values of ice cloud optical depth and regions. This research for the first time provides a clear picture on the global distribution of ice clouds with a wide range of optical depth. Through radiative transfer modeling, we have gained better knowledge on ice cloud radiative effects and their dependence on ice cloud properties. These results not only improve our understanding of the interaction between clouds and climate, but also provide observational basis to evaluate climate models.