Changes in climate depend essentially on three basic parameters, the amount of incident sunlight, the fraction of this sunlight that is reflected by the Earth, and the trapping of the Earth's infrared radiation by greenhouse gases.The Earth's reflectance of the Sun's radiation back to space—or albedo—is the least well studied of the three.

The albedo depends primarily on cloud properties, and ground-based and satellite studies published within the past 3–4 years have shown a surprisingly significant interannual and decadal variability in this parameter. The variability in reflectance is tied to changes in cloud location, amount, and thickness. However, clouds are very poorly parameterized in climate models. Thus,the scale of these variations presents a fundamental, and as yet unmet, challenge to understanding and predicting the Earth's climate.