Remote Sensing of Earth ’ s Polar Regions

the public’s imagination, through the stories of past heroic explorations, the endearing wildlife, the awe-inspiring geography of the Antarctic ice sheets, and the unique modes of human habitation in the Arctic. Outcomes from the forthcoming International Polar Year (IPY) will highlight the scientific importance of the regions, emphasizing their key roles in the global climate system. Over the past two decades, the polar regions have consistently shown the first unmistakable evidence of human industrial activity on Earth’s atmosphere and climate. The first was the 1985 discovery1 of the ozone “hole,” an annual disappearance of most of the ozone layer over Antarctica and the Southern Ocean, whose cause was quickly identified as an interaction between industrial chlorofluorocarbon pollutants and unique ice clouds that form in the extremely cold Antarctic stratosphere.2 More recently, both satellite data3 and submarine research cruises4 have documented the dramatic decrease in both geographic extent and thickness of Arctic sea ice, such that if no action is taken to curb industrial greenhouse gas emissions, Arctic sea ice could disappear altogether in the summer season by the end of this century.5 The Antarctic Peninsula has experienced a more rapid climate warming than any other region on Earth,6 with noticeable adverse impacts on the region’s ecosystem. The causes of these climate changes are complex and will be the subject of much scientific inquiry for a long time. Although ultimately related to human industrial activity,7 the specific mechanisms often involve exotic and nonlinear couplings between the radiant energy trapped by greenhouse gases and the unique features of polar meteorology. Climate studies in the 1980s, for example, suggested that a polar amplification of global warming results primarily from two straightforward feedback mechanisms, the ice albedo and cloud radiation. In ice-albedo feedback, a small warming causes a slight melting of snow and ice surfaces; this melting decreases their albedo (or reflectance), allowing the absorption of more solar energy, which in turn accelerates both the melting and the warming of the local climate. In cloud-radiation feedback, a warmer atmosphere holds more water vapor, which leads to the formation of more and optically thicker clouds; in the cold polar cli-