Alaskan permafrost cliff. General view of a 35-metre-high riverbank exposure of the ice-rich syngenetic permafrost (yedoma) containing large ice wedges along the Itkillik River in northern Alaska, USA. Changes in permafrost and related features are being studied for how they are affected by climate change and how they can contribute to global warming by directly and indirectly releasing gases such as methane and carbon dioxide. Photographed on 12 August 2011.