Global climate change is widely considered the greatest threat confronting societies and governments today. Over the last decade a consensus has developed among natural and physical scientists over the likely causes of global climate change. Businesses, governments, and citizens have begun to respond by developing a variety of strategies, policies, and institutional arrangements designed to reduce human contributions to climate change and promote adaptation to the environmental impacts that are beginning to emerge. These "policy" responses are truly diverse in form and scale, from voluntary carbon markets and business certification programs, to command and control type regulations, to international treaties.