Climate change refers to the variation in the Earth's global climate or in regional climates over time. It describes changes in the variability or average state of the atmosphere over time scales ranging from decades to millions of years. These changes can be caused by processes internal to the Earth, external forces (e.g. variations in sunlight intensity) or, more recently, human activities. In recent usage, especially in the context of environmental policy, the term 'climate change' often refers only to changes in modern climate, including the rise in average surface temperature known as global warming. This new book presents leading-edge research in this fast-moving field.