This book is an overview of the science and policy implications of climate change. Almost all scientists agree that the Earth's climate is changing, having warmed by 0.6 to 0.9 degrees Celsius (1.1 to 1.6 degrees Fahrenheit) since the Industrial Revolution. During the 20th Century, some areas became wetter while others experienced more drought. Most climate scientists conclude that humans have induced a large part of the climate change since the 1970s. Although natural forces such as solar irradiance and volcanoes contribute to variability, scientists cannot explain the climate changes of the past few decades without including the effects of elevated greenhouse gas (GHG) concentrations resulting from fossil fuel use, land clearing, and industrial and agricultural emissions. The United States contributes almost one-fifth of net global greenhouse gas emissions. The principal questions remaining for the majority of scientists concern not whether greenhouse gases will result in climate change, but the magnitude, speed, geographic details, and likelihood of surprises, and the appropriate timing and options involved in addressing the human components of climate change. These questions are addressed in this book.