This document discusses greenhouse gases and global climate change. It defines greenhouse gases as gases that cause the greenhouse effect and trap heat in the lower atmosphere. It then defines global climate change as identifiable changes in Earth's climate that last for decades or longer, and are usually caused by either natural processes or human activities that release greenhouse gases. The document goes on to explain that current climate changes are happening due to both natural causes like volcanoes and plate tectonic movement, as well as human activities that release greenhouse gases. It notes that the effects of human-caused greenhouse gases have been detected throughout the climate system and are extremely likely the dominant cause of observed warming since the mid-20th century.