This is the first of three study sessions on the rapid changes in the world’s climate since the start of the 20th century, which scientists have detected. Climate change is a global problem that is having widespread effects on the weather on every continent and in every nation. It is altering environments and impacting on biodiversity, human health and sustainable economic and social development. We begin this study session by introducing some key terms and concepts in climate science, and describing the main features of current and future projections of how the global climate is changing. You will then learn about the causes of climate change, both naturally occurring and resulting from human activity. Finally we discuss Ethiopia’s climate and some reasons for concern about Ethiopia’s vulnerability to climate change. In Study Sessions 10 and 11 you will focus on Ethiopia in greater detail as we review the frequency and severity of extreme weather events that are due to climate change, and describe the impacts on health, agriculture and the Ethiopian economy.