We know that Earth’s climate is changing, but how do we know that human actions are a primary factor today, given that climate has changed throughout geologic time? We begin by examining the many natural factors that control climate change on timescales ranging from millions of years to just a few years. These natural influences include external factors such as the position of the continents, the orbital parameters of the Earth/Sun/Moon system, and volcanic eruptions. We also consider variability arising from within the climate system due to phenomena like El Niños and La Niñas. We then shift focus from geologic timescales to the natural and human influences on the climate of the last 2,000 years. How have scientists identified human “fingerprints” in observations of climate change? Are droughts, heat waves, and wildfires being affected by climate change? The final course segment examines projected climate changes over the next 100 years, based on different emissions scenarios.