As humans burn fossil fuels and raise the concentration of greenhouse gases in our atmosphere, the climate is changing—fast. All around us, we are experiencing climate change in the form of hotter temperatures and more extreme weather events, including droughts, floods, and heat waves. What does the future have in store for us? To answer this question, we must query the geological past. The last time that carbon dioxide was at present-day levels was over 2 million years ago—a time well before humans walked the Earth. Ancient warm climates like this one hold the key to understanding how higher greenhouse gas concentrations alter land and sea ice, and patterns of warming and rainfall. We can piece together what happened in the deep past by using natural archives of climate change as well as computer simulations. As we’ll discuss, the lessons of the past can directly inform our future climate trajectory, giving us a glimpse of how the planet will behave along different emissions scenarios.