Chapter 8 explores the questions: what were the causes and consequences of an abrupt warming event of the past, and how does this inform our understanding of climate change today? This exploration is focused on an event ~56 million years ago, known as the Paleocene-Eocene Thermal Maximum (PETM). Geological evidence indicates that greenhouse gas-induced climate warming, driven by the geologically rapid release of geosphere-stored carbon, was the cause of the PETM, with effects observed throughout the ocean-atmosphere-biosphere systems. In addition to warming, ocean acidification was one of the major impacts of the PETM on the hydrosphere. The Earth system recovered from the greenhouse gas-induced PETM over a period of ~100,000 years, as CO2 was removed from the atmosphere by rock weathering and carbon burial; during this time, however, the biosphere experienced an increased rate of extinction and new species appearances. The PETM is often cited as an analog for modern climate change, although human activities are adding CO2 to the atmosphere nearly 10x faster than natural drivers did during the PETM.