The effects of climate change are being experienced firsthand. Changes in the Earth’s atmosphere, oceans, and lands are having environmental, social and economic effects. One of the world’s leading organizations dedicated to understanding climate systems and impacts is the Intergovernmental Panel on Climate Change (IPCC). This report examines the work of the IPCC over the past 23 years, and the anticipated fifth IPCC report that will be released later this year. This report also describes some of the primary scientific mechanisms of climate science that are key to understanding the work of the IPCC.