Both abrupt changes in the physical climate system and steady changes in climate that can trigger abrupt changes in other physical, biological, and human systems present possible threats to nature and society. Abrupt change is already underway in some systems, and large scientific uncertainties about the likelihood of other abrupt changes highlight the need for further research. However, with recent advances in understanding of the climate system, some potential abrupt changes once thought to be imminent threats are now considered unlikely to occur this century. This report summarizes the current state of knowledge on potential abrupt changes to the ocean, atmosphere, ecosystems, and high latitude areas, and identifies key research and monitoring needs. The report calls for action to develop an abrupt change early warning system to help anticipate future abrupt changes and reduce their impacts.