Many aspects of Earth's climate system have changed abruptly in the past and are likely to change abruptly in the future. Although abrupt shifts in temperature are most dramatic in glacial climates, abrupt changes, resulting in an altered probability of drought, large floods, tropical storm landfall, and monsoon rainfall, are all important concerns even in the absence of significant anthropogenic climate forcing. Continued climate change will likely increase the probability of these types of abrupt change and also make abrupt changes in ocean circulation and sea level more likely. Although global warning may have already triggered abrupt change, current understanding and modeling capability is not sufficient to specify details of future abrupt climate change. Improved adaptation strategies are warranted, as well as efforts to avoid crossing climate change thresholds beyond which large abrupt changes in sea level, ocean circulation, and methane-clathrate release could greatly amplify the impacts of climate change.