Greenland ice-core records provide an exceptionally clear picture of many aspects of abrupt climate changes, and particularly of those associated with the Younger Dryas event, as reviewed here. Well-preserved annual layers can be counted confidently, with only ~1% errors for the age of the end of the Younger Dryas ~11,500 years before present. Ice-flow corrections allow reconstruction of snow accumulation rates over tens of thousands of years with little additional uncertainty. Glaciochemical and particulate data record atmospheric-loading changes with little uncertainty introduced by changes in snow accumulation. Confident paleothermometry is provided by site-specific calibrations using ice-isotopic ratios, borehole temperatures, and gas-isotopic ratios. Near-simultaneous changes in ice-core paleoclimatic indicators of local, regional, and more-widespread climate conditions demonstrate that much of the Earth experienced abrupt climate changes synchronous with Greenland within thirty years or less. Post-Younger Dryas changes have not duplicated the size, extent and rapidity of these paleoclimatic changes.