Studies from 1994–1998 on the Tanana Flats in central Alaska reveal that permafrost degradation is widespread and rapid, causing large shifts in ecosystems from birch forests to fens and bogs. Fine-grained soils under the birch forest are ice-rich and thaw settlement typically is 1-2.5 m after the permafrost thaws. The collapsed areas are rapidly colonized by aquatic herbaceous plants, leading to the development of a thick, floating organic mat. Based on field sampling of soils, permafrost and vegetation, and the construction of a GIS database, we estimate that 17% of the study area (263,964 ha) is unfrozen with no previous permafrost, 48% has stable permafrost, 31% is partially degraded, and 4% has totally degraded. For that portion that currently has, or recently had,permafrost (83% of area), ∼42% has been affected by thermokarst development. Based on air photo analysis, birch forests have decreased 35% and fens have increased 29% from 1949 to 1995. Overall, the area with totally degraded permafrost (collapse-scar fens and bogs) has increased from 39 to 47% in 46 y. Based on rates of change from air photo analysis and radiocarbon dating, we estimate 83% of the degradation occurred before 1949. Evidence indicates this permafrost degradation began in the mid-1700s and is associated with periods of relatively warm climate during the mid-late 1700s and 1900s. If current conditions persist, the remaining lowland birch forests will be eliminated by the end of the next century.