Many boreal forests grow in regions where climate is now warming rapidly. Changes in these vast, cold forests have the potential to affect global climate because they store huge amounts of carbon and because the relative abundances of their different tree species influence how much solar radiation reflects back to space. Both the carbon cycling and albedo of boreal forests are strongly affected by wildland fires, which in turn are closely controlled by summer climate. Here we use a forest disturbance model in both a retrospective and predictive manner to explore how the forests of Interior Alaska respond to changing climate. Results suggest that a widespread shift from coniferous to deciduous vegetation began around A.D. 1990 and will continue over the next several decades. This ecological regime shift is being driven by old, highly flammable spruce stands encountering a warmer climate conducive to larger and more frequent fires. Increased burning promotes the spread of early successional, deciduous species at the expense of spruce. These striking changes in the vegetation composition and fire regime are predicted to alter the biophysics of Alaska's forests. The ground will warm, and a surge of carbon emission is likely. Our modeling results support previous inferences that Alaska's boreal forest is now shifting to a new ecological state and that positive feedbacks to global warming will accompany this change.