PREVIOUS studies(1-5) have demonstrated that the predictions of global climate models are highly sensitive to large changes in vegetation cover, such as the complete removal of tropical or boreal forests. Although these studies have illustrated the potential effects of massive deforestation on the climate system, vegetation changes of this scale are very unlikely to occur. Investigating past environments may better illustrate the possible interactions between climate and vegetation cover. For example, palaeobotanical evidence indicates that 6,000 years ago boreal forests extended north of the modern treeline(6), apparently in response to high-latitude warming resulting from variations in the Earth’s orbit(7,8). The expanded boreal forests, which took the place of tundra, must also have affected climate by significantly reducing the surface albedo(5). Here we use a global climate model to examine the relative effects of orbitally-induced insolation variations and of the northward extension of boreal forests on the mid-Holocene climate. Orbital variations alone warm the high latitudes by 2 degrees C or more in summer, autumn and winter. The subsequent northward extension of boreal forests gives rise to an additional warming of approximately 4 degrees C in spring and about 1 degrees C in the other seasons. This suggests that large positive feedbacks between climate and boreal forests may have taken place in the recent geological past.