Climate driven northward boreal forest expansion into the tundra biome controlled by permafrost will play a major role in global emissions trajectories. Yet our limited understanding of the interplay between vegetation and permafrost makes predictions of changing boreal forest extent difficult. We analyse fossil pollen, stable carbon isotopes, and lignin and levoglucosan biomarkers from Tortonian speleothems (8.68 ± 0.09 Ma) from the Lena River Delta (N72.27°, E126.94°) in Arctic Siberia to infer palaeotemperature, precipitation, vegetation and fire regimes. The Tortonian provides a potential analogue for near future climate warming under extreme emissions scenarios, with global mean global temperature ca. 4.5°C above modern and atmospheric CO2 concentrations similar to present. We find evidence for a mixed forest regime, capable of maintaining wildfires, in a region currently dominated by tundra. Future transition to a similarly temperate regime would have large-scale impacts on the global carbon cycle.