The past is a guide to the future? Comparing Middle Pliocene vegetation with predicted biome distributions for the twenty-first century.

During the Middle Pliocene, the Earth experienced greater global warmth compared with today, coupled with higher atmospheric CO2 concentrations. To determine the extent to which the Middle Pliocene can be used as a 'test bed' for future warming, we compare data and model-based Middle Pliocene vegetation with simulated global biome distributions for the mid- and late twenty-first century. The best agreement is found when a Middle Pliocene biome reconstruction is compared with a future scenario using 560 ppmv atmospheric CO2. In accordance with palaeobotanical data, all model simulations indicate a generally warmer and wetter climate, resulting in a northward shift of the taiga-tundra boundary and a spread of tropical savannah and woodland in Africa and Australia at the expense of deserts. Our data-model comparison reveals differences in the distribution of polar vegetation, which indicate that the high latitudes during the Middle Pliocene were still warmer than its predicted modern analogue by several degrees. However, our future scenarios do not consider multipliers associated with 'long-term' climate sensitivity. Changes in global temperature, and thus biome distributions, at higher atmospheric CO2 levels will not have reached an equilibrium state (as is the case for the Middle Pliocene) by the end of this century.