VEGETATION AND FIRE: FEEDBACKS TO PLIOCENE ARCTIC CLIMATE

The mid-Pliocene (~3.6 mya) had similar CO₂, geography and topography to present, yet higher global temperatures and a decreased latitudinal temperature gradient. Mechanisms contributing to this polar amplification of temperature are not well understood and thus undermine predictions of future climate. Combining proxy reconstructions and model simulations, we tested if Pliocene amplified Arctic warming may be attributed to feedbacks induced by high latitude boreal-type forest, growing at up to 80°N during the mid-Pliocene.

Climate, vegetation and fire regime were reconstructed at four early to mid-Pliocene localities in the Canadian High Arctic. These were then used to test the influence of climate-vegetation-fire feedbacks in the Arctic using climate simulations of the mid-Piacenzian Warm Period (MPWP). Climate reconstructions suggest mean annual temperatures (MAT) ~3°C with warmer winter temperatures contributing most to the higher MAT, and mean summer temperatures ~15°C. Reconstructed precipitation was variable between sites, but significantly higher than present. Atmospheric CO₂ concentration averaged over the deposition of one key site, Beaver Pond, was 439 ± 47 ppm. Charcoal analyses indicate fires in the Arctic were widely distributed and likely frequently recurring during the Pliocene.

Climate simulations of the MPWP for northern high latitudes found ~3°C warming attributable to strong surface albedo feedback from higher boreal tree line. Consistent with paleorecords, simulations found elevated fire in the northern high latitudes under MPWP climate. The enhanced emissions of fire aerosols strongly altered the Arctic radiation budget, primarily through changing cloud microphysics. Model runs show enhanced cloud formation and a cloud cooling effect in the Arctic region during the warm months, with a mild warming effect in the sub-Arctic, likely related to the enhanced transport of black carbon, during cool months. Thus, our preliminary results suggest amplification of Arctic Pliocene surface temperature was strongly influenced by vegetation albedo and that fire emissions contributed to reduced seasonality of surface temperatures. Our results highlight the complexity of both direct and indirect climate-vegetation feedbacks in understanding the sensitivity of Arctic climate.