Although modern Arctic climates appear to be more sensitive to increased atmospheric CO2, little is known about climate forcing mechanisms during previous warm intervals in the Arctic. The Pliocene epoch (5.3 – 1.8 MA) represents the last sustained warm interval when global temperatures greatly exceeded modern temperatures. This epoch is of particular interest as an analog for future warming because Earths continents and climate system were very similar to today. Although most proxy-derived estimates of mean annual temperature agree that terrestrial temperatures were much greater during the Pliocene, previous estimates of atmospheric CO2 suggest that levels were similar to modern levels (between 280 and 360 ppm). This begs the question- what was the physical mechanism responsible for such increased warming during the Pliocene?

Here we combine isotopic proxies from fossil vegetation collected from an early Pliocene-aged (4-4.5 MYA) peat deposit at Strathcona Fiord on central Ellesmere Island, Nunavut (78 deg N, 82 deg W and 200 MASL). Our temperature estimates derived from oxygen isotopes in the cellulose of fossil larch suggest that mean annual temperatures in the Arctic during this period were an astounding 15⁰ C greater than today. This estimate is supported by the rich assemblage of macrofossils suggesting boreal forest environment in the high arctic during the Pliocene. Using additional measurements of carbon isotopes from sedges and mosses, we explore the interaction between atmospheric water vapor and CO2 as potential mechanisms of enhanced warming during this interval. Carbon isotopic values were slightly more enriched but less variable in the sedges (-25.0‰ ± 1.6) than in the mosses (-30.5‰ ± 5.4), suggesting major changes in water availability during the Pliocene at this site. Preliminary estimates, based on carbon isotopic measurements of fossil plants and modern estimates of stomatal conductance, indicate that Pliocene atmospheric CO2 levels were only slightly higher than current day (350 to 400 ppm). These estimates of only slightly higher levels of atmospheric CO2 are inadequate to explain the significant Arctic warmth observed in the Pliocene. Thus additional climate forcing mechanisms must be invoked to explain the enhanced climate sensitivity of the Arctic during the Pliocene.