Climate-biosphere interactions on glacial-interglacial timescales

Potential positive feedbacks of the biosphere on glacial-interglacial climate change have been extensively investigated in recent years. In this paper, we summarize these feedbacks and the evidence that they may play a quantitatively significant role. We then attempt to assess the role of biosphere feedbacks in glacial/interglacial climate change by evaluating five lines of empirical evidence: (1) synchroneity of warming during the last glacial termination ( expected if the biosphere is important, because of short response times); (2) changes in the delta(18)O of O-2, which may reflect the relative fertility of the land and ocean biospheres, (3) changes in the triple isotope composition of O-2, which constrain global rates of photosynthesis in the past; (4) the relationship between atmospheric CO2 and dust accumulation at Vostok, and (5) indications for the occurrence or absence of Pleistocene-style glacial cycles before the evolution of the land biosphere. The evidence is compatible with a significant role for the biosphere in driving glacial-interglacial change, but unambiguous empirical support is not yet in hand.