The most informative analogues for future anthropogenic climate change are likely to be those with boundary conditions similar to today. The late Pliocene is the most recent time in earth history with elevated global temperatures and CO (sub 2) estimated to be similar to that anticipated by the end of this century [1, 2, 3]. Furthermore, Pliocene continental positions and vegetation distributions are thought to be broadly similar to today. Consequently the IPCC fourth assessment report highlighted the Pliocene as an important time period for further study. Recently our understanding of Pliocene CO (sub 2) and temperature has improved, with publication of multiple records from alkenone and boron isotope reconstructions for CO (sub 2) [2, 3], and Mg/Ca, U (super K') (sub 37) and TEX (sub 86-) reconstructions for sea surface temperature. However, none of the published CO (sub 2) records have sufficient temporal resolution to resolve orbital scale variations in CO (sub 2) , or to determine the relationship between the apparent reduction in atmospheric CO- (sub 2) and the intensification of northern hemisphere glaciation. Here we present new high resolution records of CO (sub 2) and temperature from ODP Site 999 over the critical interval from 3.3 to 2.6 Ma using alkenone palaeobarometry and the U (super K') (sub 37) and TEX (sub 86) palaeothermometers. By combining these with a full analysis of the biotic response to changing conditions and reconstructing haptophyte cell sizes [4], critical for the alkenone palaeobarometer, we present well constrained, coupled records of the response of the climate system to changing CO (sub 2) .