On the nature of lead-lag relationships during glacial-interglacial climate transitions

Analysis of leads and lags between different paleoclimate records remains an important method in paleoclimatology, used to propose and test hypotheses about causal relationships between different processes in the climate system. The robust lead of Antarctic temperature over CO(2) concentration during several recent glacial-interglacial transitions inferred from the Antarctic ice cores apparently contradicts the concept Of CO(2)-driven climate change and still remains unexplained. Here, using an Earth system model of intermediate complexity and generic scenarios for the principal climatic forcings during glacial-interglacial transitions we performed a suite of experiments that shed some light on the complexity of phase relationships between climate forcing and climate system response. In particular, our results provide an explanation for the observed Antarctic temperature lead over CO(2) concentration. It is shown that the interhemispheric oceanic heat transport provides a crucial link between the two hemispheres. We demonstrate that temporal variations of the oceanic heat transport strongly contribute to the observed phase relationship between polar temperature records in both hemispheres. It is shown that the direct effect of orbital variations on the Antarctic temperature is also significant and explains the observed cooling trend during interglacials. In addition, an imbedded delta(18)O model is used to demonstrate that during glacial-interglacial transitions, the temporal evolution of deep calcite marine delta(18)O in different locations and at different depths can considerably deviate from that implied by the global ice volume change. This finding indicates that the synchronization of different marine records by means of foraminiferal calcite delta(18)O yield large additional uncertainties. Based on our results, we argue that the analysis of leads and lags alone, without a comprehensive understanding and an adequate model of all relevant climate processes, cannot provide direct information about causal relationships in the climate system. (C) 2009 Elsevier Ltd. All rights reserved.