Quantification of Past Climate
The basis of the North Atlantic INTIMATE work is records of North Atlantic climate conditions in the period between 60,000 to 8000 years ago. This period is dominated by significant climatological contrasts; from the highly variable climate of the Dansgaard-Oeschger (D-O) events and the consistently cold millennia of the Last Glacial Maximum (LGM) through to dramatic climatic changes at the glacial termination to mild and stable interglacial conditions in the Holocene.
The records come from studies of multifarious archives of past climate, e.g. ice cores, tree rings, or marine or lacustrine sediments. The diversity of data is illustrated by the papers in the special INTIMATE issue of Quaternary Science Reviews (vol. 27, issues 1-2, 2008) and examples include temperature estimates derived from ice cores (Dahl-Jensen et al., 1998), ocean sediment cores (Sanchez-Goni et al., 2008; Rasmussen and Thomsen, 2008), and pollen analysis (e.g. Ilyashuk et al., 2009) as well as continental climate conditions derived from varved lake sediments (Brauer et al., 2008) and past sea level estimates (Siddall et al., 2008).
Each record provides valuable information on how local or regional climate conditions changed and – in some cases – how local ecosystems responded to the changes. Palaeoclimate reconstructions have until recently been based on proxy data (in particular pollen) described on a qualitative basis. Although such an approach has some value in providing a general scheme of events, there are inherent problems including the interpretation of proxy data, disentangling different climate signals, temporal sensitivity of proxies to climatic change and the value of qualitative terms. For instance, quantified reconstructions of summer temperature using fossil Coleoptera remains and their known present-day distribution in the North Hemisphere provide an absolute record of climatic change (Atkinson et al., 1987; Coope et al., 1998), and indicate that absolute temperature changes across northern Europe were considerably more complex than the relative scheme suggests (Witte et al., 1998). Since this pioneering work, similar quantified estimates have been obtained in the North Atlantic region from Chironomidae (non-biting midges) (Brooks and Birks, 2000), pollen (Nakagawa et al., 2002) and terrestrial plant C-13 (Beerling, 1996). The mission of INTIMATE is to facilitate the integration of these climate records so that they can be studied together. One of the main activities to support this is the establishment of protocols for time-stratigraphic correlation in the North Atlantic region (Björk et al., 1998; Walker et al., 1999; Lowe et al, 2008), laying out a common framework for discussing contemporaneous events in different records.
While the individual records are valuable for the understanding of our climate, the study of the integrated records paves the way for a deeper understanding of the processes and feedbacks active in the climate system. For example, when records from neighbouring locations are precisely compared (see WG1), it is possible to identify possible leads and lags between the records and to set up time lines of events for past periods of climate change (Steffensen et al., 2008). Time lines like these are of paramount importance for the understanding of the dynamics of the climate system because they are the starting points for making hypotheses about not only the dynamics, but the mechanisms, of past climate change, and allow for testing of these hypotheses (see WG3), adding to our understanding of the ice-sea-atmosphere interactions and feedbacks during periods of abrupt and extreme change.