Are paleo-proxy data helpful for constraining future climate change?

How sensitive is our climate system to CO2? This is a key issue in a world of rising greenhouse gas concentrations. Estimating the temperature sensitivity of the Earth to changes in atmospheric CO2 has therefore been the subject of intensive research. Yet, uncertainty in our knowledge of this sensitivity is still large—as expressed by the broad 2-4.5°C range of climate sensitivity (ΔT2x) estimates (Meehl et al., 2007). Commonly ΔT2x is defined as the equilibrium global-mean temperature change for doubling the pre-industrial CO2 concentration. The direct radiative effect is a warming by 1°C but what makes the total warming uncertain is the strength of the fast climatic feedbacks—mainly ice-albedo, water vapor, lapse rate and cloud feedback. Here, we discuss how paleo-data can be used to reduce uncertainty in the range of ΔT2x. One way to compute climate sensitivity is to use climate models that calculate the feedbacks and thus ΔT2x. Another apfeedbacks (Holland et al., 2001). This complexity is also highlighted by these PMIP2 results, where models show a large range of sea ice and associated ocean circulation responses to LGM forcing. Climate model simulations combined with proxy reconstructions have shown that LGM sea ice and ocean stratification can provide additional constraints on interpretation of the LGM Atlantic meridional overturning (Shin et al., 2003; Otto-Bliesner et al., 2007). LGM sea ice extent has been shown to be important in modulating the atmosphereocean interactions and water mass formation in the subpolar North Atlantic. The cold, salty Antarctic bottom waters at LGM form in coastal leads and just equatorward of permanent sea ice cover, due to brine rejection during sea ice production. Additionally, the suppression of air-sea gas exchange due to glacial sea ice expansion in the Southern Ocean has been suggested to play a possible role in regulating past atmospheric CO2 (Morales Maqueda and Rahmstorf, 2002; Stephens and Keeling, 2000).