Factor Separation Method and Paleoclimates

Berger, A. [UCL] Claussen, M. [UCL] Yin, Qiuzhen [UCL]

The relative contribution of the individual forcings, the feedbacks and the synergisms can be quantified by using different techniques, such as the Alpert– Stein Factor Separation Methodology (here abbreviated FS; Stein and Alpert, 1993) or other feedback analyses. In an attempt to better understand the role of the temperature–albedo feedback of the greenhouse gases (water vapour and CO2), and of the insolation at the Last Glacial Maximum (LGM), several sensitivity experiments have been made with a radiative convective model (Berger et al., 1993) and results discussed using both the classical feedback analysis and FS methodology. The LGM cooling is simulated to be 4.5 ◦C, of which 3◦C is in response to the insolation–albedo forcing and 1.5 ◦C is from the CO2 forcing. In these experiments, the water vapour feedback (WVF) is included, but the synergisms appear to be very small. The direct influence of the insolation–albedo forcing is a cooling of 1.8 ◦C, on top of which theWVFadds 1.2 ◦C.The remaining 1.5 ◦Cis due to theCO2 forcing, of which 0.9 ◦C comes from its direct influence and 0.6 ◦C is due to theWVFfeedback. The FS methodology and a generalisation of the classical linear feedback analysis technique are also used to identify the individual contributions of climatic factors and of their synergism to the Holocene climate change signal. From the Ganopolski et al. (1998) experiments it can be shown that for the temperature differences between 6 ka BP (6000 years before present) and the present in boreal latitudes, the synergism due to changes in the vegetation cover, sea-surface temperatures and the sea-ice extent plays a more important role than the pure contributions of the ocean and of the vegetation themselves. Contrary to temperature, precipitation changes in summer over the Northern Hemisphere continents – North Africa in particular – are mainly related to the pure contribution of vegetation changes. Advantages and disadvantages of the different methods in such kinds of analyses are discussed.