Abstract
In addition to the changes in the Earth's radiation balance mentioned in the first lecture, glacial cycles have induced a second positive feedback by strongly changing the albedo, the reflection coefficient of incident solar radiation, particularly at the level of glaciated land surfaces. The postglacial retreat of the Laurentian and Fennoscandian ice caps has thus favored the climatic transition of the Northern Hemisphere's high latitudes.
To estimate this contribution over time, it is necessary to quantify continental freeze-up during the Tardiglacial period. A first approach is to convert sea-level records into terms of ice-cap extension. A more direct method is to map the geomorphological traces left by continental glaciers. These marks have been studied since the 19th century, but it is only recently that they can be dated by measuring the time rocks are exposed to cosmic rays. Cosmonuclides(10Be, 26Al, 36Cl, 14C, 3He, 21Ne) are formed in erratic boulders and mustard rocks and analyzed using gas pedal mass spectrometry. In addition to conventional radiocarbon dating, these in situ cosmonuclide measurements have enabled us to reconstruct the maximum extension of the ice caps and their gradual retreat during the Tardiglacial period. With the help of representative ice-cap models, it is then possible to map land ice surfaces and altitudes, and convert them into units of radiative forcing.
Similar work is carried out to estimate the extent and spatial and temporal variations of sea ice. A number of micropalaeontological and geochemical indicators analyzed in marine sediments from polar zones enable us to reconstruct the annual and seasonal extension of this pack ice, whose effect on the albedo of high latitudes is cumulative with that of continental ice caps.
