Jean-Pascal van Ypersele de Strihou

Entering the Glaciation With a 2-d Coupled Climate Model

A 2-dimensional model which links the atmosphere, the mixed layer of the ocean, the sea-ice, the continents, the ice sheets and their underlying lithosphere has been used to test the Milankovitch theory over the last glacial-interglacial cycle. Sensitivity tests have shown that the orbital variations can induce, in such a system, feedbacks sufficient to generate the low frequency part of the climatic variations over the last 122 ka BP. These variations at the astronomical time scale are broadly in agreement with ice volume and sea level reconstruction independently obtained from geological data. At the beginning of the integration during isotopic Substage 5e when there was no northern american nor eurasian ice sheets, June insolation at high latitudes seemed to correlate well with summer temperature at the surface of the ice sheets and with the ablation rate. The relation is more complicated when large ice sheets exist. Broadly, the June-July insolation from 60 to 70-degrees-N. leads the ice volume by roughly 5000 years over the last glacial-interglacial cycle. The simulated climate was shown to be sensitive to the ice albedo-temperature feedback, to the precipitation-altitude negative feedback over the ice sheets, to the ice sheet slope and continentality effects, and to the albedo of the ice sheets as a function of the snowfall frequency at their surface. The formation and waxing of the ice sheets are particularly sensitive to ablation, more than to snowfalls. Imperfections in the simulated climate were recognized, in particular at the last glacial maximum when the cooling was not large enough and the northern hemisphere ice sheets did not extend far enough to the south, weaknesses which are partly solved in a further experiment (Gallee et al., 1989) by using the proper CO2 atmospheric concentration given by the Vostok ice core in addition to the astronomical forcing.

Potential role of solar variability as an agent for climate change

Numerical experiments have been carried out with a two-dimensional sector averaged global climate model in order to assess the potential impact of solar variability on the Earths surface temperature from 1700 to 1992. This was done by investigating the model response to the variations in solar radiation caused by the changes in the Earths orbital elements, as well as by the changes intrinsic to the Sun. In the absence of a full physical theory able to explain the origin of the observed total solar irradiance variations, three different total solar irradiance reconstructions have been used. A total solar irradiance change due to the photospheric effects incorporated in the Willson and Hudson (1988) parameterization, and the newly reconstructed solar total irradiance variations from the solar models of Hoyt and Schatten (1993) and Lean et al. (1995). Our results indicate that while the influence of the orbital forcing on the annual and global mean surface temperature is negligible at the century time scale, the monthly mean response to this forcing can be quite different from one month to another. The modelled global warming due to the three investigated total solar irradiance reconstructions is insufficient to reproduce the observed 20th century warming. Nevertheless, our simulated surface temperature response to the changes in the Suns radiant energy output suggests that the Gleissberg cycle (≈88 years) solar forcing should not be neglected in explaining the century-scale climate variations. Finally, spectral analysis seems to point out that the 10- to 12-year oscillations found in the recorded Northern Hemisphere temperature variations from 1700 to 1992 could be unrelated to the solar forcing. Such a result could indicate that the eleven-year period which is frequently found in climate data might be related to oscillations in the atmosphere or oceans, internal to the climate system.

Ice sheets and sea-level changes as a response to climatic change at the astronomical time scale

Understanding how and why global climate is changing is investigated at the astronomical time scale related to the glacial-interglacial cycles of the Quaternary Ice Age.

The role of population growth in global CO2 emissions

Abstract The principle of “differentiated responsibilities” of North and South in protecting the climate system against global warming is recognized in the Rio Framework Convention on Climate Change. We focus here on the quantification of one aspect of this issue: population growth versus growth in CO 2 /capita emissions. We first mention several problems raised by the way the Ehrlich-Holdren equation (Environmental impact=Population times Per capita impact) is used in the context of greenhouse gas emissions. In particular, we remind the importance of using the lowest possible aggregation level with this equation. We then apply this equation to population and fossil fuel-related CO 2 -emission data for nine regions of the world over the 1950–1990 period. The results of a scenario analysis using these data show that the increase in developed countries CO 2 emission per capita had a significantly larger impact on world total emission increase than LDCs (Less Developed Countries) population growth during that period. It is also shown that population growth in developed countries had a larger effect than LDCs population growth.