Time (1000 years before present)

FIGURE 7.3 Solar insolation (in watts per square meter) at the top of the Earth's atmosphere during the summer at 30° latitude in the Northern Hemisphere (June) and Southern Hemisphere (December) during the past 500,000 years, based on orbital variations between the Earth and Sun (Fig. 7.2). In both hemispheres, 30° latitude is close to the region where the highest levels of solar insolation occur today (Fig. 7.1). Note that the global average insolation is nearly constant despite marked changes between hemispheres over time. Based on calculations from Berger and Loutre (1991).

ture of the ''solar constant''), the seasonal and latitudinal distribution of solar energy across both hemispheres has been altogether variable.

As we all know, daily and seasonal warming are related to periods when sunlight is maximal. Assuming that the present is the key to the past—as suggested in the 18th century by James Hutton with his concept of ''uniformitarianism'' — it is reasonable that orbital relationships between the Earth and Sun (Figs. 7.2 and 7.3) also affected global temperatures in the past. To assess the Earth's paleocli-mate record, in an experimental manner with controlled boundary conditions, it is appropriate to consider the post-Pliocene period when continental and ocean configurations have been most similar to the present (Chapter 6: Spreading Planet).

One of the most powerful innovations in paleoclimate research has been the analysis of atmospheric gases and particles trapped in ice masses around the Earth. These ice-core records, which come from both polar regions as well as high mountains at lower latitudes, contain information for interpreting temperature and precipitation variability in relation to atmospheric composition, moisture sources, and prevailing winds.

How does the chemistry of the atmosphere and ocean reflect the Earth's climate conditions?

The longest and oldest ice core collected thus far comes from the East Antarctic Ice Sheet at the Russian research station (Vostok) near the geomagnetic south pole (78° south, 106° east) at an elevation of 3488 meters above sea level. The Vostok ice core extends more than 3600 meters through the ice sheet, profiling changes in the composition of Earth's climate over the past 420,000 years at a resolution of century increments (Fig. 7.4).

240 O o 220 O 200

240 O o 220 O 200

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