Flowing Planet

Since the measuring device has been constructed by the observer ... we have to remember that what we observe is not nature in itself but nature exposed to our method of questioning. —Werner Karl Heisenberg (1958), Physics and Philosophy fluctuating ice sheets

The geological framework imposed by the tectonic movement and ultimate positioning of the continents has exerted a major influence on Earth's climate. Not only have vast oceans been created as continents drifted apart, but pathways for air and sea currents to circulate heat and moisture around the planet have been produced.

Along with the progressive isolation of Antarctica in its polar position during the Cenozoic (Table 6.1), the Earth system has been cooling. However, superimposed on this long-term cooling trend are short-term climate fluctuations (Fig. 6.5). These climate shifts, particularly since the Pliocene, underscore the ocean-atmosphere coupling that connects environments across the Earth.

How is Antarctica related to the global climate?

Atmospheric and oceanic engines of the climate system are fueled by incoming solar radiation. The average flux of solar energy that reaches the top of the Earth's atmosphere—called the ''solar constant''—is taken to be 1367 watts per square meter. The ''solar constant,'' however, is a misnomer because it varies in relation to solar activity with changes that may range several watts per square meter during the 11-year sunspot cycle or other periods.

FIGURE 7.2 Variations in the Earth's orbit around the Sun that influence glacial-interglacial climate cycles over millennial time scales (Fig. 7.3). As hypothesized first by Milankovitch (1938), changes in Earth's insolation are influenced by precession of the equinoxes every 23,000 years as the period when the Earth is closest to the Sun moves between December and June. Every 41,000 years, the obliquity or tilt of the Earth's axis wobbles between 22.1° and 24.5° away from the plane of its orbit. Every 100,000 years, the eccentricity of the Earth's elliptical orbit around the Sun reaches its maximum. Together, these orbital changes influence the growth and decay of the Earth's ice sheets during glacial and interglacial climate phases as identified during the past half-million years (Fig. 7.4). Modified from Kennett (1982).

icc-uccay oiuuai cuniiyuiaLiun ,

/1 X. Distance Distance ^z1

FIGURE 7.2 Variations in the Earth's orbit around the Sun that influence glacial-interglacial climate cycles over millennial time scales (Fig. 7.3). As hypothesized first by Milankovitch (1938), changes in Earth's insolation are influenced by precession of the equinoxes every 23,000 years as the period when the Earth is closest to the Sun moves between December and June. Every 41,000 years, the obliquity or tilt of the Earth's axis wobbles between 22.1° and 24.5° away from the plane of its orbit. Every 100,000 years, the eccentricity of the Earth's elliptical orbit around the Sun reaches its maximum. Together, these orbital changes influence the growth and decay of the Earth's ice sheets during glacial and interglacial climate phases as identified during the past half-million years (Fig. 7.4). Modified from Kennett (1982).

force behind glacial-interglacial oscillations in the Earth system. The combined effect of changing precession, obliquity, and eccentricity is illustrated by Fig. 7.3 in relation to summer solar radiation at 30° latitude in both hemispheres during the past 500,000 years. Although the average annual solar radiation in the Earth system has been nearly uniform throughout this period (reflecting the general na-

For scale, a 1-watt increase in the solar constant would be equivalent to adding a single small Christmas tree light every square meter across the surface of the Earth. Even such tiny changes in solar output have a measurable effect on global temperatures, as suggested by the co-occurrence of the Maunder Minimum in sunspot activity and the most intense cold period of the ''Little Ice Age'' between 1640 and 1710, which is well chronicled in paintings from Europe.

At all latitudes, the annual solar radiation budget is influenced by the tilt of the Earth's axis during its 365-day orbit around the Sun. Incoming radiation and heating from the Sun is least variable in the zone along the equator (Fig. 7.1). Daily, in these low-latitude regions, there are 12-hour periods of sunlight and darkness as the Earth completes a revolution around its axis. Toward higher latitudes, solar radiation variability increases, ultimately to the poles, which have 24-hour periods of continuous summer sunlight and winter darkness (Plate 4).

The general profile of solar radiation across the Earth also varies over millennial time scales with changes in the tilt of the Earth's rotational axis and shape of its elliptical orbit around the Sun (Fig. 7.2). Every 23,000 years, precession of the equinoxes changes the month when the Earth is closest to the Sun (perihelion) from December to June. On a 41,000-year cycle, the obliquity or tilt of the Earth's axis wobbles between 22.1° and 24.5° away from the plane of the Earth's orbit. Approximately every 100,000 years, there is a slight change in the eccentricity of the Earth's elliptical orbit around the Sun.

These long-term shifts in the orbital geometry between the Earth and the Sun were first calculated in 1941 by the Yugoslavian astronomer Milutin Milankovitch (1879-1958), who recognized that solar radiation cycles could be a dominant

Month

FIGURE 7.1 Contours of incoming solar radiation (with units of watts per square meter) on the Earth's surface each month across all latitudes today. Modified from Pickard and Emery (1982).

Month

FIGURE 7.1 Contours of incoming solar radiation (with units of watts per square meter) on the Earth's surface each month across all latitudes today. Modified from Pickard and Emery (1982).

0 0

Post a comment