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face and promotes evaporation, primarily around 30° latitude in both hemispheres (Fig. 8.5).

Consequently, the mid-latitude zone of high evaporation accounts for high salinities in the ocean and many of the great deserts on Earth, such as the Sahara in Africa. Descending dry air over the polar regions also causes evaporation, which contributes to Antarctica being the largest desert on Earth. The nature of the Antarctic desert is enhanced by the persistently cold temperatures over the continent. Together, these Antarctic conditions produce minimal levels of snow accumulation that progressively decrease away from the oceanic moisture source—from the circumpolar coastline toward the center of the ice sheet (Fig. 8.6).

Moreover, high elevations of the Antarctic ice sheets (Fig. 8.6) also establish a highly coupled system with direct connections between the stratosphere (Fig. 1.5) and the depths of the ocean (Fig. 7.8) over relatively short time scales compared to all other continents. This ocean-atmosphere coupling is facilitated by the katabatic winds (Plate 4), which are dense cold-air masses falling from the polar plateau and warming as they travel at high velocities (sometimes exceeding 40 meters per second) toward sea level. In turn, these katabatic winds often create huge open-water areas in the middle of the sea ice called polynyas—covering tens to hundreds of thousands of square kilometers and generating dense shelf water masses that sink into the deep sea.

In each hemisphere, wind belts correspond with the zones of ascending and descending air masses. These winds are named for the directions they come from, as opposed to currents, which conventionally are named for the directions they move toward. For example, the East Wind Drift and West Wind Drift around Ant-

FIGURE 8.6 Approximate snow accumulation rates at different elevations around Antarctica decreasing from 20-80 grams per square centimeter per year in the circum-Antarctic coastal zone to less than a gram per square centimeter per year at elevations above 2000 meters on the polar plateau. Atmospheric moisture transport is maximal in the circum-Antarctic coastal zone and decreases with increasing distance from the ocean and elevation onto the polar plateau (Jacobs, 1992). Circumpolar distribution of ice-free coastal oases occur along the Antarctic ice-sheet margin (Berkman et al. 1998): 1, McMurdo Dry Valleys; 2, Terra Nova Bay; 3, Cape Adare and Cape Hallett; 4, Cape Denison; 5, Bunger Hills; 6, Larsemann Hills and Vestfold Hills; 7, Soya Coast; 8, Untersee Oasis; 9, Hope Bay; and 10, Marguerite Bay. The three-dimensional view of Antarctica, with elevation contours in meters above sea level, was derived from Drewry (1983).

FIGURE 8.6 Approximate snow accumulation rates at different elevations around Antarctica decreasing from 20-80 grams per square centimeter per year in the circum-Antarctic coastal zone to less than a gram per square centimeter per year at elevations above 2000 meters on the polar plateau. Atmospheric moisture transport is maximal in the circum-Antarctic coastal zone and decreases with increasing distance from the ocean and elevation onto the polar plateau (Jacobs, 1992). Circumpolar distribution of ice-free coastal oases occur along the Antarctic ice-sheet margin (Berkman et al. 1998): 1, McMurdo Dry Valleys; 2, Terra Nova Bay; 3, Cape Adare and Cape Hallett; 4, Cape Denison; 5, Bunger Hills; 6, Larsemann Hills and Vestfold Hills; 7, Soya Coast; 8, Untersee Oasis; 9, Hope Bay; and 10, Marguerite Bay. The three-dimensional view of Antarctica, with elevation contours in meters above sea level, was derived from Drewry (1983).

arctica (Fig. 8.3) are coming from the east and west, respectively. Moreover, these two wind regimes represent the general character of the polar easterlies and westerlies, which are blowing in both hemispheres (Figs. 8.4 and 8.5). Between the polar easterlies and the westerlies is the polar front zone, as shown around Antarctica (Fig. 8.3).

Equatorward of the westerlies are the northeast and southeast trade winds, which were named by the early mariners who recognized their commercial importance (Figs. 8.4 and 8.5). Between the two trade wind zones is a band of cloudy, rainy weather at the equator, which is called the doldrums by mariners or the Intertropical Convergence Zone by meteorologists.

Seasonally, the global wind belts shift poleward during the spring and equator-ward during the fall—causing distinct wet and dry seasons. Within a given lati-

tinct appearance of warm water mollusks in beaches along the Pacific coast of South America signal that ENSO events turned on around 6000 years ago (Plate 5), during the mid-Holocene when the Earth's climate began stabilizing into its present mode (Chapter 7: Flowing Planet). Cores from corals that have been growing continuously in the Pacific suggest that ENSO events occurred around every 15 years during the 17th century and that their frequency has increased toward the present—coincidentally as the Earth system has been warming.

greenhouse respiration

Heating from the Sun (Fig. 7.1) circulates around the Earth because of ocean currents (Fig. 7.9), winds (Fig. 8.4) and latent heat exchanges associated with the hydrological cycle (Fig. 8.5). Over millennia, changes in insolation also will influence the Earth's climate (Figs. 7.2-7.4). Ultimately, however, the blanket of mixed gases in the lower atmosphere (primarily the troposphere—Figs. 1a and 1b) is why the Earth system retains the solar heating and remains relatively warm.

To understand the average surface temperature of Earth (nearly 150,000,000 kilometers from the Sun), it is helpful to consider sister planets in the solar system. Venus is almost twice as far from the Sun (nearly 110,000,000 kilometers) as Mercury (nearly 58,000,000 kilometers). Nonetheless, Venus has a stable average temperature around 480°C, whereas the surface of Mercury fluctuates broadly from around - 180°C to only 430°C. Relative to Mercury, the tremendous warmth of Venus (which is hot enough to melt lead) clearly is influenced by factors other than its distance from the Sun. The answer lies in the composition of the planets' atmospheres.

How do atmospheres influence the surface temperatures of planets in our solar system?

Mercury is without an atmosphere. In contrast, Venus is enshrouded in a dense cloud of carbon dioxide (nearly 96% of the atmosphere), which produces surface pressures that only occur on Earth at ocean depths below 1000 meters. Venus's carbon-dioxide-rich atmosphere allows incoming solar radiation to pass through but absorbs outgoing radiation, trapping it within. Absorption of outgoing radiation—like the daytime heating of an indoor plant nursery—is called the ''greenhouse effect.''

To understand the greenhouse effect, it is necessary to start with the Sun, which has a surface temperature around 6000°C. This solar energy radiates through space in particles (photons) as well as in waves—embracing the ''duality of light.'' Waves are further represented by their wavelengths (distances from crest to crest or trough to trough), like the different colors in the humanly visible region of the electromagnetic spectrum (Fig. 8.7).

The maximum wavelength emitted by the Sun or any other radiating body is tudinal zone, however, environments vary between hemispheres, with more land north and ocean south of the equator (Fig. 1.4). These land-sea contrasts influence weather and climate patterns because rock warms and cools faster than water, which has an unusually high heat capacity and ability to moderate temperature changes (Table 7.1).

This differential heating between the land and sea causes winds to blow onshore and offshore depending on solar radiation. On a daily or seasonal basis, relatively moist air over the sea is sucked onshore as the sun heats the land and causes the overlying air to rise rapidly. These warm, moist air masses then condense at higher altitudes, often causing high levels of rainfall. Conversely, more arid winds blow offshore from the land as night or winter cooling causes overlying air masses to fall faster than those over the sea. These onshore-offshore processes are associated with the shifting winds and currents of the monsoons (from the Arabic word mausim, ''a season''), which seasonally deluge areas in India, Asia, and even North America.

Today, the most pronounced oscillation in the global weather system is associated with a weakening of the southeast trade winds every 2 to 7 years. Normally, high pressure over South America and low pressure over Indonesia causes the south equatorial current to flow westward across the Pacific Ocean. To replace the westward flowing surface waters, cold nutrient-rich bottom waters upwell along the coast of South America. Conversely, surface seawater in the west Pacific is nearly 8°C warmer and a half meter higher than in the east Pacific. This ocean-atmosphere coupling leads to dry coastal areas in Peru and lush rainforests in Indonesia.

When the sea surface becomes abnormally warm in the east Pacific, however, the normal pressure gradient across the ocean is reduced—causing the southeast trade winds to weaken. The accompanying shift in the low pressure regions from west to east also causes a reversal of the south equatorial current across the Pacific Ocean. As warm water piles up in the eastern Pacific, upwelling nutrient-rich waters are shut down along with the rich marine ecosystems which they support. Commonly referred to as El Nino (the boy or Christ child), because of its occurrence during the Christmas season, this southern oscillation also leads to droughts in Indonesia and fires in Australia while the Americas are blanketed in torrential rains.

In 1997-98, the El Nino Southern Oscillation (ENSO) caused the sea surface to rise by nearly 40 centimeters near the Americas—creating a warm pool of water with temperatures above 20°C and a volume that was 30 times greater than all of the water in the United States, including the Great Lakes. Additional estimates from the National Oceanic and Atmospheric Administration indicate that the abnormal amount of heat in this warm pool was nearly 100 times greater than the heat produced from all of the fossil fuel energy consumed in the United States in 1995.

ENSO impacts propagate across the Earth today, and there are hints that this ocean-atmosphere phenomenon is coupled with global climate conditions. Dis-

box 8.1 temperature-radiation relationships

WIEN'S LAW

Maximum wavelength (Lmax) = constant #1/Absolute temperature (°K)

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