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Percent of Earth's Surface Area

FIGURE 1.3 Hypsographic curve of Earth surfaces today, which illustrates the percentage of the total area on our planet that is occupied at various land elevations and ocean depths relative to sea level. Note that the ocean accounts for more than two-thirds of the Earth's total area with an average depth of 3865 meters and a maximum depth that is deeper than the elevation of the highest mountain. Modified from Thurman (1978).

deepest point in the ocean at the bottom of the Marianas Trench, the vertical range is less than 20 kilometers. Known life exists within this narrow band of surface habitats on Earth.

How was the Earth system transformed into a habitable environment?

Across the Earth's surface, water is the common environmental feature that makes life possible. Water is the major constituent of metabolizing cells in all known life forms. Water also was the basis for the ''primordial soup'' in which life initially evolved on Earth and, possibly, other celestial bodies in our solar system. In fact, in his famous simulation of Earth's early atmosphere, Stanley Miller demonstrated in 1953 that merely sending electrical sparks through a gaseous mixture of methane, ammonia, and hydrogen with water will produce amino acids and other building blocks of organic (carbon-based) life forms.

Water exists in all physical states (liquid, solid, or gas) on the Earth's surface; however, only liquid water is capable of supporting known biological processes. Even bacteria frozen into Antarctic lakes or floating in clouds have liquid microhabitats that facilitate their metabolism. Nonetheless, the fact that life is present in these extreme environments gives rise to speculation about extraterrestrial biology on planets, moons, and comets with water in any state.

In the Earth system, the largest water reservoir is the ocean, which covers nearly 70% of the planet's surface with an average depth of almost 4 kilometers (Figs. 1.3 and 1.4). Considering the Earth's surface area is around half a billion square kilometers, a ''back-of-the-envelope'' calculation indicates that the total volume of the ocean today is between 1 and 2 trillion cubic kilometers—which accounts for more than 97% of the water on the planet. The remaining water in the Earth system cycles among the ocean, atmosphere, and land.

Among the nonmarine reservoirs (Fig. 1.5), nearly 80% of the water is frozen into the ice sheets and glaciers around the world today. Seventeen thousand years ago, when the Earth was in the midst of the Last Glacial Maximum, global sea level was 120 meters or about 3% lower than today. Most of the seawater during this period was locked into massive ice sheets that expanded across the northern hemisphere continents—with twice the ice volume of the Earth system today. For perspective, 90% of the ice on Earth today exists in Antarctica, and if it all melted into the ocean, it would raise global sea level only about 60 meters.

The second largest nonmarine reservoir occurs in the ground, with more than 20% of the freshwater on Earth. Together, all of the other reservoirs—lakes, inland seas, soils, atmosphere and rivers (in order of decreasing volumes)—account for less than 1% of the freshwater in the Earth system today. Compared to the ocean, which has persisted for billions of years, the smaller reservoirs of water are replenished and depleted rapidly over time scales that are proportional to their volumes—underlying their different dynamics and interactions across the planet.

FIGURE 1.4 Relative coverage of land (dark) and ocean (hatched) areas on the Earth's surface at each latitude north and south of the equator (Fig. I). Today, nearly 70% of the ocean exists in the southern hemisphere. Modified from Duxbury and Duxbury (1984).

across time

The concept of time is at the heart of understanding the Earth system as a complex of interacting phenomena, pulsing at different rates since the origin of our planet around 4.5 billion years ago (Fig. 1.6). Earth events and their extension along timelines are analogous to activities which occur during different periods in our lifetime, as riddled by the Sphinx:

What walks on four legs in the morning, two legs in the afternoon, and three legs at night?

Each of these life stages has a discrete history that is unique unto itself, and at the same time is linked with and often preconditioning future circumstances—as in the Earth system and human civilization.

Lakes (0.33%) Inland Seas (0.27%) Soil Moisture (0.18%) Atmosphere (0.03%) " Rivers (<0.01%)

FIGURE 1.5 Water reservoirs in the Earth system today, excluding the ocean, which alone accounts for 97.24% of the nearly 1,360,000,000 cubic-kilometers of water that flows through the various reservoirs in the global hydrological cycle. During the current interglacial (warm) period, the Earth's ice caps have shrunk by more than 60% compared to their volumes in the last glacial (cold) period, which occurred only 17,000 years ago (Chapter 7: Flowing Planet). Today, nearly 90% of the Earth's ice exists in Antarctica, accounting for more than 60% of the freshwater on the planet. Based on data from the United States Geological Survey (http://wwwga.usgs.gov/edu/earthwhere water.html).

What are the relative space and time dimensions of phenomena in the Earth system?

After originating around 4.5 billion years ago, the Earth cooled and water pooled on its surface. Primitive life from the ''primordial soup'' began using the Sun's energy to synthesize carbon dioxide and water into basic sugars and molecular oxygen [Eq. (1.1)].

PLANTS OR BACTERIA Carbon dioxide (CO2) + Water (H2O)

sunlight or chemical energy Q (1.1)

Sugars (CH2O)n + Molecular oxygen (O2) nANIMALS

With food and an increasingly oxygenated atmosphere (Fig. 1.6), simple cells evolved into multicellular organisms. Eventually, animals evolved that could breathe the oxygen and consume the sugars. As they respired carbon dioxide back into the system, animals also created a feedback with the plant photosynthesizers. These early changes in the Earth system reveal the paired evolution of life and habitats during the Precambrian era—spanning more than 80% of Earth's history.

Soft-tissue life forms eventually gave way to animals with shelled external skeletons. At the start of the Paleozoic (an era whose name refers to ''ancient animals''), around 570 million years ago, there was an evolutionary explosion

Lakes (0.33%) Inland Seas (0.27%) Soil Moisture (0.18%) Atmosphere (0.03%) " Rivers (<0.01%)

FIGURE 1.5 Water reservoirs in the Earth system today, excluding the ocean, which alone accounts for 97.24% of the nearly 1,360,000,000 cubic-kilometers of water that flows through the various reservoirs in the global hydrological cycle. During the current interglacial (warm) period, the Earth's ice caps have shrunk by more than 60% compared to their volumes in the last glacial (cold) period, which occurred only 17,000 years ago (Chapter 7: Flowing Planet). Today, nearly 90% of the Earth's ice exists in Antarctica, accounting for more than 60% of the freshwater on the planet. Based on data from the United States Geological Survey (http://wwwga.usgs.gov/edu/earthwhere water.html).

FIGURE 1.6 Earth system history from its origin 4.5 billion (4,500,000,000 or 4.5 X 109) years ago to the present. The vital role of life in transforming habitats around the Earth, commonly known as the ''Gaia hypothesis'' (Lovelock, 1979), is reflected by the photosynthetic production of oxygen molecules (O2) by primitive plants that facilitated the evolution of higher life forms (modified from Sumich, 1996). Relative changes in the Earth's temperature (Baron, 1992) and sea-level (Tucker, 1992) illustrate global climatic periods that have been influenced by phenomena that are internal and external to the Earth system (Chapter 6: Spreading Planet).

FIGURE 1.6 Earth system history from its origin 4.5 billion (4,500,000,000 or 4.5 X 109) years ago to the present. The vital role of life in transforming habitats around the Earth, commonly known as the ''Gaia hypothesis'' (Lovelock, 1979), is reflected by the photosynthetic production of oxygen molecules (O2) by primitive plants that facilitated the evolution of higher life forms (modified from Sumich, 1996). Relative changes in the Earth's temperature (Baron, 1992) and sea-level (Tucker, 1992) illustrate global climatic periods that have been influenced by phenomena that are internal and external to the Earth system (Chapter 6: Spreading Planet).

of species with hard body parts. The Paleozoic extended for the next 320 million years with the appearance of fish and most invertebrate (without backbone) groups, including the ubiquitous trilobites, which inhabited seas around the world throughout this era. The Paleozoic also involved at least two distinct ice ages and the formation of a single supercontinent, called Pangea (all land), which al lowed reptiles, amphibians, insects, and vascular plants to extend life's conquest onto land.

Heralding the Mesozoic (''middle animals''), around 250 million years ago a catastrophic Earth system change caused the extinction of more than 90% of Paleozoic marine animals, including all of the trilobites. The Mesozoic saw the dawn of dinosaurs as well as pronounced changes in terrestrial vegetation with ferns, cycads, and the earliest flowering plants. Pangea also began to separate into northern and southern hemisphere supercontinents during this era.

Around 65 million years ago there was another catastrophic global environmental change, possibly from a meteorite impact, which caused the dinosaurs to go extinct. Just as previous eras were distinguished by the appearance and extinction of various life forms as a result of changes in global habitat conditions, the Cenozoic (''new animals'') has been recognized as the age of mammals. During this era, the current configuration of continents emerged, including the isolation of Antarctica over the south polar region.

Subdivisions of the Cenozoic, as in all of the preceding eras, provide more refined distinctions among periods of environmental and biotic change. In the Cenozoic, there have been two major periods: the Tertiary (65 to 1.8 million years ago) and the Quaternary (1.8 million years ago to the present). During this latter period, the Homo genus began evolving into modern humans (Homo sapiens sapiens). The Quaternary also is recognized for its glacial-interglacial variations associated with global climate cooling and warming.

The Quaternary can be divided further into the Pleistocene and Holocene, which represents the current climate epoch from 10,000 years ago to the present. In the same manner that climate periods have been distinguished along the timeline of the Earth system, the ''common era'' began 2000 years ago following a particular event in the history of human civilization. During all periods, there are shorter intervals—each with their own histories—embedded one within another across time (Fig. 1.6) in the same manner as concentric spheres across space (Figs. 1.1 and 1.2) in the Earth system.

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