Greenhouse Effect Oceans and Ocean Currents

The Earth has a natural temperature control system: the atmosphere and the oceans constitute the two most important components, regulating the global climate jointly.

The Earth's atmosphere is a mixture of gases, vapors, and aerosols (78% nitrogen, 21% oxygen, 1% other gases; carbon dioxide accounts for just 0.03% to 0.04%, and water vapor varies in amount from 0 to 2%). These atmospheric gases are critical to, and are known as, "greenhouse gases." Carbon dioxide and some other minor gases present in the atmosphere absorb some of the thermal radiation leaving the earth's surface and then emit radiation from much higher and colder levels out to space. These radioactive gases are known as "greenhouse gases" because they act as a partial blanket for the thermal radiation from the earth's surface and thus enable it to be substantially warmer than it would otherwise be - analogous to the effect of a greenhouse. This blanketing is known as the natural greenhouse effect. Without these greenhouse gases, the Earth's average temperature would be roughly 0°C. The "greenhouse" effect is the natural phenomenon that keeps life on the Earth. It is due to the sun's energy, which arrives at the atmosphere and Earth's surface, is partly absorbed warming the earth, and then is partly radiated back through various phenomena. In this delicate balance, allowing the organisms' life, the gas composition of the atmosphere plays fundamental roles as both an insulating layer and a heat-trapping layer (Figs. 15.1 and 15.2). Current terrestrial life is allowed at an average temperature of 15°C (59°F), which depends on the gas composition of the troposphere. The troposphere is the thin layer (10,000 m) of atmosphere that is directly at contact with the Earth's surface and in which high-altitude habitants and flying organisms can survive.

The oceans, the planet's largest reservoir of water, play a fundamental role in the distribution and availability of water throughout the earth. Evaporation from oceans transfers huge amounts of water vapor to the atmosphere, where it travels aloft until it cools, condenses, and precipitates in the form of rain or snow. These weather phenomena can also occur far from the evaporation sites: the equatorial sun warms the oceanic surface and enhances evaporation in tropical areas; winds then blow and drive water vapor in the form of clouds, creating the ocean/atmosphere dynamics that regulate Earth's climate. The oceanic evaporation leaves the tropical water saltier: salt concentration, together water temperature, creates the big currents, collectively known as the "Ocean Conveyor." The currents distribute vast quantities of heat around the planet, playing a fundamental role in governing climate: this oceanic pump is the

Fig. 15.1 Energy balance between the sun and earth's surface throughout the atmosphere layer

Fig. 15.2 Balance of radiations (W m-2) incoming to or outgoing from the whole earth over 24 h. Greenhouse gases are mixed throughout the atmosphere (depicted here as a light layer) (the source of the major part of records are from the IPCC (Intergovernmental Panel on Climate Change) 1st (1996a, b) and 3rd (2001) reports)

Fig. 15.2 Balance of radiations (W m-2) incoming to or outgoing from the whole earth over 24 h. Greenhouse gases are mixed throughout the atmosphere (depicted here as a light layer) (the source of the major part of records are from the IPCC (Intergovernmental Panel on Climate Change) 1st (1996a, b) and 3rd (2001) reports)

most important mechanism for reducing equator-to-pole temperature differences. The Gulf Stream, for example, moderates climate in the North Atlantic region, increasing the northward transport of warmer waters by about 50%. Oceanic waters then release their heat to the atmosphere at colder northern latitudes, especially during the winter, so the ocean and ocean/atmosphere gradients increase. The Conveyor warms North Atlantic regions by as much as 5°C (Gagosian 2003). Beginning cooler, the waters are also denser than warm waters and sink to the abyss, forming the cool Labrador Stream, which flows into the South Atlantic. This enormous ring, circulating throughout the oceans (Fig. 15.3), is the major factor that affects global climate patterns.

Changes in the oceans' circulation or water properties can disrupt this hydrological cycle on a global scale, causing flooding and long-term droughts in various regions. The El Niño phenomenon demonstrated a few years ago how an oceanic change can dramatically induce precipitation falls or hurricanes in different parts of the planet.

The records of past climate collected from a variety of sources, such as deep sea sediments and polar ice cores, show that the Conveyor has slowed and shut down several times in the past. This shutdown reduced heat delivery to the North Atlantic and caused substantial cooling throughout the region.

Fig. 15.3 Global conveyor belt thermohaline circulation. The warm water from the equatorial ocean surface is carried toward northerrn Europe. In the North Atlantic, water releases heat into the atmosphere, becomes colder, and begins to sink
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