Category Acid Deposition and Energy Use

THE OCEANS

The oceans cover 70.8% of the surface of the earth, although there is a much greater fraction in the Southern Hemisphere (80.9% of the area) than the Northern Hemisphere (60.7%), and through their fluid motions and high heat capacity they have a central role in shaping the earth’s climate and its variability. The average depth of the ocean is 3795 m. The oceans are stratified opposite to the atmosphere, with warmest waters near the surface. Consequently, in the ocean, convection arises from cooling at the surface and transport of heat upwards occurs through colder and denser waters sinking and being replaced by lighter and more buoyant waters. Another vital factor in convection is the salinity of the water because this also affects density...

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THE HYDROLOGICAL CYCLE

Driven by exchanges of energy, the hydrological cycle involves the transfer of water from the oceans to the atmosphere, to the land, and back to the oceans both on top of and beneath the land surface. In the tropics in summer, land warms relative to the ocean and sets the stage for monsoon development. Water is evaporated from the ocean surface, cooling the ocean. As water vapor, it is transported perhaps thousands of kilometers before it is involved in clouds and weather systems and precipitated out as rain, snow, hail, or some other frozen pellet back to the earth’s surface. During this process, it heats the atmosphere. Over land, soil moisture and surface waters can act through evaporative cooling to moderate tempera­tures...

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THE ATMOSPHERE

In the atmosphere, phenomena and events are loosely classified into the realms of ‘‘weather’’ and ‘‘climate.’’ Climate is usually defined as average weather and thus is thought of as the prevailing weather, which includes not just average conditions but also the range of variations. Climate involves variations in which the atmosphere is influenced by and interacts with other parts of the climate system and the external forcings. The large fluctuations in the atmosphere from hour to hour or day to day constitute the weather but occur as part of much larger scale organized weather systems that arise mainly from atmospheric instabilities driven by heating patterns from the sun.

Much of the incoming solar radiation penetrates through the relatively transparent atmosphere to rea...

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REGIONAL PATTERNS

The annual mean absorbed solar radiation (ASR) and outgoing long-wave radiation (OLR) are shown in Fig. 3. Most of the atmosphere is relatively transpar­ent to solar radiation, with the most notable exception being clouds. At the surface, snow and ice have a high albedo and consequently absorb little incoming radiation. Therefore, the main departures in the ASR from what would be expected simply from the sun-earth geometry are the signatures of persistent clouds...

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Effects of Clouds

Clouds also absorb and emit thermal radiation and have a blanketing effect similar to that of greenhouse gases. However, clouds are also bright reflectors of solar radiation and thus also act to cool the surface. Although on average there is strong cancellation between the two opposing effects of short-wave and long-wave cloud heating, the net global effect of clouds in our current climate, as determined by space-based measurements, is a small cooling of the surface. A key issue is how clouds will change as climate changes. This issue is complicated by the fact that clouds are also strongly influenced by particulate pollution, which tends to make more smaller cloud droplets, and thus makes clouds brighter and more reflective of solar radiation...

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The Greenhouse Effect

Some of the infrared radiation leaving the atmo­sphere originates near the earth’s surface and is transmitted relatively unimpeded through the atmo­sphere; this is the radiation from areas where there are no clouds and which is present in the part of the spectrum known as the atmospheric ‘‘window’’ (Fig. 2). The bulk of the radiation, however, is intercepted and reemitted both up and down. The emissions to space occur either from the tops of clouds at different atmospheric levels (which are almost always colder than the surface) or by gases present in the atmosphere that absorb and emit infrared radiation. Most of the atmosphere consists of nitrogen and oxygen (99% of dry air), which are transparent to infrared radiation...

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THE GLOBAL ENERGY BALANCE

The incoming energy to the earth system is in the form of solar radiation and roughly corresponds to
that of a black body at the temperature of the sun of approximately 6000 K. The sun’s emissions peak at a wavelength of approximately 0.6 mm and much of this energy is in the visible part of the electromag­netic spectrum, although some extends beyond the red into the infrared and some extends beyond the violet into the ultraviolet. As noted earlier, because of the roughly spherical shape of the earth, at any one time half the earth is in night (Fig. 1) and the average amount of energy incident on a level surface outside the atmosphere is one-fourth of the total solar irradiance or 342 Wm-2...

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THE EARTH AND CLIMATE SYSTEM

Our planet orbits the sun at an average distance of 1.50 x 1011 m once per year. It receives from the sun an average radiation of 1368 Wm-2 at this distance, and this value is referred to as the total solar irradiance. It used to be called the ‘‘solar constant’’ even though it does vary by small amounts with the sunspot cycle and related changes on the sun. The earth’s shape is similar to that of an oblate spheroid, with an average radius of 6371 km. It rotates on an axis with a tilt relative to the ecliptic plane of 23.5° around the sun once per year in a slightly elliptical orbit that brings the earth closest to the sun on January 3 (called perihelion). Due to the shape of the earth, incoming solar radiation varies enormously with latitude...

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