Changing Paleoclimates and Mass Extinctions

The Mechanism Behind Modern Climate Zones

by Donald L. Blanchard


The mechanism behind modern climate distribution is well understood -- to a level of complexity far beyond the scope of this article. What is presented here is the barest summary of our climate system, with emphasis placed on those elements of present climate which the proposed paleoclimate model postulates as variable over long spans of geological time.

Our climate today is controlled primarily by energy from the sun, in the form of solar radiation, and by patterns of global atmospheric circulation. Radiant solar energy, in the form of heat, is most concentrated where the Sun shines down from directly overhead. Due to the Earth's 23½° obliquity (the angle between the Earth's axis and a line perpendicular to the plane of the Earth's orbit -- or the Plane of the Ecliptic), the Sun is directly overhead at the Equator only during the Spring and Fall Equinoxes: around March 21 and September 21. At the beginning of northern hemisphere Summer, around June 21, the Sun passes directly overhead at 23½° north latitude, while on about December 21, it is at 23½° south latitude. This apparent migration of the Sun in the sky (it is actually the Earth that moves) not only affects what part of the Earth's surface receives the most heat, it also influences the circulation of the atmosphere.

The Earth's atmosphere circulates in six giant rotating toruses, called Hadley Cells, which are aligned approximately parallel with the Earth's equator. The two cells in the center are called the north and south tropical cells; the next two outward are the temperate cells, and the two at the ends are the polar cells. The boundary between the two tropical cells is called the Intertropical Convergence Zone, or ITCZ. This tends to lie near that latitude where the sun's radiation is the most intense. Heat from the sun warms the ground, which transmits some of its heat to the air above. Because warm air is lighter than cold air, and therefore rises, a band forms that is essentially a globe-encircling curtain of rising warm, moist air. As the air cools adiabatically (from the decrease in atmospheric pressure as the air rises), moisture condenses out to fall as rain, providing moisture all year around to the Earth's tropical climatic zones.

At high elevation, this air, now stripped of its humidity, spreads poleward into both hemispheres, cooling as it goes, until it is sufficiently dense to fall back to the surface. As the now dry air returns to the surface, the increase in air pressure compresses the air, heating it back to nearly the same temperature as it left the surface at the convergence zone. In actual fact, the condensation of moisture which occurs when the air rises releases its heat of vaporization into the air, effectively raising the air temperature, even as it is being cooled adiabatically. (This added heat is the energy source that powers typhoons and hurricanes.) Most of the added heat is still present when the air descends to the surface, causing it to be even hotter than it was at the ITCZ.

The hot, dry return air normally descends over the Horse Latitudes -- dreaded in the days of sailing ships for their lack of wind -- which lie roughly between 20° and 30° north and south latitude. These zones are marked by nearly globe-encircling bands of very hot, dry deserts in both hemispheres. Surface air from the Horse Latitudes returning to the tropics is deflected to the west by Coriolis force, forming the Trade Winds, which complete the circulation of the tropical pair of Hadley Cells.

The descending air at the outer (poleward) edge of these two cells entrains air from the temperate regions, driving the temperate Hadley Cells, which rotate in the opposite direction from their tropical neighbors. Surface winds in this zone are deflected by Coriolis force, and blow out of the west and poleward; these are known as the Prevailing Westerlies. Where these winds meet cold air coming from the poles, another pair of convergent zones are formed. These rising boundaries, the temperate/polar convergence zones, or simply the Polar Fronts, in turn drive the polar Hadley Cells. The air in the polar cells, stripped of whatever moisture it contained, descends at or near the north and south poles, and produces the same calm and dryness there as occurs at the Horse Latitudes, only at the poles it is far colder.

It is along the three convergent zones -- the north and south Polar Fronts, and the Intertropical Convergence Zone (the ITCZ) -- that most of the world's stormy weather occurs.

Storm Systems:
Not all of the air rising along convergent zones is at the same temperature. Depending on surface conditions and the amount of heat conducted from land or water to the air, some pockets will be decidedly warmer than others. Because warm air is less dense than cold, warm pockets rise faster. If the rising air is laden with moisture, the heat of vaporization of that moisture will be added to the rising air column as the water condenses, causing the air to rise faster still. This reduces the air pressure at the surface, creating low pressure cells. When a low pressure cell forms at a distance from the equator, air drawn in to fill the partial vacuum will be deflected by Coriolis force from the Earth's rotation. (This is the same effect that causes the little whirlpool to form when water drains from a sink or basin.) Air drawn in from closer to the Equator is deflected to the east, while that from the poles is deflected west. The result is that the inrushing air rotates spirally into the low pressure cell: clockwise in the Southern Hemisphere, counter-clockwise in the north.

When low pressure cells form along the Polar Front, they are called cyclones, and are the primary source of most temperate zone rain and snow storms. Fingers of air from both the temperate and polar Hadley Cells are drawn towards and wrap around the Low, producing an interdigitation of warmer and colder air masses.

Cyclones and the alternating bands of warm and cold air they produce gradually move from west to east. Thus any fixed point in mid-temperate latitudes will commonly see a succession of alternating bands of warmer and colder air masses pass over them. The boundaries between these masses are called fronts. A cold front is simply the leading edge of the advancing colder air mass; a warm front, the leading edge of warmer air. Cloud cover and precipitation in temperate climates occurs primarily along these fronts. Most storms are associated with cold fronts; warm fronts only produce precipitation when the warm air masses contain large quantities of moisture.

Seasonal variations in climate result from Earth's 23½° obliquity. In each hemisphere, as summer approaches, the Sun's overhead path tracks farther away from the Equator, drawing the ITCZ along behind it. Where the rising boundary passes over large expanses of ocean, warmer pockets along the boundary cause an increase in its upward flow, creating what weathermen fondly refer to as a 'tropical depression'. Moisture condensing as the air rises releases its heat of vaporization into the air, increasing the upward flow further, and lowering the air pressure. Coriolis force causes the air drawn into the low pressure region to spiral, and a tropical storm is born. Given enough time and enough heat and moisture, a hurricane or typhoon will form. As the Coriolis force is zero at the Equator, these storms generally occur only in the summer and early fall months when the ITCZ is drawn away from the Equator.

Tropical storms and their associated frontal systems travel from east to west, following the trade winds, bringing monsoon rains to the east coasts of continents. On the west coasts of continents, the descending boundary of the tropical Hadley Cells, with its hot, dry down-welling air, is pushed farther from the Equator during the summer months, bringing summer drought to Mediterranean climates between 30° and 40° latitude.

File created on June 1, 1997.
Last updated November 2004.