Earth's Climate System Review
Basics
Earth's orbit
orbit is elliptical in shape (though nearly circular)
average distance to sun: approx. 93 million miles (150 million km)
perihelion (nearest to sun) approx. 147 million km, every Jan 3
aphelion (farthest from sun) approx. 152 million km, every July 4
Earth's orbit lies in the ecliptic plane
Earth's axis of rotation is tilted 23.5° from being perpendicular to ecliptic
axis points in same direction (toward North Star) as Earth orbits sun
(though this direction changes gradually over thousands of years)
Solar Energy: From Sun to Earth
the electromagnetic spectrum of energy
visible light is part of a continuous spectrum of energy
from long wavelength (low frequency) to high wavelength (high frequency)
radio waves, microwave, infrared, visible light, ultraviolet, x-ray, gamma ray
visible light ranges from (longest) red thru orange, yellow, green, blue, to violet
insolation (energy received from the sun at the top of the atmosphere)
the solar constant (1372 W/m2)
some of that is reflected, some lost by scattering, some is absorbed and re-radiated
as long wave infrared
uneven distribution of insolation
differences by latitude and by season
global net radiation (incoming vs. outgoing)
more incoming solar radiation than outgoing energy at low latitude
more heat loss than incoming solar radiation at high latitude
how is this balanced so equator doesn't burn up and poles don't freeze solid?
ans: atmospheric and oceanic circulation (winds and currents)!!!!!
the uneven heating of the Earth is what drives circulation and weather!!!!!
Atmospheric Energy Balance
Incoming Radiation
- a little ultraviolet
- lots of visible light
- some near-infrared (shorter wavelength infrared)
What happens to it?
Scattering Light waves are deflected by gas molecules and small particles of dust, ash, and ice in the atmosphere. Clouds also scatter solar radiation that passes through them (diffuse radiation).
Shorter wavelengths (blue) are scattered more than longer wavelengths (red). So when we look at air that sunlight is passing through, the air doesn't appear clear, it is blue because there is more blue light scattered than red light.
Refraction Light (or any other kind of) waves are refracted (bent) as they pass from one medium to another such as from space to the atmosphere or from the air into water. Rainbows are caused by light being refracted as it passes through myriad raindrops, each acting like a tiny prism which refracts light. Short wavelengths are bent more and the longer wavelengths are bent less, thereby spreading light into its spectrum of wavelengths/colors.
Reflection Clouds and Earth's surface reflects much of the incoming solar radiation back to space.
light-colored, smooth surfaces reflect the most
ice and snow are the most reflective surfaces on Earth
deserts reflect about 35% of incoming radiation
grasslands
forests reflect about 15%
oceans reflect the least and absorb the most solar radiation
dark-colored, rough surfaces reflect the least
Albedo is the intrinsic brightness, or reflectivity of a body (like the Earth).
Earth's overall albedo is 31% - Nearly a third of incoming solar radiation is reflected/scattered back into space.
21% is reflected by clouds
7% is scattered and reflected by gases and particles in the atmosphere
3% is reflected by Earth's land and ocean surfaces
Note also that while clouds and fine particles and aerosols (from volcanoes and pollution) in the atmosphere reflect solar energy, causing the ground to be cooler, they also absorb long wavelength (infrared) radiation from the sun or the ground, warming the atmosphere.
Insolation at Earth's Surface The yearly average amount of solar radiation that strikes the Earth's land and ocean surfaces is generally greater at low latitudes and less at high latitudes, as one would expect due to the curvature of the Earth and decreasing angle of the sun at higher latitudes.
However, there are some differences. The equatorial belt receives less solar radiation at the surface than at around 20 degrees latitude. The reason is that the equatorial belt is where the rain forests lie (the Amazon, Congo, and Indonesian rain forests). There is lots of rain to nourish the forests - lots of clouds to reflect sunlight. The desert belts lie around 20 degrees latitude (Sahara, Arabian, Indian, Kalahari, deserts, etc.). The air is dry and there are few clouds to reflect the sunlight, so it all reaches the surface.
Absorption The remaining 69% (100-31) of incoming solar radiation is absorbed by the Earth.
atmospheric gases, clouds, and dust absorb 24% of incoming solar radiation
land and water surfaces absorb the remaining 45% (24+45=69%)
The electromagnetic energy is turned into thermal energy (raising the temperature of the materials that have absorbed it) or chemical energy (the carbohydrates produced by photosynthesis in plants, etc.). The thermal energy is re-radiated as longer wavelength infrared radiation.
Heat stored in the Earth and atmosphere my be transferred by means other than infrared radiation.
Conduction: the atom-to-atom transfer of heat is a relatively slow process. It is relatively fast in a piece of metal. In the soil and bedrock it is slower but is the principal means of energy transfer in the ground. Other processes are largely responsible for heat transfer in the atmosphere.
Convection is the circulation of the atmosphere that results when heated, low density (lighter) air rises and is replaced by the sinking of cooler, denser (heavier) air. Convection occurs in other fluids such as the oceans as we will see later.
Advection is the horizontal transfer of heat via atmospheric and oceanic circulation (prevailing winds and ocean currents).
The Greenhouse Effect The gases of the atmosphere are transparent to visible light. The dominant gases of the atmosphere (nitrogen and oxygen) are transparent to infrared radiation. Water vapor, carbon dioxide, and methane are "greenhouse gases" which absorb infrared. This energy is then held as thermal energy, increasing the temperature of the atmosphere. The heat is held temporarily in the atmosphere but eventually is released from the atmosphere as long wavelength infrared. But the absorption slows the passage of the outgoing infrared radiation and keeps the atmosphere warm.
This process in the atmosphere is similar to an actual greenhouse, or car for that matter. Short wavelength radiation passes through the glass, but long wavelength radiation cannot pass back out - so it stays much warmer inside.
Energy Balance from Equator to Poles At low latitude there is more incoming solar radiation striking the Earth (atmosphere and surface) than radiates back to space as long wavelength infrared. At high latitude this is reversed: there is more outgoing infrared radiation than incoming solar radiation. The yearly averaged incoming and outgoing radiation are in balance only at around 36 degrees (N & S) latitude.
So why don't the equatorial regions burn up and higher latitudes freeze solid? Because atmospheric and oceanic circulation carry heat from low latitudes to high latitudes.
The Seasons
changing daylength (sunrise to sunset hours) except at equator
6 hour difference in daylength at 40° latitude
8 hour difference in daylength at 50° latitude
poles 6 month day / 6 month night
changing sun altitude (angle of sun above the horizon at noon)
at 40°N: Dec 21=21° above horizon; June 21=73° above horizon
changing declination (latitude of the subsolar point)
Dec 21 sun overhead at Tropic of Capricorn, 23.5° S
Mar 21 & Sept 22 sun overhead at equator
June 21 sun overhead at Tropic of Cancer, 23.5° N
note: the changing daylength and altitude of the sun are what cause changes in seasonal temperatures
What causes the seasons?
seasons are caused by the revolution of the Earth about the sun on its tilted axis
Revolution: once every 365.2422 days (from equinox to equinox)
Earth's axis tilted 23.5 degrees from being perpendicular to ecliptic
and tilt remains parallel to itself (it doesn't rotate around as Earth revolves about sun)
If Earth's axis had no tilt, all latitudes would have 12 hr days and 12 hr nights and the same weather throughout the year - no seasons. But because of the revolution & the tilted axis, more or less than 50% of a hemisphere is in light at any moment depending on the season, except at the equinoxes when exactly 50% of both hemispheres are in daylight. At the equinoxes, daylength is exactly 12 hr light / 12 hr dark at all latitudes except at the poles.
Atmospheric and Oceanic Circulation
Driving Forces of the Wind
Gravitational Force - compresses the atmosphere
Pressure Gradient Force - air moves from high pressure (more compressed) toward low pressure (less compressed)
Coriolis Force - air in motion appears to be deflected as Earth spins west to east
Friction Force - drag against the Earth's surface slows the wind; slows surface winds the most, high level winds the least
The net effect of these forces is that near surface winds spiral outward away from high pressure centers and inward toward low pressure centers.
- In the northern hemisphere, where winds are deflected to the right,
winds spiral clockwise around high pressure
and counterclockwise around low pressure
- In the southern hemisphere, where winds are deflected to the left,
winds spiral counterclockwise around high pressure
and clockwise around low pressure
Low pressure centers are zones of convergence, with winds spiralling inward. These are called cyclones.
Tornadoes and hurricanes are strong cyclonic storms.
High pressure centers are zones of diveregence, with winds spiralling outward. These are called anticyclones.
Principal Pressure Regions
Equatorial Low-Pressure - Intertropical Convergence Zone (ITCZ) The equatorial region is the most strongly heated area on the Earth. It is there that we find the most vigorous upward convection. Low pressure is found all along the equator. Winds converge on the intertropical convergence zone from the northeast (northeast tradewinds) and the southeast (southeast tradewinds). The trades are fairly strong and consistent. Right at the ITCZ winds are weak and variable. Sailors in the days of sailing ships called this the doldrums.
Subtropical Highs At latitudes around 25° to 30° north and south of the equator there are several more-or-less continuous and stationary centers of high pressure. For example the Bermuda High (or Azores High) in the north Atlantic Ocean, the Pacific High (or Hawaiian High) in the north Pacific, and highs over the south Atlantic, south Pacific, and south Indian oceans. Winds diverging from these highs toward the equator form the tradewinds. Winds diverging from the highs towards the poles are deflected to the east in northern and southern hemispheres forming the prevailing westerlies in midlatitudes.
Subpolar Lows A series of low pressure centers encircle Antarctica summer and winter. In the northern hemisphere, the Icelandic Low in the north Atlantic and Aleutian Low in the north Pacific spawn cyclonic storms in winter but weaken or die out in summer as the subtropical highs strengthen in the north Atlantic and north Pacific.
Polar Highs High pressure dominates in the polar regions because the air is very cold and dense. Antarctica is the coldest place on Earth because it lies over the south pole, because it is continental, and because the ice sheet is very thick and so the surface elevation is also high. High pressure dominates Antarctica year-round. The north pole, however, lies in the Arctic Ocean. The ocean has a moderating effect on the arctic. High pressure is less well developed in the north polar summer, but devlops over land as the Canadian and Siberian Highs in winter.
Climate Belts Simplified
It is commonly known that when materials (such as atmospheric gases) are heated they expand thereby becoming less dense or "lighter." In a room with a radiator or other such heater, as the air around the radiator becomes heated it expands and rises to the ceiling and is replaced by cooler denser air. This is an example of convection. A similar process occurs near the coast in summer. Land heats up faster during the day than the water does. Air over the land is heated, expands, and rises. It is replaced by cooler, denser air from over the water forming a cool sea breeze. Convection also occurs on a global scale driven by the uneven heating of the Earth.
equatorial: hot & wet
The equatorial regions are the most strongly heated areas on the Earth's surface. It is there that we find the most vigorous upward convection. Hot air is capable of holding much water vapor. Hot, humid air rises over the equator. As it rises to high altitude it expands because the air pressure decreases (there is less mass of air above it). As air expands due to this decreasing pressure, it also cools. Since cool air is able to hold less water vapor than warm air, condensation occurs. This is why the equatorial regions normally have very high rainfall. It is here that we find tropical rainforests such as those in the Amazon, Congo, and Indonesia. Areas of upward convection are dominated by low atmospheric pressure.
desert belts: hot & dry
The rising air at the equator is replaced by low-level air from higher latitudes north and south of the equator. To balance the air moving toward the equator at low altitude, the convecting air moves away from the equator, toward the north and south, at high altitude. It is now cool because of expansion and dry because it has dropped off excess moisture. To complete the convection loop, in the regions around 25 degrees north and 25 degrees south of the equator, this cool and dry air descends back to the surface (subtropical highs). As it descends, the pressure increases (because there is now more air overhead). The increased pressure increases the temperature of the air and therefore increases the capacity of the air to hold water vapor. Now the air is very dry and has the capacity to soak up much evaporation. Consequently, these latitudes are very dry with high evaporation and low rainfall. These are the desert belts, including the Sahara, Mojave and Sonoran deserts of the U.S. southwest and Mexico, the Kalahari and Namib in southern Africa, the Australian desert, and the Atacama Desert on the west coast of South America. Areas of descending air are dominated by high atmospheric pressure.
midlatitude: temperate - cool and moist
The midlatitudes are a battleground between very cold, dense polar air and warm air moving poleward from the subtropical highs. The boundary between them is called the polar front. The polar front is an undulating boundary. The undulations are called Rossby Waves. In the midlatitudes, these undulations are sites where cold air pushes equatorward and warm air pushes poleward. Cyclonic circulation (convergence, counterclockwise in northern hemisphere) develops around the southward bulges of the polar front. As these undulations of the polar front sweep southward and eastward (northern hemisphere) cold dense air pushes under warmer, less dense air. As the warmer air rises, it expands, cools, and condenses some of its water vapor. That is why cold fronts bring clouds and rain. During the summer the polar front lies farther north and we seldom see summer cold fronts on Long Island.
polar: cold & dry
The polar regions are the coldest on Earth. The air is very cold, dense, and dry. High pressure dominates. Much of Antarctica is essentially a desert because so little precipitation falls (though what does fall remains frozen).
Ocean Circulation
surface currents
Wind drives both waves and surface currents in the oceans. Warm surface waters at the equator are driven westward by the easterly trade winds. When the equatorial currents reach the western edge of the ocean basin (east coast of some continent) they are diverted to the north and south along the continents. In the central Atlantic this northward flowing branch is called the Gulf Stream. It carries warm water from the equator, Caribbean and Gulf of Mexico, northward along the east coast of North America, across the North Atlantic, and to arctic Ocean off northwestern Europe. The westerlies help to drive the Gulf Stream eastward across the Atlantic toward Europe. The warmth of the Gulf Stream moderates the climate of northwestern Europe. The waters in the North Atlantic cool. The cooled waters then flow southward along the coast of Europe and Africa. This southern current is called the Canary Current. It brings cool waters down to northwest Africa and keep the coast here relatively cool. Eventually this current approaches the equator where the waters warm again and the easterly winds drive them westward across the Atlantic again to start another clockwise loop. Such large surface current loops, called subtropical gyres, are found in all the open ocean basins. They generally flow clockwise in the northern hemisphere and counterclockwise in the southern hemisphere.
deep ocean currents
As water chills in the North Atlantic it becomes more dense. Also, when the cold water begins to freeze to form sea ice the ice that forms is from pure water; the salt is left behind in the remaining sea water. The sea water gets saltier. The saltier the water the denser it becomes. These cold, salty, dense surface waters sink down to the bottom of the Atlantic. The sinking waters are replaced by less dense surface waters from the south. The sinking waters flow southward along the bottom of the ocean as surface waters flow northward to replace them. The North Atlantic Deep Water (NADW) continues south until they meet a northward flowing bottom current of even denser waters that formed off the coast of Antarctica. The North Atlantic Deep Water then rides up above the Antarctic Bottom Water and continues southward at intermediate depths until they eventually rise to the surface near Antarctica. From there they follow other currents that carry them throughout the oceans.
There is an ocean conveyor belt that mixes waters through all the ocean basins from the sea bottom to the sea surface, connecting the surface, intermediate, and bottom water currents. This mixing moves heat, dissolved gases, and nutrients through the oceans in one grand cycle. Breakdown of this conveyor belt may have been responsible for sudden changes in the Earth's climate in the past.
As the Earth warms, apparently largely due to the release of greenhouse gases from industrial and agricultural activity, the rate of melting and release of icebergs from Greenland into the far north Atlantic and Arctic Ocean. The increased inlux of fresh water into the sea will make these surface waters less dense and slow or stop the formation of deep water (sinking of surface water). This should slow the northward movement of the Gulf Stream. Because heat carried into the far north Atlantic helps to moderate the climate of densely populated northwestern Europe, a weakening or cessation of the Gulf Stream would cause major social, agricultural, and economic problems.
