Principles of Ecology
320 Harned Hall
|A field of eucalyptus trees in Brazil. The trees are native to Australia and grow very fast in Brazil. The wood is used mainly for disposable diapers. Plantations of these trees have brought jobs to the area but have also drastically reduced the size of one of Brazil's most diverse environments, the Atlantic Coastal forest, endangering many endemic species.|
Lecture 3 The Global Environment
Overview - Link to Course Objectives
Climate is the long-term pattern of weather in a locality, region, or even over the entire globe. Weather is the temperature and moisture conditions for a specific place and time. They differ in scale in both space and time. Note that, unlike many definitions of weather, there is no mention of averages. This is not to exclude averages from the description of climate, as they are important descriptors of climate. The reason for not mentioning average in the definition is that averages are not sufficient to describe climate. Maximum and minimums are also important. Extreme events may also be important. Hurricanes are very important components of the climate of southern Florida but they do not affect long-term averages very much.
Solar Radiation and the Earth
- Basic unit is the photon, a packet of energy that travels as a wave
- waves have lengths (distance from successive peak to peak) and frequency (number of peaks passing a fixed point per unit time)
- since the speed of light is a constant, the frequency is related to the wavelength in that shorter wavelengths have greater frequencies
- the wavelength (and frequency) of electromagnetic radiation is determined by the amount of energy in the photon - greater energy causes shorter wavelengths
- all bodies with a temperature above absolute zero (just zero on the Kelvin scale) radiate energy
- the higher the temperature, the shorter the wavelength
Consider an iron bar in a blacksmith's furnace. As it heats it begins to give off visible radiation. It starts to glow red and, as it heats more, it glows with a white light. There are some lessons to be learned here.
Why red first? Heating is not uniform throughout the rod, and each region emits photons directly related to the temperature at that spot. Red light has the longest wavelength of visible light and so, as the iron heats, the hottest areas will appear red, the first wavelengths of electromagnetic spectrum we can detect with our eyes. As the rod continues to heat, more areas are hot enough to emit visible radiation but, once again, the actual temperature is not uniform and so we get a wide range of photons with wavelengths in the visible spectrum and we perceive the overall effect as white light, since there is no one wavelength of light that is white.
Second Lesson - Objects with the temperature of our sun emit most of their radiation in the visible spectrum. This is not a coincidence. We evolved eyes to detect at these wavelengths as they dominate the spectrum available to us because we are close to the sun. Note that we do not detect all of the common wavelengths given off by the sun. We do not see ultraviolet, which has wavelengths shorter than the shortest we can see. Other organisms (not just animals!) can detect these but we name the visible spectrum for what humans can see, not for what all living organisms can see. A collective name for the radiation given off by the sun is shortwave radiation (from about 100 to 2000 nanometers).
A subset of shortwave radiation (roughly the same as visible radiation) is used by photosynthetic plants, algae, and bacteria to power photosynthesis. These wavelengths are referred to by biologists as PAR (Photosynthetically Active Radiation).
Third lesson - radiation is emitted by the rod before we can see it glow. In fact, radiation is emitted from all objects, including you. However, the wavelengths emitted by you are too long for our eyes to detect. When special cameras that do detect at our wavelengths are used, we do glow. This is the basis of night vision. Images are made from the glow of objects that emit in the Infrared range of wavelengths and translated into wavelengths that we can see by the night vision apparatus.
Fourth lesson - infrared means below red. Thus, it is radiation with wavelengths below our power to detect as the photons do not have sufficient energy to initiate the physiochemical reactions we call vision. Infrared is divided into two types, near and far. Near Infrared has wavelengths near to visible (from about 700 to 4000 nanometers). Far Infrared is even longer wavelength (about 4000 to 1 million nm). This is the range emitted by objects at the temperatures we have on Earth and so we also refer to this as Thermal Radiation.
The radiation discussed here is not the entire spectrum. Gamma ray and X-ray radiation have shorter wavelengths than the radiation discussed here and can have wavelengths as short as a millionth of a nanometer. Radar, radio, TV, and cell phone use radiation with wavelengths longer than far infrared (up to hundreds of meters long). To see the entire spectrum, go to this site at the Laboratory for Atmospheric and Space Physics.
Heat and Radiation
Recall that heat is kinetic energy, the energy of moving and vibrating atoms and molecules. Temperature is determined by the average speed of that motion (e. g. hotter gasses and liquids have faster moving molecules). Electromagnetic radiation is converted to heat when photons are absorbed by atoms and move faster as a result (we measure this as heating). Heat energy is converted to electromagnetic variation when a photon is emitted by a moving atom that slows as a result of the loss of energy (we measure this as cooling).
Global Heat Budget
The sun contributes almost all of the energy that drives climate and, ultimately, living systems. The sun's contribution is in the form of electromagnetic radiation
Earth receives energy as photons from the Sun. The energy balance at the surface of the Earth is result of losses to space and the incoming radiation from the Sun. Thus, we can do a budget, just like a household budget is the record of income and expenditures. The global budget is complicated by the fact that Earth is surrounded by an atmosphere.
The atmosphere is densest at the surface of the Earth because gravity pulls the molecules of gas toward the center of the Earth. Gravity is resisted by the speed at which the molecules move (their temperature). So the atmosphere is thickest at the surface and becomes thinner as you ascend. But, the temperatures found as you ascend change abruptly and this separates the atmosphere into four general layers
Troposphere - surface to about 8 km at the poles and to about 17 km at the equator. The air cools as you go up at a rate of about 6.5°C per kilometer for 12 km but then stops cooling (this is called the Tropopause and it the boundary layer between the troposphere and the stratosphere) - This is the layer of most clouds and precipitation. The average temperature of the surface of the Earth is about 15°C and the tropopause is about -60°C.
Stratosphere - from tropopause to Stratopause at about 50 km up. The stratosphere warms as you go up because oxygen is absorbing the sun's energy and heating the layer (from -60°C to about -10°C). The upper is hotter because it gets more input from the sun but the lower layers are somewhat shaded by the upper layers. The stratosphere is where we fly airliners and there are two important climatic factors in this layer.
- Ozone Layer - as Oxygen absorbs sunlight the energy of the photons can do one of two things. They can cause the molecules to speed up (increasing their temperature) or they can be absorbed by electrons, including those involved in the O2 bond. This addition of energy makes the oxygen molecules reactive and some break apart. The singlet oxygens are highly reactive and, if they happen to bump into oxygen molecules, ozone (O3) molecules are formed. Ozone is important because it absorbs ultraviolet radiation and the layer in the Stratosphere greatly reduces the amount of UV that reaches organisms at the surface. UV, of course, can break chemical bonds and wreaks havoc with complex organic molecules like proteins and DNA.
- Jet Streams - these occur at the lower areas of the stratosphere and are currents of air that move very fast (some over 300 kpm). The jet streams cause movement in the troposphere and affect the movement of the high and low pressure areas of the troposphere we follow on the weather forecasts.
Mesosphere - from stratopause to the Mesopause at about 100 km. This is an area of cooling gasses and it drops from -10° to -100°C. However, at the mesopause, the upper boundary of the mesosphere, the temperature begins to rise again.
Thermosphere - from mesopause until the atmosphere becomes too thin to measure, over 3000 km up. It heats because nitrogen and oxygen are maximally exposed to the sun and they can heat up to temperatures over 2000°C. However, the atmosphere is so thin here that the amount of energy involved is not very great and, were you to be exposed to it, it would not feel like exposure to such a temperature at the surface. The thermosphere is divided into two different layers
- Ionosphere - mesosphere to 600 km. Here, the gas molecules are broken apart by the absorption of sunlight and they become ionized as they leave behind or take electrons. The ions have a strong effect on longwave radiation, especially very long radiation and the can cause reflection of radio waves or can, under some circumstances, disrupt radio waves.
- Exosphere - 600 to end of atmosphere.
Thus, our atmosphere is a very complex boundary between the surface of the Earth and space and its presence is vital to creating the conditions suitable for life at the surface.
At the surface of the Earth, energy is received from two sources
- the sun (51 units)
- thermal radiation from the atmosphere (96 units)
and energy is lost through three processes
- evaporation (23 units) - evaporation is the loss of faster moving water molecules from a larger body of water. As they leave, they become part of the atmosphere and the energy of their motion is lost from the surface of the Earth.
- convection by the atmosphere (7 units - called thermals in the book)
- Heat energy is transported either through Conduction (transfer of heat energy from molecule to molecule in a solid), Radiation (conversion to photons and loss through space) or Convection (transfer to molecules in a fluid - either gas or liquid). Air is heated by the Earth and expands, which decreases its density, and it rises as cooler, denser air displaces it at the surface. This convection constitutes a loss of energy from the surface of the Earth,
- thermal radiation (117) - The infrared "glow" of the Earth's surface is a loss of energy
- As you can see, the largest flow of energy is thermal radiation (117units from the Earth, 96 back from the atmosphere
- The 96 units from the Atmosphere is the famous Green House Effect as gasses in the atmosphere absorb thermal radiation from the Earth and return most of it to the surface. Our effect on the concentration of greenhouse gasses is the basis of the worries about global warming.
- Note that the budget is balanced at the surface (51+96=147 units of energy gained and 23+7+117=147 units lost as thermal radiation)
Large-Scale Patterns of Wind, Currents, and Precipitation
Seasonality in temperature is a result of the tilt of Earth's axis
- the tilt is 23.5° which enables us to divide the earth into latitudinal regions
- Arctic and Antarctic Circles
- dividing line between those regions that get 24 hr of sunlight at height of summer, 24 hr of darkness in depths of winter and those regions that get some dark and light periods every day
- Circumference midway between N and S poles
- regions of earth where the sun is directly overhead at least once a year
- go from 23.5° N (Tropic of Cancer)
- to 23.5° S (Tropic of Capricorn)
- tilt divides up the year as well as the Earth
- Solstices (Summer and Winter)
- shortest and longest days of the year
- Equinoxes (Autumnal and Vernal)
- days on which there are 12 hrs of sunlight and 12 hrs of darkness
Wind patterns are caused by the effects of insolation and the rotation of the Earth
unequal heating causes winds as warm air rises and colder air move in to fill space vacated by heated air or warm air moves in to fill space vacated by sinking, cooling air
- on global scale, Hadley predicted hot air rising at equator and moving north where it sinks and then moves toward equator again
- such circulating movement is a Convection Cell
- zone of heating is where hot air rises
- rising air condenses the water it holds and rain is plentiful in this area
- air is moist because it has been in contact with surface of Earth, especially over oceans
- Subsidence Zone is where air in upper atmosphere cools, becomes more dense, and sinks Earthward
- air is dry as there is no source of moisture and what the air contained was lost by rains when air first rose to upper levels of troposphere (lowest layer of atmosphere - the one in which weather happens
- in reality, the circulation is not a single cell in each hemisphere, but is complicated by cooling of air before it reaches the poles
- hot air rises at equators and moves to about 30° N and S, where it cools and subsides
- the rising hot air leads to a low pressure zone (Equatorial Low) and the cooling, descending air leads to high pressure zones at 30°N and 30°S (N and S Subtropical Highs)
- hot air also rises at about 60° N and S and moves both N and S
- Movement toward equator cools at 30° and subsides with air from equatorial heating
- Movement toward pole cools at pole and subsides there
- this results in three convection cells in each hemisphere (from north to equator):
- Polar Cell
- 60° N or S rising air to polar subsidence
- Ferrel Cell
- 60° N or S rising air to 30° N or S subsidence
- Hadley cell
- equator to to 30° N or S subsidence
- The zone of rising air between the north and south Hadley cells is called the Intertropical Convergence Zone and is the area of highest rainfall, on average, on the planet
- caused by moving N or S on rotating Earth
- earth is rotating from left to right, if you look at globe on a page with N pole facing top of page
- person standing motionless on equator revolving faster than is person standing at polar circle
- so as one moves toward equator, one enters a faster region from a slower one
- objects in motion seem to be deflected to left as they are moving slower (to the right) than their surroundings
- moving away from equator, one enters a slower region from a faster one
- objects in motion seem to be deflected to right as they are moving faster (to the right) than their surroundings
Regions of Winds result from combination of convection cells and coriolis effect
- Horse Latitudes - 30° N or S subsidence zones - no winds as air is sinking but not rushing to fill a void
- Doldrums- equatorial zone of rising air - no winds as air is rising but not rushing to fill a void
- Northeast and Southeast Trade Winds - zone between horse latitudes and doldrums where air is moving toward equator and is deflected to the left (east), so it appears to come from the east (northeast in northern hemisphere, southeast in southern hemisphere)
- called trade winds because they were much used by trade shipping
- Westerlies - zone between horse latitudes and 60° N and S zones of rising air where air is moving away from equator and is deflected to the right (west), so it appears to come from the west (northwest in southern hemisphere, southwest in northern hemisphere)
Regions of Precipitation
- Intertropical Convergence Zone results in lots of rain in the tropics
- Subsidence zones result in dry regions at 30° N and S, where most of the deserts occur
- Rising air at 60° N and S results in a second peak in rain at these high latitudes, where most of the northern forests are
- Less rainfall at 60° N and S than at the equator because cooler air carries less moisture
- Polar subsidence zone means poles are desert-like but cold. Snow accumulates because it doesn't melt.
- Steady winds blowing over water cause water to move in large masses called currents
- because moving water is replaced by surrounding water, surface currents pushed by wind tend to form large circular movements called Gyres that circulate around an oceanic basin
- pushed by westerlies the gyres move east at high latitudes north or south
- trade winds push the currents west just north and south of the equator
- counterclockwise in southern hemisphere, clockwise in northern hemisphere
- results in moving warm waters northward or southward where, when they contact land, they can warm the climate of the land
- example is the Gulf Stream in the North Atlantic, which so warms western Europe that palm trees grow in southern Ireland
- can also move cool water south (must do so to replace water moving north) with the opposite effect on the land's climate
- California Current is responsible for the coolness of weather in central and northern California
- Where winds push water away from a continent, surface water can not fill in so water is drawn up from the bottom of the ocean basin
- this is a zone of upwelling and it brings nutrients from the lower waters to the surface where algae can use the nutrients to photosynthesize
- Upwelling zones are cooler than surrounding water and also very productive
Local conditions often alter global patterns
- Mountain chains cause Rain Shadows
- Windward side of mountains have air pushed up from lower elevations
- rising air cools (adiabatic effect) and drops moisture
- Leeward side of mountains have descending, heating air
- dry because it has already lost its moisture
- Important Rain Shadows
- Some Caribbean Islands - lush and wet on windward side, deserts on leeward side
- Eastern side of Andes are wet but westward side, a narrow band of land between the mountains and the sea, is the driest place on Earth (Atacama Desert)
- Washington and Oregon are divided into western, coastal regions and eastern, continental regions by the Cascade Range of the Rocky Mountains
- Western Washington and Oregon (coastal regions) are wet most coastal areas get over 65 inches of rain per year (up to 200 for some parts of the Olympic Peninsula)
- Eastern Washington and Oregon are dry and most areas get less than 12 inches of rain per year
- ENSO (El Niño Southern Oscillation) in the Pacific affects weather globally
- region of warm surface water in equatorial western Pacific Ocean
- prevailing winds push warm water west along the equator
- moist air over warm area rises and drops lots of rain over western Pacific region
- eastern side of Pacific has strong upwelling, leading to great fisheries production
- ENSO conditions alter this pattern
- winds fail and warm water is not pushed west
- warm water stays east and so rains fall on eastern side of Pacific (from Peru to California), often causing flooding
- dry in Australia and Indonesia, often to the point of drought and fires in dry Indonesian forests can put so much smoke in the air that other parts of the world receive less sunlight
Climate, Electromagnetic radiation, photon, lengths, frequency, PAR (Photosynthetically Active Radiation), Near Infrared, Far Infrared, Thermal Radiation, Global Heat Budget, Troposphere, Tropopause, Stratosphere, Stratopause, Ozone Layer, Jet Streams, Mesosphere, Mesopause, Thermosphere, Ionosphere, Exosphere, Conduction, Radiation, Seasonality, Arctic and Antarctic Circles, Equator, Tropics, Solstices, Equinoxes, winds, Convection Cell, Subsidence Zone, Equatorial Low, Subtropical Highs, Polar Cell, Ferrel Cell, Hadley cell, Coriolis effect, Horse Latitudes, Doldrums, Northeast and Southeast Trade Winds, Westerlies, Currents, Gyres, Gulf Stream, California Current, Rain Shadows, ENSO (El Niño Southern Oscillation
Last updated January 13, 2007