Understanding Our Earth, Part III
Now this day, my Sun Father,
Now that you have come out standing
To your sacred place.
That from which we draw the water of life
Prayer meal -
Here I give unto you.
Your long life,
Your old age,
Your strong spirit,
Of all these, to me may you grant.
-- EGYPTIAN PRAYER TO SUN GOD
A number of ancient cultures worshipped the sun. They realized on some level that all life owed it's existence to the sun. While this may have been based, in part, on a need to fulfill a spiritual void, and they did not have the scientific understanding that we have gained, primitive cultures were more connected to the natural world around them and therefore recognized in their own way the importance of the sun in the natural order of things. So, even without the benefit of "science", they were right, the sun is the source of life on this earth.
Our sun is a middle-aged star that lights and warms our planet. It is a gigantic thermonuclear reactor of 72% hydrogen and 28% helium. The extreme temperatures and pressures fuse hydrogen nuclei into helium nuclei releasing enormous amounts of energy in many forms of electromagnetic radiation, including heat and light. Solar energy is necessary for life for 2 main reasons: warmth and light.
THE "GOLDILOCKS PLANET" (warmth)
Like the bowl of porridge (from the story Goldilocks) that was not too cold (Mars) or too hot (Venus), but "just right", the Earth seems to be in just the right position in our solar system to maintain the conditions of life. Life and the organic molecules upon which it is built have a narrow temperature tolerance. At higher temperatures most organic compounds break down and become nonfunctional. (Just like your body when you have a temperature that is too high for too long, molecules necessary for life degrade and you die.) At lower temperatures the chemical reactions of life (metabolism) cease or slow down too much to maintain a living system.
The sun warms our planet. This warming (incoming solar radiation) powers the cycling of matter (biogeochemical cycles such as the carbon cycle) and is responsible for our climate and weather systems that distribute heat and fresh water throughout our biosphere. As discussed in the previous section, "The Second Element - Wind", our atmosphere contains greenhouse gases (water vapor, carbon dioxide, methane, etc.) that contribute to capturing and holding in the sun's energy. Without this "thermal blanket" of greenhouse gases, the captured heat would all radiate back out into space (infrared radiation) leaving this planet nothing more than the "3rd Rock from the Sun". So as we have said before, global warming (greenhouse effect) is a natural, necessary process that allows for life, but human activities are upsetting this delicate energy balance by increasing the rate of global warming.
Sunlight doesn't strike the earth's surface evenly and therefore doesn't heat the surface evenly. At any one time one-half the earth is dark, the other half in direct sunlight. Some of the surface is water, or snow and ice or soil and rock that absorbs or reflects light and heat in varying amounts. Cloud cover reflects sunlight, but also helps hold in heat radiating out from the earth. (Notice how a cloudy day sometimes feels warmer?) The tropics receive more sunlight or solar radiation than the higher latitudes (polar regions). Life on earth would be very difficult indeed if the energy (and water) were not redistributed. The equatorial regions would be hotter, the polar regions colder, and most land masses would be deserts with only the coastal regions receiving enough moisture to sustain life.
This energy imbalance is evened out by the movement of air driven by the sun. When our atmosphere is warmed by solar radiation, it expands and rises, creating currents and pressure differences (winds) that ultimately carry water vapor and energy into upper regions to flow and descend, redistributing water and energy from low to high altitudes, from low latitudes (equator) to high latitudes (the temperate zones and ultimately the polar zones) and from the oceans inland determining our weather patterns, climates and the final distribution of plants, animal, and humans. Solar radiation drives the winds that sustain the water cycle (hydrologic cycle) that determines where life can exist on land.
Oceans also receive solar radiation with some of it reflected, and some of it absorbed heating the water. Because of this oceans play an important role in the redistribution of heat on the planet helping keep the earth's temperatures more moderate. The colder polar waters are dense and sink and flow towards the equator. Warmer surface water in the tropics tends to stay on top. The movement of the earth rotating on it's axis combined with the seasonal tilt differences from winter to summer plus the movement of air currents (produced by the uneven heating) contributes to the mixing of these colder and warmer waters vertically (upwelling) and creating currents from the equator to the polar regions and back. (The Gulf Stream that flows from the equator along the east coast of North America, carries the warmer water that makes the climate of Great Britain so mild.) Without this interaction of ocean currents and air currents, heat storage and movement throughout the planet would not be moderated and there would be extremes in temperatures much like on the moon.
ENERGY FOR LIFE (light)
Not all light energy that reaches the earth is absorbed or reflected by the physical environment. Some of this light energy is captured by green plants on land and algae and phytoplankton in water. These are called producers (or autotrophs), because they make their own food using sunlight energy and compounds from the environment. Only producers (green plants, etc.) make their own food. All other life forms must feed on other organisms to obtain what they need and are called consumers.
Most producers capture sunlight to make sugars (and other compounds) through the process of photosynthesis. Photosynthesis captures the energy in the photons of light and changes it into the chemical bond energy of sugars and other compounds. Now the solar energy is in a form (chemical energy) that is useable by other organisms.
Only about 1% to 2% of all the sunlight that reaches the earth's surface is used in photosynthetic activity. All organisms depend on the sunlight energy trapped in the chemical bonds of the sugars and other compounds to run the activities of life. Life on earth depends on this one-way flow of energy from the sun -> green plants -> consumers.
All organisms (producers and consumers) are potential sources of food for other organisms. In the process of one organism feeding on another, the organic compounds are broken down and the solar energy stored in the chemical bonds is released to provide energy to run the cell's activities. This process is called cellular respiration.
Both producers (plants) and consumers (animals) use this process to supply energy to run the processes of life. Take notice that the process of photosynthesis is the reverse of the process of cellular respiration. What this means is that green plants are capable of both capturing solar energy and storing it as chemical energy for their own use. Consumers can't. They can only use the chemical energy provided for them by the plants and others they feed on. In a sense, we are all vegetarians because of our dependence on plants to capture and convert solar energy.
"We are trapped starlight."
So how does this energy flow through the ecosystem? The sequence of one organism feeding on another is called a food chain:
This "chain" of feeding is how energy and nutrients move through the ecosystem. Each feeding level in the chain is called a "trophic" level (from the Greek word trophos, "nourishment"). The first trophic level is always the producers (or green plants). All the other trophic levels (second level, third level or fourth level) are consumers. Real ecosystems are more complex and consist of food webs, which are interconnected networks of food chains.
In food chains energy is stored in the chemical compounds of the biomass of an organism. Biomass is the dry weight of all the organic matter (chemical compounds containing the energy from the sun) in that organism. This biomass is transferred through food chains from one level to another. Some energy is lost to the environment as heat with only a small amount actually converted into the tissues (chemical compounds) of the organism. (You really are what you eat!)
The percentage of energy transferred from each trophic level to another (by way of the biomass) is very small; only 10% of what is eaten becomes biomass. If we feed 100 kilograms of grain to a cow, only 10 kilograms transfers to the cow. If we feed 10 kilograms of cow to a human, only 1 kilogram becomes transferred to the human.
100 kilograms of grain => 10 kilograms of human
If we skip the cow and eat "lower" on the food chain by eating more grains, vegetables, fruits, etc., the earth could better feed the growing human population. We could also potentially feed 10 times more people than we do now.
FOOD FOR THOUGHT
Technically there is no limit on solar energy. But there is a limit on how much makes it up the food chain to support life. Net primary productivity (NPP) is a measurement of the rate at which energy for use by consumers is stored in new biomass. It is now estimated that humans use, waste or destroy about one fourth (25%) of the entire earth's potential NPP and 40% of just the land based NPP. Through our habitat destruction and overuse of resources for growing or raising our own food species, we are upsetting the one way flow of solar energy through the ecosystem. Can we afford to do this? What will happen to us (and the rest of nature) when human population doubles from 6 billion (who are hard to feed now) to 12 billion in this century?
Fire - fire from our sun that fuels all life, drawing up plants and raising waters to the sky to fall again replenishing. The inner furnace of your metabolism burns with the fire of the Big Bang that first sent matter-energy spinning through space and time. And the same fire as the lightning that flashed into the primordial soup catalyzing the birth of organic life.
You were there, I was there, for each cell of our bodies is descended in an unbroken chain from that event.
-John Seed and Joanna Macy