Recent Global-Rural Urban Mapping Project (GRUMP) data reveals that about 3% of the world’s surface is “urbanized” (although the actual definition of “urban” as well as “rural” is questioned and argued over). Most of the nearly 9 billion humans who reside on this planet live in these urbanized areas. Recent data indicates that the figure today is about 56%live in “urbanized” regions and that by 2050 the urban population is expected to grow to as much as 75%.
One has only to look at images of the Earth available on Google Earth to see the sprawling, coalescing urban desolation spreading across our planet. But there is no disagreement about the fact that it is in these “urbanized” areas where most of the world’s population resides and which are responsible for 3/4 of the global warming that we are experiencing.
Sitting in front of a window on a sunny but cold winter’s day, the sun’s rays pass through glass almost unaltered. Placing your hand on the glass one can feel it remains cool. But that same radiant energy that does not heat the glass strikes your face or hands and warms them. Like a pane of window glass the atmosphere does not heat as solar radiation passes through it. But—like your face or hands— it does heat the Earth surface it strikes. It is the warmed earth surface that heats the gases above it.
Some of those earth surfaces absorb and reradiate solar radiation better than others. Dry, barren earth and rock, beach sand, and black asphalt are all powerful absorber and “re-radiators” of solar heat, as is concrete, steel, most metals, roofing materials, etcetera, all are in this category. Recall how on a hot summer day, you can not walk barefoot across an asphalt coated parking lot…or even on the hot sand of a beach. These materials have absorbed solar radiation.
These areas heat up and then release that heat as “earth radiation” which passes upward through the air. This earth radiation is the energy that is absorbed by certain “heat trapping” gases in the atmosphere to warm it.
Some of the natural “heat trap” gases are better than others in absorbing earth radiation…these are the so-called “greenhouse” gases: nitrous oxide (N2O), methane (CH4), carbon dioxide (CO2) and water vapor (H2O). (Other pollutant gases polyflourinated compounds used as refrigerants in air conditioners may be many times more effective)
Both carbon dioxide and water vapor are natural common constituents in the air. Nitrous oxide is a product of combustion, production of chemical fertilizers and agriculture, as well as bacterial decay. Methane is produced by decay of landfill wastes, wetlands, rice fields, animal wastes, fossil fuel extraction (i.e. natural gas).
So it is clear that urban areas constructed of materials such as concrete, asphalt and metal are much better absorbers of heat. They absorb solar heat and the radiate it away. They are most effective in heating the atmosphere. Urban areas are hotter than rural areas!
But there is another reason too. Trees! And their demise in our landscape!
My grandfather’s farmstead had a tin roofed car port—open on all sides. On a summer day the car port was just as shady below this roof, as it was under the near-by eighty-year-old Mulberry tree. The temperature difference between the two shady areas was enormous. No one would dream of sitting in the shade of the car port on a hot day. But the deep cool shade of the old Mulberry tree beckoned us all. I was curious about the reason for this discrepancy. Grandpa when asked, explained that the tin roof absorbed heat and the tree didn’t.
I tested that explanation out. On the next sunny day I conducted an experiment. Standing on a chair, I was just about tall enough to touch the metal of the tin roof. — it was very hot! Hot enough to burn my fingers and (I guessed) to cook eggs on too. But the green Mulberry tree leaves—exposed to the exact same sun intensity were cool to the touch. That summer observation remained a puzzle to me—well until my last year in High School. Finally, in Mr. Snow’s biology class I finally learned that Grandpa’s explanation was correct—but incomplete.
In Dr Snow’s class, I learned about photosynthesis and that green plants use the energy of sunlight to convert water and carbon dioxide gas into sugars. The carbon bonds of the these carbohydrates require energy sources to form and then store that energy—converting H2O and CO2 to carbohydrates so that energy which heated up the surface of the tin roof was used to create substances like sugar, starches and cellulose in the green plants where that energy was stored. (Years later, Grandpa had to cut that Mulberry down and cut it into firewood..The energy that was stored in the wood all those hot summers was released as heat and radiant energy in the old cast iron fireplace in Grandpa’s big kitchen).
The tree leaves remain cool for that reason—they are storing solar energy, not as heat, but as cool chemical bonds. Trees absorb solar radiation and store it. Some explain this by claiming that trees and forests are “heat sinks” were solar radiation is stored as leaves, and wood.
As a consequence forests and wooded areas are generally cooler than urban areas. Urban areas are comprised of substances such as asphalt, concrete, metal, glass, plastic and other man-made substances that simply absorb heat and then reradiate that back out into the surrounding air.
A large portion of global warming is the result of greenhouse gas emissions. But a good part of the problem has to come from the fact that we are creating a synergistic effect by cutting down forests as we expand urban areas. The sum of our denuding the earth of forests and expanding urban areas are greater than their individual effects. Forests absorb and save the carbon, they also keep the Earth cooler so there is less earth radiation to be trapped by green house gases.
There is more to this story…..but this is a start. Think about saving that beautiful tree…
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