2006-04-28 / Columnists

Drawing On Science


Global Warning I: The Greenhouse Effect
Global Warning I: The Greenhouse Effect

by Stephen Yaeger

Global warming is a controversial topic. Many scientists believe that the increase in global temperatures is partly due to human activities, while there are others who oppose the concept of global warming saying it just isn't happening. Because of the nature of the topic I have decided to divide it into three separate columns.

To better understand global warming let's first take a look at how the sun's energy is distributed, absorbed and reflected once it contacts Earth. The sun's energy powers our weather, provides energy for the synthesis of food in green plants and algae, and warms the Earth's surface. The sun gives off heat energy as part of the electromagnetic spectrum (EMS) in the form of visible light having short wave lengths. When sunlight reaches Earth it is partly used to heat the planet. The heat energy must be moved around to make it useful. There are three ways in which heat energy moves: Radiation from the Earth is in the form of infrared energy, having long wavelengths therefore less energy. (In the EMS short wavelengths have higher energy than longer wavelengths.) In conduction heat energy is transferred directly from a hot object to a cooler object when they come in contact with each other; when cool air comes in contact with warm water the water transfers heat to the air. The water becomes cooler while the air warms up. Convection occurs when cooler, denser water, for example, sinks and warmer, less dense water rises. The warm water then cools and sinks, again pushing up the warmer water; these up and down movements are called convection currents. Now let's see what happens to the sun's energy radiating to the Earth.

The sun radiates energy in all directions. Only about 1/2,000,000,000 (one-two billionth) of the energy reaches Earth. This is known as insolation (INcoming SOLar radiATION). Let's consider this small number as 100% of the sun's energy reaching Earth and see what happens to it. 26% and 4% are reflected back into space by the atmosphere and the Earth's surface respectively for a total of 30%; 19% and 51% are absorbed by the atmosphere and surface respectively for a total of 70%.

Keep in mind that the Earth, in turn, radiates infrared energy from its surface. The Earth must maintain what is called a heat balance . Of the total energy absorbed by Earth (again, let's call it 100%) 21% and 49% are radiated back into space by the surface and atmosphere respectively and another 30% is conducted into the lower atmosphere warming it up. The warmer, less dense air rises and the higher, cooler and denser air fall creating convection currents of air. Warm/cool air moves between land and water transferring heat also. This horizontal wind movement is known as advection.

Sunlight, then, consists of short wave lengths, passing through the atmosphere to heat the Earth's surface. It is absorbed by the soil, which becomes warmer. To maintain a heat balance the Earth's surface and atmosphere radiate infrared waves. These longer waves are, for the most part, absorbed by the carbon dioxide and water vapor in the atmosphere, as well as other prticles, and remain trapped, so that they can't pass through the atmosphere back into space. What is happening is that the short light waves are allowed to pass through the upper atmosphere, but the longer infrared waves remain inside the atmosphere. This is known as the greenhouse effect and the gases are called greenhouse gases . Something similar happens in a greenhouse, but here the glass roof and sides act as greenhouse gases preventing the infrared waves from escaping. Anyone who has ever visited a greenhouse knows this. We tend to keep our houses warm if we allow sunlight in through a south-facing window. This is also a greenhouse effect.

It is this trapped infrared energy that contributes to the warming of our planet. In recent years there has been some evidence that the greenhouse effect is increasing at an alarming rate. This is the result of too much infrared energy being trapped due to an increase in greenhouse gasses, which, in turn, causes an increase in global temperature over the years. According to the National Academy of Sciences, Earth temperatures have increased 1 o F in the past 100 years. In the last decade there has been accelerated warming based on the evidence indicating an increase in carbon dioxide, methane, and nitrous oxide due to human activity. Atmospheric carbon dioxide has been increasing steadily since the Industrial Revolution coupled with improved methods of deforestation and mining. Excess emission of CO2 is due to wood and fossil fuel (coal, oil, gasoline, etc.) consumption. Methane is emitted when organic material decays, landfill garbage decomposes and fossil fuels burn. Nitrous oxide results from the use of agricultural and industrial activities, combustion of solid waste and fossil fuels. If that's not enough consider the man-made gases: hydrofluorocarbons , perfluorocarbons , and chlorofluorcarbons (linked to ozone depletion in the stratosphere) resulting from various industrial processes.

Ozone is a (greenhouse) gas that naturally occurs in the stratosphere. It shields Earth's surface from destructive ultra violet radiation. Depletion of ozone in this atmospheric layer presents a major problem to health. In the lower troposphere UV is a danger to the ecosystem as well as being a health hazard. According to research by the NASA Goddard Institute for Space Studies ozone in the troposphere is responsible for 1/3 - 1/2 of Artic warming trends during winter and spring. This trend is due to the industrialized nations' production of ozone in the Northern Hemisphere, which reaches the Arctic easily during winter and spring. From December through May in 1880 normal global temperatures were recorded. From 1950 - 1990 increasing temperatures were detected. Not only do these gases contribute to global warming, but they're also not exactly the type of gases we want to breathe.

Next month: Facts supporting global warming.

Questions/comments?

E-mail Steve: Drawingonscie nce@ aol. com

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