by Reason McLucus
Earth’s Energy System
By Reason McLucus
Those who talk about global warming take a very superficial view of earth’s energy system. Even many who call themselves scientists seem to think earth is a simple system of solar radiation in, heating of earth's matter and radiation out. They believe that earth must radiate as much energy out as it absorbs or it will become hotter. They ignore the role of the biosphere and the complex energy role of water. Earth would be uninhabitable if it didn't convert much of the solar radiation it receives into other forms of energy besides heat.
Much of the energy on earth exists as potential energy rather than just the specific form of kinetic energy known as heat energy. The standard definition of “heat energy” is the motion of atoms/molecules. Gas molecules actually move around more as they become “hotter”. Molecules that are part of a solid cannot move independently so they vibrate along with the other molecules that are part of the solid. Kinetic energy includes wind and objects moving through the air or along the ground. This motion is not a form of heat energy, but friction caused by moving through the air, particularly for falling objects, may raise the temperature of the object.
Earth’s climate cannot be understood without examining its complex energy system.
Solar radiation provides most of the energy for earth. Radiation hitting solid and liquid portions of earth is converted into heat energy. Some radiation also heats the atmosphere directly. Plants use solar radiation to store energy in the form of chemical bonds of complex carbon molecules. This energy is a from of potential energy. Cold blooded animals, and even some warm blooded animals, use solar radiation for heat. Humans use radiation to produce vitamin D and changes in skin pigment. A significant amount of solar radiation is reflected back into space.
Geothermal energy provides some localized heating of earth. Natural radioactivity provides a very limited amount of heat. Human activity generates more heat than either source, but in areas of human settlement which cover only a small portion of the earth. The fossil fuels humans use for energy are essentially from solar radiation that plants stored in the past. Geothermal energy could have a significant affect in polar regions, such as the undersea volcano near the recently collapsed Larson B ice shelf in Antarctica.
Humans can affect solar heating through alterations of microclimates including alterations in the biosphere and construction including roadways comprised of black asphalt. Humans can reduce solar heating by encouraging plant growth including planting trees. Replacing plant covered areas with roads and buildings causes those areas to become hotter because radiation is converted to heat rather than being stored in carbon molecules.
Elimination of trees by itself can lead to a process that causes areas to become heat producing deserts. These deserts may in turn affect the air circulation patterns that move heat energy around the globe. There is less certainty about impact on ocean currents that also circulate heat energy around the globe. The earth receives sufficient energy from the sun to melt both polar ice caps, but the heat is received and circulated unevenly. Thus melting of ice caps could indicate an increase in heat circulation to polar areas rather than an increase in the amount of heat energy on earth.
Water plays a major role in earth’s energy system. In solid form as ice and snow it reflects most radiation striking it back into space. Dirt or soot in the snow may convert radiation into heat energy. Liquid water has a more complicated role. In the liquid state, water is transparent to solar radiation but also reflects some radiation into space depending on the angle at which the radiation strikes it. Plants in water like plants on land store radiation in carbon molecules. Fish are heated by solar radiation, or the heat in the water. They use the heat energy for growth and to move around. If the ocean fish populations decline, this heat energy will not be converted to increasing the mass of the fish or powering their movements. The oceans will warm as a result.
Traditional physics recognizes three basic forms of heat transfer: conduction, radiation and convection. Physicists ignore a fourth form of heat energy transfer, evaporation of water, because it does not directly change temperature. Those who talk about global warming concentrate on the radiation and ignore the fact that earth isn’t a good radiator. Most of the earth is covered by water which isn’t a good radiator because it is either reflective or transparent to radiation. Substances which are transparent or reflective are poor radiators.
Conduction plays a far more important role in atmospheric heating than does radiation. As land, human structures and water warm up they conduct heat to cooler air that is in what physicists call “thermal contact”with the land, etc. The warmed air rises allowing cooler air to replace it. The process of conduction causes the substance receiving heat to become the same temperature as the substance losing heat. The process depends primarily on the difference in temperature of the two substances. The greater the difference the more heat is transferred.
The amount of heat transferred by radiation on the other hand depends only on the characteristics and temperature of the substance losing heat. The substance emitting the radiation will only convert a fraction of its energy into radiation which should mean that a substance will transfer less heat energy by radiation than by conduction. If those who talk about radiation heating of the atmosphere have evidence that natural radiation can heat the atmosphere they should provide that evidence instead of simply stating the belief as if they had gotten it from the Bible or Koran.
The atmosphere does not trap heat by trapping radiation. The term "greenhouse gases" is used because some falsely believe that greenhouses remain warm by trapping radiation. Actually greenhouses remain warm because they keep heated air from escaping. Air doesn't transmit heat very well. Heat energy passes through solid objects. Air molecules carry heat around with them until they encounter cooler objects to transfer the heat to or they rise and convert the heat to another form of kinetic energy and potential energy.
Convection heat transfer involves the movement of heated air or water to cooler areas and then conducting heat to the cooler substances there. Examples of major convection heat transfer systems are the North Atlantic Oscillation and the Gulf Stream. Low pressure and high pressure systems are minor convection systems which may redistribute hot or cold air.
Evaporation heat transfer differs from conduction and radiation in that it only transfers heat energy into the air, but may not change the temperature of the air. Evaporation reduces the temperature of the substance the water evaporates from. Air temperature may also drop if heat from the air contributes to the evaporation.
Water has some peculiar thermal properties. It has a high specific heat of 1.0 (water needs 1 calorie per gram to raise the temperature by 1 C). That is it takes more heat energy to raise the temperature of water than to raise the temperature or air, metals, etc. Air has a specific heat of about 0.2 depending upon the degree of confinement. Water also has a high heat of vaporization of 540 calories per gram at its boiling point. This means that in order for water to evaporate each gram must absorb the amount of heat necessary to heat 540 grams of water 1 C. This amount of heat energy must be released for water vapor to condense back to a liquid state. (Water at lower temperatures such as is found in the ocean requires more energy to evaporate, but I will use the 540 calorie per gram figure to avoid complicating the discussion.)
When water evaporates from the ocean, lakes, etc. it carries this heat energy into the atmosphere and may carry it up 10 miles or farther before releasing it and condensing. Plants and animals also transfer excess heat energy to the air by evaporating water. Plants evaporate water through their leaves. Animals exhale water vapor when they breathe. Some animals including humans cool themselves by evaporating water from their skin. Soil also may transfer heat to the air by evaporating water.
This process doesn’t directly raise the temperature of the air, but does move heat energy up into the atmosphere. Increases in the water vapor content of the air slows temperature changes because of water’s high specific heat. The condensation of water in the air does raise air temperature. With water’s high heat of vaporization and specific heat, each gram of water vapor that condenses in the atmosphere releases sufficient heat energy to heat over 2 kg of air by 1 C. Water’s high heat of fusion of 80 calories per gram increases the heating when water droplets freeze in the air. Water must release 80 calories per gram of heat energy to freeze. This freezing typically occurs during winter months or at higher altitudes during summer.
The energy water vapor carries into the atmosphere provides the energy involved in the production of lightning, especially the lightning that occurs above clouds referred to by names such as “sprites” and “blue jets”.
As water vapor rises in the atmosphere a portion of its heat energy changes to a different form of kinetic energy to allow it to rise. This kinetic energy then changes to potential energy. When water falls as rain some of this potential energy turns to kinetic energy and may move dirt or water around. Water that becomes hail may carry enough energy to damage objects on the ground. Water falling in higher altitudes retains some of its potential energy when it hits the ground. If that water subsequently flows through hydroelectric dams, the generators convert some of that potential energy into electrical energy.
Atmospheric water that forms clouds can affect both air and ground temperature. Clouds block the rise of warm air keeping heated air closer to the ground. High air pressure also keeps heated air close to the ground. A common misconception is that clouds cause heating by blocking radiation, but any radiation would still move out from under the cloud cover in microseconds.
On the other hand clouds reflect solar radiation back into space preventing it from heating the earth’s surface. Clouds, particularly the particles in clouds that water droplets condense on, absorb some radiation and may produce sufficient heat to evaporate the water in the clouds. This process is sometimes referred to as the sun “burning off” the cloud cover. Radiation that evaporates water in clouds does not heat the atmosphere itself.
Snow and ice are part of an important feedback loop that may affect temperatures and climate. Snow reflects sunlight back into space reducing the amount of heating. Thus as snow cover increases temperatures will remain below freezing longer. Below freezing temperatures mean precipitation that falls will wall as snow or ice rather than rain. Lower temperatures are more likely to be close enough to the dew point to encourage more snowfall. The amount of water the air can hold increases with higher temperatures. The dew point is the lowest temperature the air can fall to without having to give up water.
When snow cover declines more land is open to sunlight and is heated by the sun resulting in higher air temperatures. The higher air temperatures mean the temperature will be more likely to remain above freezing and precipitation will fall as rain instead of snow.
Those who blame humans for “global warming” ignore the possibly significant impact human intervention in this feedback loop may be having on climate. For example, in North America prior to significant human settlement, vast areas would be covered with snow with no open ground to allow solar heating of the air. Urban heat islands allow snow to evaporate faster in the cities with some of the heat gradually spreading to the countryside.
Clearing snow off rural highways may not seem like a significant act, but the cleared roads provide the opportunity for solar heating of the ground and air. As snow heats the pavement some of the heat spreads to the surrounding ground allowing additional melting. Pavement can become hot enough from solar radiation that snow will not only melt, but some of it may even absorb enough heat to evaporate.
This human affect on the snow feedback loop won’t eliminate winter, but may reduce its length and allow more days with above freezing temperatures during winter. Early snows will be less likely to trigger early cold weather. Early snows that remain on the ground will slow warm ups after early movement of cold Arctic air into temperate areas.
Those who talk about “global warming” oversimplify earth’s energy system. Earth is not a simple system involving radiation in and radiation out. Solar radiation received by earth is converted into other forms of energy and used for such activities as powering the biosphere.
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