Discovery Channel – Global Warming, What You Need To Know, with Tom Brokaw
Maintaining a Balance
On a sunny day, the earth usually reaches a certain maximum temperature for the day but does not keep getting hotter. This is so because the earth, like any other heated object, radiates heat. The energy that is not reflected back into space is absorbed in the earth’s climate system. However, if the earth kept receiving heat, it would continue to heat up continuously. To maintain a constant temperature, the earth must radiate back to space the same amount of energy that it receives. By doing this, the earth maintains thermal equilibrium.
This balance is based on the following three things.
1. Incoming solar radiation. Currently, an average of 342 W is received on every square meter of surface on which the sun shines directly. The amount of incoming radiation can change if the sun’s output varies or as a result of changes in the earth’s orbit.
2. Reflection. Currently, about 30 percent, or 107 W/m2. The amount of reflected light can change if the albedo, or reflectance, of surfaces on the earth, including clouds, aerosols, ice, or vegetation, varies.
3. Absorption. Currently, about 70 percent, or 235 W/m2. The amount of invisible infrared radiation absorbed by the atmosphere depends on the concentration of greenhouse gases.
What If the Earth Did Not Have an Atmosphere-Nature’s Greenhouse?
Thermal equilibrium is established between the sun, sending about 1370 W for every square meter of surface, and the earth (being a sphere), receiving about 342 W. Thermal engineers know that the temperature of an object floating in space-such as a satellite-will be determined by the heat input from the sun, the amount of light reflected, and the amount of heat emitted. The sun, like any other random object floating in space, is obligated to follow the same laws of thermodynamics.
Based on its thermal properties, a 30 percent reflective, 100 percent emissive object in earth’s orbital position should be –19°C (–2°F). Without an atmosphere, the earth would be much colder than it actually is.*
This, of course, is not the average temperature of the earth. The reason for the difference is that the atmosphere absorbs and retains heat. The reason that the earth is not an uninhabitable slush ball is because of the natural greenhouse effect.
* The amount of energy, E, emitted by a warm object at temperature, T (in kelvins), is given by the Stefan Boltzman’s law: E = σT 4, where σ = 5.67 × 10–8 W/m2 K4. At equilibrium, the energy received by the earth equals the energy given off by the sun: π RE 2(1 – α) S = 4πRE2σT 4 where π = 3.14 and RE is the radius of the earth (which cancels out of the equation). With albedo α = 0.3 and solar constant S = 1370 W/m2, solving for T gives –19°C (–2°F). The earth is much warmer than this calculated value because of the natural greenhouse effect.
Satellites Detect Earth’s Infrared Radiation from Space
When an object gets warm, we can actually “feel” it from a short distance away. What we feel is the heat radiating across space. (During radioactive heat transfer, we would feel the warmth even if there was no air between us and the object.)
Where did that part of earth’s heat energy go? The “missing” energy was gobbled up by carbon dioxide and other gases. Ozone also took a bite out of the energy at 10-μm wavelength. The point of this is that the energy missing from the satellite measurements has been absorbed by the atmosphere and is raising the temperature of the earth. This graph is a direct snapshot of the earth subjected to the greenhouse effect.
Venus and Mercury
Our solar system provides an opportunity to observe how carbon dioxide in a planet’s atmosphere can affect its temperature. Mercury is, on average, 58 million km (33 million miles) from the sun, and Venus is, on average, 108 million km (67 million miles) from the sun. Mercury receives about 3½ times more solar radiation than Venus. Based on distance alone, we would expect mercury to be more than 35 percent hotter than it actually is. It turns out, though, that Venus, despite being farther from the sun, is warmer than Mercury. The average temperature on Venus is over 460°C (860°F), which is far greater than the 170°C (338°F) found on Mercury. The temperature on Venus is hot enough for rocks to glow visibly and for lead to melt. The reason is that the atmosphere of Venus is 95 percent carbon dioxide, whereas Mercury’s atmosphere is very thin, with negligible amounts of carbon dioxide. Venus is as hot as it is because of a runaway green house effect taking place there. Venus does not represent a realistic scenario for what might happen to the earth, but it does help us to underscore the influence that absorption in the atmosphere can have on temperature.
Carl Sagan, an American scientist working in the 1960s, determined that the atmosphere of Venus is extremely hot and dense. He related global warming on the earth as a growing, human-induced danger analogous to the transformation of Venus into an inhospitable, overheated planet as a result of the buildup of greenhouse gases in its atmosphere. Sagan’s predictions about the surface of Venus were confirmed by the Mariner 2 spacecraft, whose mission he helped plan.
James Hansen, a NASA scientist, refined Sagan’s calculations and included the effects of the sulfate aerosols in Venus’s atmosphere. Carbon dioxide is an invisible gas. Sulfates and not carbon dioxide are what gives Venus its characteristic hazy cloud cover. Aerosols such as suspended sulfates play a role in the heat balance in the atmosphere of both Venus and earth.
The Glass of the Earth’s Greenhouse
In 1958, Charles Keeling began monitoring carbon dioxide levels in the atmosphere. He chose the remote location of Hawaii’s Mauna Loa Observatory to eliminate possible effects of local industry or surrounding vegetation cycles. The observatory was at an elevation 3.35 km (11,000 ft) above sea level, which put it at about the halfway point into the troposphere. Keeling chose that location because he believed that it would give an excellent representation of the entire earth. He developed a method of collecting air samples in fl asks and then analyzing them in the laboratory to a level of precision of parts per million (ppm). Keeling is credited with having extended the state-of-the-art of measuring small gas concentrations at the time to accomplish this.
Since carbon dioxide is stable in the atmosphere for extended periods of time, and because it mixes very thoroughly with other atmospheric gases, Keeling considered measurements above the Pacifi c to be truly global in nature. When Keeling started making measurements in 1959, the carbon dioxide level was 316 ppm. This is about 13 percent higher than preindustrial levels. Today, the carbon dioxide reading is over 380 ppm, representing a 35 percent increase above preindustrial levels.
Keeling’s measurements show a steady increase in the carbon dioxide level. A saw tooth overlay shows a recurrent cyclic increase and decrease each year coinciding with the alternating growing and winter seasons in the northern hemisphere (which has a much larger presence of plants than the southern hemisphere). Carbon dioxide was higher during the winter and lower in the summer each year. Keeling’s chart showed the role of photosynthesis in removing carbon dioxide from the atmosphere during the warmer months when plants were growing. In the fall, leaves and other remnants from the growth process fall to the ground and decompose, releasing carbon dioxide back into the atmosphere. Despite its ups and downs, the most significant feature of the Keeling curve is that the overall trend is upward.
THREE METHODS OF HEAT TRANSFER
Radiation is unique in that it does not require contact between the objects transferring heat. The sun heats the earth by radiation through space. The earth keeps from overheating by radiating some of the heat that it has absorbed to space. The earth radiates mostly at a temperature that produces invisible infrared light. The greenhouse effect occurs when some of this radiation is intercepted and absorbed by greenhouse gases in the atmosphere.
Convection Sometimes fluids act a conveyer belts to facilitate the flow of heat from one place to another. Prevailing wind patterns transfer heat to restore an imbalance of temperatures on the surface of the earth. Storms perform the same function, although in a more abrupt and often destructive manner. Ocean currents such as the Gulf Stream move enormous amounts of heat. This helps to keep regions such as Europe at a much more temperate climate than they otherwise would be.
This occurs when there is direct contact between materials such as the atmosphere and the oceans. Conduction proceeds until thermal equilibrium is achieved. This happens when the temperature of both adjoining materials is the same. This can take place even though one of the materials contains more heat than the other. This is the case when the air and the oceans reach equilibrium. They are at the same temperature, but a given mass of water has a much greater amount of heat stored in it than a comparable mass of air.
Carbon dioxide does not absorb visible light from the sun. The enhanced greenhouse effect is the result of gases in the atmosphere (such as carbon dioxide) that absorb invisible infrared radiation coming from the warm surface of the earth.
