Greenhouse effect. The greenhouse effect is a process by which certain gases in a planet's atmosphere give it a higher temperature than the planet would have otherwise:
Light is one form of electromagnetic (EM) radiation. From lower energy to higher energy, categories of EM radiation include: radio waves, microwaves, infrared, visible light, ultraviolet, x-rays, and gamma rays. Blackbody radiation is EM radiation given off by all matter, as a function of the matter's temperature. Hotter objects give off more total EM radiation, and also give off a greater fraction of that radiation as higher energy radiation.
Since the Sun has a surface temperature of 5,700° C, about 42% of the blackbody radiation it radiates is visible light. (About 8% is ultraviolet and about 50% is infrared.) EM radiation from the Sun reaching the Earth is either reflected back into space or absorbed by the Earth, warming it. The Earth also radiates blackbody radiation but almost entirely as infrared light (since it is much cooler than the Sun).
Certain gases in the Earth's atmosphere are transparent to the Sun's visible light but tend to absorb the outgoing infrared light emitted by the Earth. These are called greenhouse gases and include primarily water vapor and carbon dioxide. (Other natural and manmade gases also contribute.) By absorbing some of this outgoing EM radiation, the atmosphere becomes warmer and also radiates more blackbody radiation. The atmosphere and the Earth itself becomes warmer than it would be without the greenhouse gases.
The Earth's atmosphere (by volume) is about 77.8% nitrogen, 20.9% oxygen, 0.9% argon, 0.4% water vapor, 0.037% carbon dioxide, and 0.003% other gases. The water vapor and carbon dioxide naturally occurring in the atmosphere produce a greenhouse effect which gives the Earth a temperature 30° C more than it would have without these gases, making life possible.
Carbon Dioxide. Natural sources of carbon dioxide include animals, natural fires, and releases of carbon dioxide stored in the ocean or in minerals. Plants also produce carbon dioxide, but they consume more than they produce. Manmade sources include the burning of wood or fossil fuels (coal, oil, and natural gas), clearing of forests, and cement production.
From 1750 to 2003 the amount of carbon dioxide in the atmosphere has increased by 35%. This correlates with carbon dioxide that has been released by manmade sources (primarily burning of fossil fuels), although only 53% of the carbon dioxide released from manmade sources since 1940 has remained in the atmosphere. It is generally assumed that the increased carbon dioxide is the result of human activities, and that the amount of carbon dioxide in the atmosphere will continue to increase.
Global Warming Hypothesis. The global warming hypothesis states that the increase in atmospheric carbon dioxide (caused by mankind) will increase the greenhouse effect, significantly raising global temperatures and having disastrous consequences. Supporters of this hypothesis argue:
Some people go on to argue that we need to take drastic action to avoid these predicted consequences of catastrophic increases in temperature. Specifically, they propose drastic limits on energy consumption.
Problems with the Hypothesis. The global warming hypothesis is not scientifically verified. Critics of the hypothesis argue:
These critics point out that we have not scientifically verified that mankind is causing a measurable increase in global temperatures. They say we should therefore wait for more evidence before we take actions that will cause immediate harm to people.
Consequences. Despite scientific questions, the supporters of the global warming hypothesis have persuaded many national governments to take action. In 1992, most nations agreed in principle to limits on greenhouse gas production. In December 1997, most nations signed the Kyoto Treaty to limit greenhouse gas emissions. The treaty as written would limit the U.S. to the levels of carbon dioxide production it had in 1990, but is less restrictive on developing countries. Critics argue that the U.S. government will have to impose taxes of about 50% on gas, electricity, and heating oil prices to meet these goals. They also argue that millions of jobs will be lost over the next decade if the treaty is implemented.The Clinton administration signed the treaty in 1997, but the U.S. Senate soon overwhelmingly approved a resolution indicating that they would not ratify it. The Clinton administration never sent the treaty to the U.S. Senate for ratification, but they did quietly implement some minor provisions through executive orders. In November 1999, national representatives met in Bonn, Germany, to work out details of implementation, including restrictions on land use. By the second such meeting in 2000, negotiations were falling apart due to disagreements on how to credit removals of carbon dioxide from the atmosphere. In March 2001, President Bush indicated that he would not seek to implement the Kyoto protocol.
|Figure 1Concentration of carbon dioxide in the atmosphere vs. time. Concentration is given in parts per million (ppm). Note that the rate of increase was greater after 1960 than before.|
|Figure 2Global average annual temperature vs. time. Temperature is in ° Celsius. After 1979 the dashed line indicates data compiled from selected ground-based stations, the solid line indicates composite data from satellite and balloon-based measurements. Note that most of the temperature increase occurred from 1910 to 1940, before the greatest increase in atmospheric CO2.|
|Figure 3Solar irradiance in watts per square meter. Values from 1978 forward are from satellite measurements, values before 1978 are reconstructed from proxy measures. Note the effect of the 11-year solar activity cycle.|