Greenhouse Effect

The greenhouse effect refers to circumstances where the short wavelengths of visible light from the sun pass through a transparent medium and are absorbed, but the longer wavelengths of the infrared re-radiation from the heated objects are unable to pass through that medium. The trapping of the long wavelength radiation leads to more heating and a higher resultant temperature. Besides the heating of an automobile by sunlight through the windshield and the namesake example of heating the greenhouse by sunlight passing through sealed, transparent windows, the greenhouse effect has been widely used to describe the trapping of excess heat by the rising concentration of carbon dioxide in the atmosphere. The carbon dioxide strongly absorbs infrared and does not allow as much of it to escape into space.

Sunlight warms your car Increasing atmospheric carbon dioxide Global warming Transmission of water vapor Earth's energy balance Role in the absence of water on Venus?

A major part of the efficiency of the heating of an actual greenhouse is the trapping of the air so that the energy is not lost by convection. Keeping the hot air from escaping out the top is part of the practical "greenhouse effect", but it is common usage to refer to the infrared trapping as the "greenhouse effect" in atmospheric applications where the air trapping is not applicable.

Greenhouse Effect Example

Short wavelengths of visible light are readily transmitted through the transparent windshield. (Otherwise you wouldn't be able to see through it!)

Shorter wavelengths of ultraviolet light are largely blocked by glass since they have greater quantum energies which have absorption mechanisms in the glass. Even though you may be uncomfortably warm with bright sunlight streaming through, you will not be sunburned.

Increase in Greenhouse Gases

The increase in the concentration of carbon dioxide, one of the three major atmospheric contributors to the greenhouse effect has been carefully documented at the Mauna Loa Observatory in Hawaii. The 1990 rate of increase was about 0.4% per year. The interesting cyclic variations represent the reduction in carbon dioxide by photosynthesis during the growing season in the northern hemisphere.

Current analysis suggests that the combustion of fossil fuels is a major contributor to the increase in the carbon dioxide concentration, such contributions being 2 to 5 times the effect of deforestation (Kraushaar & Ristinen).

Data from NOAA

The Mauna Loa monitoring station reports the carbon dioxide level in the atmosphere today as over 400 parts per million compared to 315 ppm in 1958 when modern measurements were initiated. Measurements of air bubbles trapped in the Greenland ice sheet indicate concentrations of 270 ppm in preindustrial times.

These are sketches of the graphs produced in the IPCC 2007 report of the increase in key greenhouse gases. They make clear that most of the increase of the last thousand years has occurred in the past 200 years. The radiative forcing of these gases is related to their concentration.

Contributors to Greenhouse Effect

Those gas molecules in the Earth's atmosphere with three or more atoms are called "greenhouse gases" because they can capture outgoing infrared energy from the Earth, thereby warming the planet. The greenhouse gases include water vapor with three atoms (H₂O), ozone (O₃), carbon dioxide (CO₂), and methane (CH₄). Also, trace quantities of chloro-fluoro-carbons (CFC's) can have a disproportionately large effect.

Note that the above data is decades old, and the atmospheric CO₂ is projected to be about 410ppm in 2020. Major efforts have been made to reduce the amount of CFC's in the atmosphere.

To attempt to quantify the effects of greenhouse gases on the global temperature, climatologists use the "radiative forcing" of the current atmospheric content of these gases.

Global Warming

An issue of major concern is the possible effect of the burning of fossil fuels and other contributors to the increase of carbon dioxide in the atmosphere. The action of carbon dioxide and other greenhouse gases in trapping infrared radiation is called the greenhouse effect. It may measurably increase the overall average temperature of the Earth, which could have disastrous consequences. Sometimes the effects of the greenhouse effect are stated in terms of the albedo of the Earth, the overall average reflection coefficient.

This graphic of the global air temperature was posted by Phil Jones on behalf of the Climatic Research Unit, UK. The key reference used was Brohan, et al.

Another depiction of the mean temperatures in the northern hemisphere was drawn from NOAA.

Essentially any kind of tabulation you access will tell the same story. The temperature has gradually risen over the last 150 years.

Because the potential consequences of global warming in terms of loss of snow cover, sea level rise, change in weather patterns, etc are so great, it is a major societal concern. On the other hand, proposed measures to reduce human contributions to greenhouse gases can also have great consequences. The large potential impact combined with the ambiguities of the science has given rise to many passionate extremes.

Stephen Schneider of Stanford seems to me to be one of the more balanced voices. His website is a good source for relevant data. He discusses the problems in the context of the Earth's energy balance and the changes in the concentrations of greenhouse gases.