Global Warming Primer
	In order to fully understand the issues surrounding Global Warming, one must have a basic knowledge of the underlying physical principles.  The purpose of this page is to provide such an understanding in the simplest way possible.
	Global warming refers to the observed increase in the average temperature of the oceans and the air near the Earth's surface in recent decades, and the projected continuation of these temperature increases predicted by climate models.
	Energy from the sun, over geological time periods, is the source of over 99.9% of the earth's energy.   About half of this solar energy is radiative energy in the visible portion of the spectrum.  Most of the remainder is in the near-infrared, with a small amount in the ultraviolet.  Very little of the visible, and the shorter wavelength (higher-energy) ultraviolet radiation is absorbed by the atmosphere, and so passes through to heat the earth's surface.  Whereas, the longer wavelength (lower-energy) infrared radiation is readily absorbed in the lower levels of the atmosphere (the troposphere) which contains approximately 75% of the mass and almost all the water vapor and aerosol in the atmosphere.
	A  simple model, based only on the absorption and emission of radiative energy, is sufficient to discuss the heating of the atmosphere.  As the atmosphere is heated by absorbing solar radiation, it also emits infrared radiation in all directions, and some of this radiated energy is lost into outer space.  If the atmosphere absorbs more energy than it emits, its temperature will rise.
	The greenhouse effect is the difference between the actual heating of the earth's surface and atmosphere due to net infrared radiation absorption, versus that which would occur if there were no atmosphere to absorb the radiation.  There are two components to this effect.  There is a natural greenhouse effect due to infrared radiation absorption by naturally occurring atmospheric gases, and there is an anthropogenic greenhouse effect which is a consequence of the absorption of infrared radiation by gases emitted into the atmosphere by human activities.
	The atmospheric gases; oxygen, nitrogen, and argon, that constitute the bulk of the atmosphere, do not absorb a significant amount of infrared radiation.
	The green house gases, water vapor, carbon dioxide and other gases containing carbon, and ozone however, are components of the atmosphere that are strong absorbers of infrared radiation..
	Carbon exists in the Earth's atmosphere primarily in the gas carbon dioxide (CO2).  Other atmospheric gases containing carbon are methane and certain fluorocarbons (which are man made).   These carbon-containing gases are all strong absorbers of infrared radiation, and their concentration has been increasing in recent decades, contributing to  to an increase in the greenhouse effect and global warming.
	Greenhouse gas concentrations are contributed to by the burning of fossil fuels, land clearing, and agriculture.
	The prevailing scientific opinion on climate change is that most of the observed global warming since the mid-20th century is due to the observed increase in anthropogenic greenhouse gas concentrations, and the corresponding increase in the greenhouse effect.
	Computer models, based on observation and climate physics, predict that global temperatures are likely to increase by several degrees °C  by the year 2100.  The uncertainty in the amount results from a range of predicted/possible future greenhouse gas emission and uncertainties regarding their impact on climate.
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