Earth's climate has always been in a state of flux, according to data gleaned from the geological record, ice core samples and other sources.
However, since the Industrial Revolution began in the late 1700s, the world's climate has been changing in a rapid and unprecedented way.
Earth's average temperature has risen by 1.4 degrees Fahrenheit (0.8 degrees Celsius) over the past century, the Environmental Protection Agency (EPA) reports. What are the causes of global warming?
The greenhouse effect
Earth's climate is the result of a balance between the amount of incoming energy from the sun, and energy being radiated out into space.
Incoming solar radiation strikes Earth's atmosphere in the form of visible light, plus ultraviolet and infrared radiation (which are invisible to the human eye), according to the Earth Observatory of the National Aeronautics and Space Administration (NASA).
Ultraviolet (UV) radiation has a higher energy level than visible light, and infrared (IR) radiation has a weaker energy level. Some of the sun's incoming radiation is absorbed by the atmosphere, the oceans and the surface of the Earth.
Much of it, however, is reflected back out to space as low-energy IR radiation. For Earth's temperature to remain stable, the amount of incoming solar radiation should be roughly equal to the amount of IR radiation leaving the atmosphere.
As Earth's atmosphere changes, however, the amount of IR radiation leaving the atmosphere also changes. And since the Industrial Revolution, the burning of fossil fuels like coal, oil and gasoline have greatly increased the amount of carbon dioxide (CO2) in the atmosphere, according to NASA's Earth Observatory.
Along with other gases like methane and nitrous oxide, CO2 acts like a blanket, absorbing IR radiation and preventing it from leaving the atmosphere. The net effect causes the gradual heating of Earth's atmosphere and surface. [Related: Effects of Global Warming]
This is called the "greenhouse effect" because a similar process occurs in a greenhouse: Relatively high-energy UV and visible radiation penetrate the glass walls and roof of a greenhouse, but weaker IR radiation isn't able to pass out through the glass. The trapped IR radiation keeps the greenhouse warm, even in the coldest winter weather.
There are several gases in Earth's atmosphere known as "greenhouse gases" because they exacerbate the greenhouse effect: CO2, methane, nitrous oxide, water vapor and ozone are among the most prevalent, according to NASA.
Chlorofluorocarbons (CFCs) were once commonly used as refrigerants and aerosol propellants (they're now largely phased out by international agreement because they caused significant damage to the ozone layer). CFCs also function as greenhouse gases.
Not all greenhouse gases are the same: Some, like methane, are produced through agricultural practices including livestock manure management. Others, like CO2, largely result from natural processes like respiration and from the burning of fossil fuels.
Additionally, these greenhouses gases don't all contribute equally to the greenhouse effect: Methane, for example, is about 21 times more effective at trapping heat from IR radiation than carbon dioxide, according to the EPA.
This difference in heat-trapping ability is sometimes referred to as a gas's "global-warming potential," or GWP.
Natural causes vs. human causes of global warming
Earth's historic climate changes have included ice ages, warming periods and other fluctuations in climate over many centuries.
Some of these historical changes can be attributed to changes in the amount of solar radiation hitting the planet. A drop in solar activity, for example, is believed to have caused the "Little Ice Age," a period of unusually colder climate that lasted from about 1650 to 1850, according to NASA.
However, there is no evidence that any increase in solar radiation could be responsible for the steady increase in global temperatures that scientists are now recording, according to the National Oceanic and Atmospheric Administration (NOAA).
In other words, natural causes cannot be held responsible for global warming. "There is no scientific debate on this point," the NOAA website states.
Indeed, virtually every credible source of scientific research from around the world indicates that human causes — primarily the burning of fossil fuels and the subsequent increase in atmospheric CO2 levels — are responsible for global warming. [The Reality of Climate Change: 10 Myths Busted]
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