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Climate always changes from natural causes. What makes today special is that human causes have entered the mix, and scientists are challenged to untangle human causes from the rest and assess their role. Climate specialists juggle a large number of factors, most of which are imperfectly known, and use them as input for computer simulations, none of which are yet reliable for policy purposes.
The Earth's climate system can be regarded as a machine that converts radiation from the sun into the heat and motion of the lower atmosphere and ocean. The major elements that add or subtract energy inputs into this machine are called forcings, and climatologists divide them into natural and anthropogenic (human). In addition to forcings are internal variations within the climate system, such as El Niño in the oceans and the North Atlantic Oscillation in the atmosphere, and others that involve both sea and air.
For purposes of global-warming research, we study factors that are important for the last 2000 years, so this discussion will ignore orbital cycles and the geotectonic carbon cycle, which dominate climate over longer periods.
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