In the climate change debate, the bottom line boils down to the principle of conservation of energy, or the energy coming into the Earth-atmosphere system equals that going out.
Climate will change over the long-term when this relationship is altered so that there is a difference between input and output. Given the magnitude of natural and anthropogenic forcing, the induced differences between the input and output are very small compared to the total values overall. Those that support a human induced cause for climate change, however, argue that the strength of human induced forcing is far greater than natural forcing.
In the latest round of testing the ability of climate models to reproduce the climate of the 20th century and beyond, the same old assumptions produce the same old results. And for the climate scenarios of the future, they produce the same old frightening projections. Meehl et al. (2012) discuss their results of testing the climate system response to external forcing in the Community Climate Model Version 4 (CCSM4) for the Coupled Model Intercomparison Project phase 5 (CMIP5).