In the context of the IPCC assessments, radiative forcing refers to an external perturbation in the radiative energy budget of the Earth"s climate system, which may lead to changes in climate parameters. A positive forcing (more incoming energy) tends to warm the system, while a negative forcing (more outgoing energy) tends to cool it. This is due to changes in drivers of climate change such as the concentration of carbon dioxide or the output of the Sun. In the IPCC reports, changes in radiative forcing are relative to the year 1750.