First-order climate forcing

Humans are significantly affecting the global climate and in a variety of ways. Research is currently being conducted in areas that extend beyond the radiative effect of carbon dioxide, e.g., the interaction between groundwater and the atmosphere and also the protective value of our rainforests. Are the IPCC's assessments too conservative in recognizing the importance of various human-climate forcings as they impact regional and global climate? Maybe so, but we have to tackle what we currently know to be the primary cause. We face difficult years ahead as we debate policy and come to grips with worldwide actions, in order to recover from damage already done and to prevent further harm that could lead to true catastrophe.

Not all drivers of climate change are of the same magnitude, either globally or regionally, so it is useful to understand the "first-order" forcings from those that are not. What is a first-order climate forcing?