What are climate forcings?

Climate forcings are factors that drive or force the climate to change. The most important forcings during the last millenium were: changes in the output of energy from the sun, volcanic eruptions, and changes in the concentration of greenhouse gases in the atmosphere.

The figure below shows time series of these forcings using ice cores to measure levels of CO2, solar and volcanic activity over the last millennium.

Source:  http://www.ncdc.noaa.gov/paleo/globalwarming/gw-forcing.html

Note from source:

The size of these forcings is expressed in terms of Watts (a flux of energy) per square meter of the Earth's surface. Positive forcing warms the Earth, while negative forcing cools the Earth. Proxies do not record these forcings perfectly, so the time series are not known exactly. However, models that use these forcing time series are able to closely match the paleoclimate record of temperature for the last 1000 years. When CO₂ is excluded, the models fail to simulate all of the warming of the 20^th century, despite the generally successful simulation of the preceding centuries using only natural forcings (i.e., solar and volcanic variations). Model results (Crowley, T.J 2000) clearly indicate that greenhouse gas forcing is necessary for explaining the global warming of the 20^th century.