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Radiative forcing is the change in the net, downward minus upward irradiances
(expressed in W.m^{-2}) at the tropopause due to a change in an external driver
of climate change, such as, for example, a change in the concentration of
carbon dioxide or the output of the Sun. Radiative forcing is computed with
all tropospheric properties held fixed at their unperturbed values, and after
allowing for stratospheric temperatures, if perturbed, to readjust to
radiative-dynamical equilibrium. Radiative forcing is called instantaneous if
no change in stratospheric temperature is accounted for. (Definition from IPCC
2007). See sections 4.1.1, 4.1.2, 5.5.1 and 5.5.2.1.

Weather forecasting centres analyse the present atmospheric configuration every day, using models to interpolate observations in order to construct physically consistent estimates of the atmospheric state. Because of changes in the structures of the models and in the procedures used, those analyses are not necessary consistent over long periods. In order to reduce the long-term biases, reanalyses are performed, using the same model and the same procedure over the whole period. However, biases are still present as the amount of available data change over time. The most widely used reanalyses are those computed by the National Center for Environmental Prediction and the National Center for Atmospheric Research (NCEP/NCAR) (http://www.cdc.noaa.gov/cdc/reanalysis/reanalysis.shtml) and of the European Centre for Medium-Range Weather Forecasts (ECMWF) (ERA-40) (http://www.ecmwf.int/research/era/).

Linear regression is a statistical procedure that represents a variable (the dependent variable) as a linear function of one or more other variables (the independent variables). The model parameters that link the dependent and independent variables are called the regression coefficients.

The relative humidity (*RH*) of an air parcel is defined as the ratio of the
partial pressure of the water vapour in the parcel to the
saturation vapour pressure
at the temperature of the air parcel. At saturation (i.e. equilibrium between the liquid and vapour phases) the relative humidity is 1 (or 100%).
See section 4.2.1.