The major contributors to increases in radiative forcing since pre-industrial times are CO2 (61%), CH4 (17%), N2O (4%) and CFCs (12%). The most recent decadal increase in radiative forcing is attributable to CO2 (56%), CH4 (11%), N2O (6%) and CFCs (24%). Stratospheric H2O is estimated to have contributed 4%. Man-made sulphur emissions, which have increased in the Northern Hemisphere over the last century, affect radiative forcing by forming aerosols and influencing the radiative properties of clouds so as to cool the earth. It is conceivable that this forcing has been of a comparable magnitude, but of opposite sign, to the greenhouse forcing. -J.F.Austin