Here the greenhouse gas forcing is increased gradually to represent the observed changes in forcing due to all the greenhouse gases from 1860 to 1990. From 1990 to 2100 it uses an increase in concentrations represented by the IS92a emissions scenario with the Copenhagen modifications of industrial gases. This is sometimes referred as "IS95a". Greenhouse gas experiment (GHG) In GHG, the concentrations of the following greenhouse gases are prescribed as a function of time: CO2, CH4, N2O and several industrial gases. From 1860 to 1990, the annual concentrations of these gases are prescribed as observed and, from 1990 onward, according to the IPCC'92 scenario IS92a. For the industrial gases, the IS92a scenario has been updated to be consistent with a Copenhagen-like emission scenario (IPCC'96). All of the time dependent, or 'transient', forcing experiments have been initialised at year 100 of CTL, nominally 1860 in the transient experiments. However, these initial values do not correspond to 'pre-industrial' (1860) but to present-day conditions. This applies not only to SST and other climate variables but also to the concentrations of CO2, CH4 and N2O. In the transient experiments, this initial shift in concentration is taken into account by enhancing the observed/projected concentrations of these gases in an appropriate way (Appendix A). The climatology constructed from this 240-year CTL run serves as a reference for the time-dependent forcing experiments GHG, GSD and GSDIO (EH4OPYC_22670GHG_,EH4OPYC_22697GSD_, EH4OPYC_22723GSDIO_). These data represent monthly averaged surface values of selected variables for the IPCC-Data Distribution Centre. (see also http://www.ipcc-data.org/ )