The main inputs for a climate model are external factors, so called “forcings”, that change the amount of the sun’s energy absorbed by the Earth or trapped in the atmosphere. Examples of these forcings are the sun’s varying radiation output, variable atmospheric concentrations of greenhouse gasses (e.g. CO₂, methane, N₂O) or aerosols (particles emitted e.g. by fossil fuel burning and volcanic eruptions influencing sunlight and cloud formation). These factors are incorporated into the climate model as best estimates of past conditions or as part of future socio-economic and emission scenarios.

Past forcings can be estimated by reconstructing ancient greenhouse gas concentrations (e.g. by analyzing air trapped in ice cores), climate gas and particle emissions during past volcanic eruptions or changes in the Earth’s orbit (i.e. cyclical variations in solar radiation reaching the Earth due to Milankovitch cycles).

Concerning future forcings, different scenarios of future developments in technology, energy and land use provide potential  pathways, so called “Representative Concentration Pathways” (RCPs), for atmospheric greenhouse gas concentrations (Fig. 1).

The main outputs for a climate model are normally temperatures and humidity of different atmospheric layers from the surface to the upper stratosphere. Climate models also produce estimates of ocean temperatures, salinity and pH from the surface to the seafloor as well as snowfall, rainfall, snow cover and the extent of glaciers, ice sheets and sea ice. They also give information about wind speed, strength and direction, as well as climate features, such as the jet stream and ocean currents. “Climate sensitivity” can also be modelled (i.e. the warming expected when the concentration of carbon dioxide in the atmosphere reaches twice the amount it was in preindustrial times).

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https://www.carbonbrief.org/qa-how-do-climate-models-work
van Vuuren, D. P., Edmonds, J., Kainuma, M., Riahi, K., Thomson, A., Hibbard, K., Hurtt, G. C., Kram, T., Krey, V., Lamarque, J.-F., Masui, T., Meinshausen, M., Nakicenovic, N., Smith, S. J., and Rose, S. K., 2011, The representative concentration pathways: an overview: Climatic Change, v. 109, no. 1, p. 5.