You add more physical parameters. Climate models are not parameter fitting from the past data, they are physical models. Of course you test them with old data

You add more physical parameters. Climate models are not parameter fitting from the past data, they are physical models. Of course you test them with old data too, but they are mainly for verification and controlling for variables that are still missing. They test these models with other planets too. Set in parameters for Mars or Venus and they precinct pretty well how those climates behave.