Coarsening a numerical mesh using methods of multiphase flow property upscaling is essential in almost any modeling application due to computational limits. In CO2 storage modeling, after injection wells have been shut off or far away from the wells, CO2 migrates upwards under the influence of gravity and capillary forces. Upscaling permeability (k), relative permeability (kr) and capillary pressure (Pc) is required for grid coarsening these models. However, it has been shown that this is a difficult problem and using conventional upscaling methods such as the common capillary limit upscaling approach leads to large modeling errors. This work presents a new upscaling method based on an effective property formula for k, power law averaging in the capillary limit for kr, and an optimization approach for Pc. The new method is tested on various example cases and coarse-grid simulations are shown to match fine-grid ones with sufficient accuracy. The challenge of upscaling the flows is found to be related to entry pressure trapping and the optimization upscaled Pc is shown to have a unique structure allowing to model the trapping. The method is global, requiring a fine-grid simulation for calibration of the optimized parameters. However, we show that the method reduces computational time dramatically if calibrated parameters are used in similar cases in which the fine-grid solution is unknown, such as for varying k realizations.