The Green's Function Model Intercomparison Project (GFMIP) explores the atmospheric response to surface temperature perturbations, focusing on the "pattern effect" where the response to different warming patterns varies. This paper presents the GFMIP protocol, developed using sensitivity tests with the HadAM3 model and other models. The protocol involves a control simulation, patch simulations, and diagnostic simulations. Key findings include nonlinearities in the atmospheric response to surface temperature changes, such as asymmetrical responses to warming versus cooling patches and changes in response magnitude with patch size. These nonlinearities suggest that the pattern effect depends on both the heterogeneity and location of warming. The protocol aims to balance scientific utility with computational efficiency, providing a standardized approach for future model intercomparisons.The Green's Function Model Intercomparison Project (GFMIP) explores the atmospheric response to surface temperature perturbations, focusing on the "pattern effect" where the response to different warming patterns varies. This paper presents the GFMIP protocol, developed using sensitivity tests with the HadAM3 model and other models. The protocol involves a control simulation, patch simulations, and diagnostic simulations. Key findings include nonlinearities in the atmospheric response to surface temperature changes, such as asymmetrical responses to warming versus cooling patches and changes in response magnitude with patch size. These nonlinearities suggest that the pattern effect depends on both the heterogeneity and location of warming. The protocol aims to balance scientific utility with computational efficiency, providing a standardized approach for future model intercomparisons.