Half a degree additional warming, prognosis and projected impacts (HAPPI): Background and experimental design by Mitchell, D, AchutaRao, K, Allen, M, Bethke, I, Beyerle, U, Ciavarella, A, Forster, P, Fuglestvedt, J, Gillett, N, Haustein, K, Ingram, W, Iversen, T, Kharin, V, Klingaman, N, Massey, N, Fischer, E, Schleussner, C, Scinocca, J, Seland, Ø, Shiogama, H, Shuckburgh, E, Sparrow, S, Stone, D, Uhe, P, Wallom, D, Wehner, M, Zaaboul, R