Volcanic, CO₂ and solar forcing of Northern and Southern Hemisphere surface air temperatures

Seasonally-resolved energy balance models having air-surface, land-ocean and Northern and Southern Hemisphere resolution are used to elucidate the possible relative importance of several external factors on the climate of the past century. The model used here allows a direct comparison of observed and simulated temperatures from the same physical domains - over land and sea separately in each hemisphere. The availability of independent temperature records in two hemispheres significantly increases the number of independent degrees of freedom in the data available to 'verify' the simulations. Independent volcanic effects are found in both hemispheres, while similar responses for the more globally distributed CO₂ and tentatively identified solar forcings are found in the two hemispheres. The empirically derived CO₂ equilibrium doubling response for air surface temperature is 1.6±0.3°C, although the statistical significance of this result is uncertain.