Last Millennium Climate Change in CMIP5 Models

Paleoclimate data suggest that anomalously cold conditions across the Northern Hemisphere (NH) and a southward shift of the Intertropical Convergence Zone (ITCZ), a major feature of the tropical atmosphere, occurred during the Little Ice Age (LIA) ~ 1350 – 1850 AD. The purpose of this research is to identify whether these climate changes are simulated by the latest Coupled Model Intercomparison Project (CMIP5) General Circulation Models, and whether they can be attributed to changes in solar irradiance, volcanic aerosols, and greenhouse gas concentrations. For each model, two types of runs are analyzed: pre-industrial control runs and last millennium runs. The pre-industrial control runs are 1,000-year runs with climate forcings held at 1850 AD levels while the last millennium runs are 1,000-year runs with solar irradiance, volcanic aerosols, and greenhouse gas concentrations that change over time in accordance with paleoclimate records. Our results indicate that while anomalous global cooling occurs during the LIA in all models, little agreement exists on the magnitude of the cooling and on the changes in interhemispheric temperature gradient. In addition, changes in the position of the ITCZ are small and vary widely between models. We perform an energy budget analysis using the approximate partial radiative perturbation method to further investigate the LIA temperature and ITCZ changes. Volcanic forcing and ice albedo feedbacks appear to be the main drivers of global cooling during the LIA in the models, while the contribution from changes in solar irradiance is small. Changes in the position of the ITCZ are well correlated with changes in cross-equatorial atmospheric heat transport, as expected from theory.