Paleoclimate diagnostics: robust relationships in past and future simulations

Abstract:

Beyond the understanding of past climate/variations, the CMIP5/PMIP3 paleoclimate simulations allow us to explore possible connections between past and future climate projections because model configurations are exactly the same in the paleo and future simulations in CMIP5. Izumi et al (2013, GRL) and Li et al (2013, GRL) showed that several relationships and robust patterns occur in climate simulations across different climate states (i.e. past (cold), present, and future (warm)), and used paleo- and modern environmental data to provide some independent evidence for those relationships and patterns. Izumi et al (2014, Clim Dyn) examined the possibility that a small set of common mechanisms controls large-scale responses using a simple energy-balance model to decompose the temperature changes shown in multiple simulations. These and other studies demonstrate the way in which paleoclimate simulations are useful adjuncts to analyses of modern-day climates in understanding the fundamental mechanisms of climate changes.

Here, we further investigate change in the surface clear-sky longwave downward radiation, which is a key control of large-scale temperature responses such as land-ocean contrast and high-latitude amplification, by decomposing the this component into to direct longwave feedback, water vapor effect, and CO₂ effect in a simple energy balance model. We also discuss the residual term in the decomposition, which measures the difference between the simulated CMIP5 surface temperature and surface temperature estimated by the energy balance model. In addition, we investigate the key mechanisms in mid-Holocene vs piControl climates in order to compare the responses from orbitally induced changes in insolation with the responses induced by changes in atmospheric CO₂ concentration. Finally, we explore other large-scale climate responses under different climate states, in particular precipitation and water-balance variables over land, focusing on the large-scale atmospheric circulation of moisture variations.