Pliocene Constraints on Earth System Senstivity

Abstract:

The magnitude of long-term global temperature rise due to an increasing concentration of carbon dioxide (CO₂) in the atmosphere is a question of relevance to society and policy makers. Previous studies have addressed this issue on the basis of the equilibrium response of the climate system to a doubling of CO₂ due to fast feedbacks, such as clouds and sea-ice albedo (Climate or Charney Sensitivity). Here however we use the concept of Earth System Sensitivity (ESS) that additionally includes slow feedbacks such as those derived from changes in the major ice sheets and vegetation distribution.

We have produced a robust estimate of ESS using the mid-Pliocene, the last interval in Earth history when CO₂ was at modern or near future levels (around 3 million years ago). The mid-Pliocene warm period (mPWP; 3.264 to 3.025 Ma) is useful for investigating the concept of ESS because it represents a world in quasi-equilibrium with high CO₂ for a sufficient period that the long term feedbacks are close to equilibrium.

Over 300 simulations have been completed using coupled-atmosphere ocean climate models and Earth System models in order to rigorously estimate ESS. We have performed ensembles of experiments which have perturbed physical components in the models and tested the sensitivity of critical model boundary conditions in order to explore the plausible range of Pliocene-based estimates of ESS. Additionally, we have used output from the international Pliocene Model Intercomparison Project (PlioMIP) to take into account structural uncertainty in our modelled estimates of ESS.

Our entire ensemble of plausible mid-Pliocene scenarios demonstrates temperature anomalies of 0-5.5°C between end member CO₂ levels of 300 ppmv and 500 ppmv. We present a range of probability density functions to demonstrate the most likely value of ESS based on the modelled scenarios we have included. Our results are compatible with initial studies of ESS (e.g. Lunt et al. 2010) that suggest ESS is between 30% and 50% greater than traditional climate sensitivity.