Speaker: Anna von der Heydt (Utrecht University)

Climate sensitivity is a key predictor of climate change. However, it is not very well constrained, either by climate models, observational, historical or palaeoclimate data. This ‘uncertainty’ has its origin in different aspects: i) There is a classical uncertainty related to measurements or proxy estimates of temperature and greenhouse gas concentrations. From model estimates, there may be a model uncertainty, which is however difficult to quantify. Using a Bayesian approach for each line of evidence (e.g. models, modern observations, palaeoclimate) we can quantify a probability distribution (PDF) of the equilibrium climate sensitivity (ECS) given all information available. Moreover, a similar approach can be used to combine all lines of evidence into a single PDF of the ECS. ii) The climate system exhibits strong internal variability and forcing on many timescales meaning that the ‘equilibrium’ will only be relative to fixing the slow feedback processes before comparing palaeoclimate sensitivity estimates with estimates from model simulations. Palaeoclimate records and modelling instead can also give insight into the so-called Earth System Sensitivity, which includes the integrated effect of slow processes and boundary conditions (e.g. geography, vegetation and land ice). iii) The background state dependence of the fast feedback processes: Information from the late Pleistocene ice age cycles indicates that the equilibrium climate sensitivity varies considerably between regime because of fast feedback processes changing their relative strength over one cycle. Moreover, deep-time climate model simulations of mostly ice free-climates indicate a similar to modern ECS including significant polar amplification suggesting that fast feedback processes other than ice must be responsible for high-latitude warming.

Tipping elements in the climate system: Regime shifts in the climate system can affect the global mean temperature and therefore ECS. A probability of tipping is necessary to quantify any impact on the uncertainty in ECS due to regime shifts, which is however very difficult to assign. Moreover, the (linear) concept of ECS needs to be extended in order to capture expected climate changes in the presence of tipping points.

Brief Biography: Anna von der Heydt is a physicist by training and obtained her PhD in the Physics of Fluids group at the University of Twente with her thesis entitled “Non-ideal turbulent flows” in 2003. Since her PhD, she has worked at the Institute for Marine and Atmospheric Research at Utrecht University on (palaeo-) climate phenomena and transition behaviour in climate using a hierarchy of climate models and stochastic dynamical systems approaches. In 2006 she received a personal national grant to study the effect of palaeo-geography on the global ocean circulation. Currently, her research group is successfully applying the first high-resolution ocean modelling techniques to palaeoceanographic problems such as the development of the Antarctic circumpolar current and has recently developed a technique to efficiently transform information of plate-tectonic models into topographic/bathymetric information for coupled climate models. A combination of past and present climate studies has evolved around natural climate variations such as the Atlantic Multidecadal Oscillation and the El Niño Southern Oscillation using elements of dynamical systems approaches. A third line of research has grown around climate sensitivity derived from both model simulations and palaeoclimate data. Anna is active in the Netherlands Earth System Science Centre (NESSC), the Centre for Complex Systems Studies in Utrecht (CCSS) and is leading a work package of a newly starting European funded project on tipping points in the Earth System (TiPES).