Friday 27 Mar 2015: Palaeo climate sensitivity - state dependent, non-linear, unpredictable?
Anna S. von der Heydt - Utrecht University, The Netherlands
Harrison 209 13:00-14:00
The equilibrium climate sensitivity (ECS) is the equilibrium change in Earth's global mean surface temperature due to a radiative forcing associated with a doubling of pCO2, the atmospheric CO2 concentration. Although known for decades, little progress has been made in constraining upper and lower limits for climate sensitivity. Originally, ECS was derived from climate models where the atmospheric CO2 concentration is doubled in typically about 100 years. Also palaeo data have been frequently used to determine ECS, and ? if slow feedback processes are adequately taken into account ? indicate a similar range as those based on climate models used in the IPCC . However, palaeo data usually span a much larger time than the 100-year model experiments, and include a wide range of climate variability that is not necessarily captured by the models. The traditional linear and equilibrium concept of climate sensitivity as is applied in typical (short time scale) climate model simulations might not apply to the climate system's non-stationary and non-linear response to changing forcing.
Proxy data for atmospheric CO2-concentration together with a model-based reconstruction of land ice albedo (as main slow feedback process) over the last 5 million years allows in principle to estimate ECS. Over this time scale, large Northern hemisphere ice sheets have appeared, first varying on Milankovitch time scales (41kyr) and later (since about 800 kyr) exhibiting strong 100 kyr ice-age cycles. Here we show, that over this long time period also the fast feedback processes vary in strength such that the derived ECS depends on the background climate state. We assess the dependency of the fast feedback processes on the background climate state using data and a conceptual climate model. Though still (locally) linear, we propose a different approach to estimate climate sensitivity, which better accounts for a possible state dependency of the fast feedbacks. This approach uses local slopes of temperature versus radiative perturbation and is most suitable for palaeo-data spanning a range of background climate states. We find the specific climate sensitivity generally lower during cold (glacial) than during warm periods .
 E. J. Rohling, A. Sluijs, H. A. Dijkstra, P. Kohler, R. S. W. van de Wal, A. S. von der Heydt, and PALAEOSENS Project members, Making sense of palaeoclimate sensitivity, Nature 491 (2012), 683?691.
 A. S. von der Heydt, P. Kohler, R. S. W. van de Wal, and H. A. Dijkstra, On the state dependency of fast feedback processes in (paleo) climate sensitivity, Geophysical Research Letters 41 (2014)