Earth System Sensitivity: a Feedback perspective

In the field of climate-change research a lot of effort is devoted to the ‘narrowing down’ of uncertainties in the estimation of Equilibrium climate sensitivity (ECS), the global mean warming as a result of an instantaneous doubling of the CO2 concentration in the atmosphere. The present study explores possible consequences of this narrowing down of ECS for the long-term Earth system sensitivity (ESS), taking into account ‘slow’ feedbacks due to the cryosphere response (permafrost melting and ice-sheet disintegration) to a warming world. Implications for international policy making, aiming at avoiding 2 degrees Celsius of global warming, are briefly discussed.

Constraining clouds and convective parameterizations in a climate model from past climate

Cloud and convective parameterizations strongly influence uncertainties in equilibrium climate sensitivity (ECS). We provide a proof-of-concept study to constrain these parameterizations in a perturbed parameter ensemble of atmosphere-only simulations by evaluating model biases in the present-day runs using multiple satellite climatologies and by comparing simulated δ18O of precipitation (δ18Op), known to be sensitive to parameterization schemes, with a global database of speleothem δ18O records covering the Last Glacial Maximum (LGM), mid-Holocene (MH) and pre-industrial (PI) periods. Relative to modern, paleoclimate simulations show greater sensitivity to parameter changes, allowing for an evaluation of uncertainties over a broader range of climate forcing and the identification of parts of the world that are parameter sensitive. Certain simulations reproduced LGM and MH δ18Op anomalies relative to the PI better than the default parameterization. Not a single set of parameterizations worked well in all climate states, thus improving simulations requires determining all plausible parameter combinations.

A global climatology of the ocean surface during the Last Glacial Maximum mapped on a regular grid (GLOMAP)

We present a climatology of the near-sea-surface temperature (NSST) anomaly and the sea-ice extent during the Last Glacial Maximum (LGM, 23 000–19 000 years before present) mapped on a global regular 1∘×1∘ grid. It is an extension of the Glacial Atlantic Ocean Mapping (GLAMAP) reconstruction of the Atlantic NSST based on the faunal and floral assemblage data of the Multiproxy Approach for the Reconstruction of the Glacial Ocean Surface (MARGO) project and several recent estimates of the LGM sea-ice extent. Such a gridded climatology is highly useful for the visualization of the LGM climate, calculation of global and regional NSST averages, and estimation of the equilibrium climate sensitivity, as well as a boundary condition for atmospheric general circulation models. The gridding of the sparse NSST reconstruction was done in an optimal way using the Data-Interpolating Variational Analysis (DIVA) software, which takes into account the uncertainty in the reconstruction and includes the calculation of an error field. The resulting Glacial Ocean Map (GLOMAP) confirms the previous findings by the MARGO project regarding longitudinal and meridional NSST differences that were greater than today in all oceans. Taken at face value, the estimated global and tropical cooling would imply an equilibrium climate sensitivity at the lower end of the currently accepted range. However, because of anticipated changes in the seasonality and thermal structure of the upper ocean during the LGM as well as uneven spatial sampling, the estimated cooling and implied climate sensitivity are likely to be biased towards lower values.