ATAT 1.0, an Automated Timing Accordance Tool for comparing ice-sheet model output with geochronological data

Earth's extant ice sheetsare of great societal importance given their ongoing and potential future contributions to sea-level rise. Numerical models of ice sheetsare designed to simulate ice sheetbehaviour in response to climate changes, but to be improved require validation against observations. The direct observational record of extant ice sheetsis limited to a few recent decades, but there is a large and growing body of geochronologicalevidence spanning millennia constraining the behaviour of palaeo- ice sheets. Hindcastscan be used to improve model formulations and study interactions between ice sheets, the climate system and landscape. However, ice-sheet modellingresults have inherent quantitative errors stemming from parameter uncertainty and their internal dynamics, leading many modellers to perform ensemble simulations, while uncertainty in geochronologicalevidence necessitates expert interpretation. Quantitative tools are essential to examine which members of an ice-sheet modelensemble best fit the constraints provided by geochronologicaldata. We present an Automated Timing Accordance Tool (ATAT version 1.0) used to quantify differences between model results and geo-data on the timing of ice sheetadvance and/or retreat. To demonstrate its utility, we perform three simplified ice-sheet modellingexperiments of the former British-Irish Ice Sheet. These illustrate how ATAT can be used to quantify model performance, either by using the discrete locations where the data originated together with dating constraints or by comparing model outputs with empirically-derived reconstructions that have used these data along with wider expert knowledge. The ATAT code is made available and can be used by ice-sheet modellersto quantify the goodness of fit of hindcasts. ATAT may also be useful for highlighting data inconsistent with glaciologicalprinciples or reconstructions that cannot be replicated by an ice sheet model.