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One of the most prominent climate tipping elements is the Atlantic meridional overturning circulation (AMOC), which can potentially collapse because of the input of fresh water in the North Atlantic. Although AMOC collapses have been induced in complex global climate models by strong freshwater forcing, the processes of an AMOC tipping event have so far not been investigated. Here, we show results of the first tipping event in the Community Earth System Model, including the large climate impacts of the collapse. Using these results, we develop a physics-based and observable early warning signal of AMOC tipping: the minimum of the AMOC-induced freshwater transport at the southern boundary of the Atlantic. Reanalysis products indicate that the present-day AMOC is on route to tipping. The early warning signal is a useful alternative to classical statistical ones, which, when applied to our simulated tipping event, turn out to be sensitive to the analyzed time interval before tipping.

We present the Utrecht contribution to the Pliocene Model Intercomparison Project Phase 2 (PlioMIP2), using the Community Earth System Model version 1.0.5 (CCSM4-Utr). Using a standard pre-industrial configuration and the enhanced PlioMIP2 set of boundary conditions, we perform a set of simulations at various levels of atmospheric pCO2 (280, 400, and 560 ppm). This allows us to make an assessment of the mid-Pliocene reference (Eoi400) climate versus available proxy records and a pre-industrial control (E280), as well as determine the sensitivity to different external forcing mechanisms. We find that our simulated Pliocene climate is considerably warmer than the pre-industrial reference, even under the same levels of atmospheric pCO2. Compared to the E280 case, our simulated Eoi400 climate is on average almost 5 °C warmer at the surface. Our Eoi400 case is among the warmest within the PlioMIP2 ensemble and only comparable to the results of models with a much higher climate sensitivity (i.e. CESM2, EC-Earth3.3, and HadGEM3). This is accompanied by a considerable polar amplification factor, increased globally averaged precipitation, and greatly reduced sea ice cover with respect to the pre-industrial reference. In addition to radiative feedbacks (mainly surface albedo, CO2, and water vapour), a major contribution to the enhanced Pliocene warmth in these simulations is the warm model initialisation followed by a long spin-up, as opposed to starting from pre-industrial or present-day conditions. Added warmth in the deep ocean is partly the result of using an altered vertical mixing parameterisation in the Pliocene simulations, but this has a negligible effect at the surface. We find a stronger and deeper Atlantic meridional overturning circulation (AMOC) in the Eoi400 case, but the associated meridional heat transport is mostly unaffected. In addition to the mean state, we find significant shifts in the behaviour of the dominant modes of variability at annual to decad

While the early Eocene has been considered in many modelling studies, detailed simulations of the middle and late Eocene climate are currently scarce. To understand Antarctic glaciation at the Eocene-Oligocene Transition (~ 34 Ma) as well as middle Eocene warmth, it is vital to have an adequate reconstruction of the middle-to-late Eocene climate. Here, we present a set of high resolution coupled climate simulations using the Community Earth System Model (CESM) version 1. Two middle-to-late Eocene cases are considered with new detailed 38 Ma geographical boundary conditions with a different radiative forcing. With 4 × pre-industrial concentrations of CO2 (i.e. 1120 ppm) and CH4 (~ 2700 ppb), the equilibrium sea surface temperatures correspond well to available late middle Eocene (42–38 Ma) proxies. Being generally cooler, the simulated climate with 2 × pre-industrial values is a good analog for that of the late Eocene (38–34 Ma). Deep water formation occurs in the South Pacific Ocean, while the North Atlantic is strongly stratified and virtually stagnant. A shallow and weak circumpolar current is present in the Southern Ocean with only minor effects on southward oceanic heat transport within wind-driven gyres. Terrestrial temperature proxies, although limited in coverage, also indicate that the results presented here are realistic. The reconstructed 38 Ma climate has a reduced equator-to-pole temperature gradient and a more symmetric meridional heat distribution compared to the pre-industrial reference. Climate sensitivity is similar (~ 0.7 °C/Wm2) to that of the present-day climate (~ 0.8 °C/Wm2; 3 °C per CO2 doubling), with significant polar amplification despite very limited sea ice and snow cover. High latitudes are mainly kept warm by albedo and cloud feedbacks in combination with global changes in geography and the absence of polar ice sheets. The integrated effect of geography, vegetation and ice accounts for a 6–7 °C offset between pre-industrial and 38 Ma Eo

In Autumn 2013, we—an international team of climate scientists, computer scientists, eScience researchers, and e-Infrastructure specialists—participated in the enlighten your research global competition, organized to showcase advanced lightpath technologies in support of state-of-the-art research questions. As one of the winning entries, our enlighten your research global team embarked on a very ambitious project to run an extremely high resolution climate model on a collection of supercomputers distributed over two continents and connected using an advanced 10 G lightpath networking infrastructure. Although good progress was made, we were not able to perform all desired experiments due to a varying combination of technical problems, configuration issues, policy limitations and lack of (budget for) human resources to solve these issues. In this paper, we describe our goals, the technical and non-technical barriers, we encountered and provide recommendations on how these barriers can be removed so future project of this kind may succeed.