MORDICUS

The scientific understanding of climate change is now sufficiently clear to show that anthropogenic global warming is already upon us, with a projected rate of change that exceeds anything seen in nature in the past 10,000 years. Uncertainties remain, however, especially regarding how climate will change in the next decades, in particular at regional and local scales for which natural (both internal and external) variability is large. Furthermore, a complete understanding and attribution of 20th century climate changes is still missing. This lack of knowledge could delay needed adaptation strategies as decision makers in diverse arenas, from water managers to public health experts, need to know if the climate events they are seeing are the result of natural variability, and hence can be expected to reverse at some point, or are the result of potentially irreversible anthropogenic climate change. This epistemic uncertainty has two main causes: the first one is the poor understanding of decadal natural variability and the degree to which it modulates anthropogenic climate change at continental to regional scales. The second concerns uncertainties related to past and future emissions of radiatively important trace gases (including greenhouse gases, stratospheric ozone, and stratospheric ozone depleting substances) and pollutants affecting atmospheric aerosol composition as well as our capability to accurately simulate the response of the climate system to that altered radiative forcing. In that context, MORDICUS has three main scientific objectives. The first one is to improve understanding of the key physical and dynamical mechanisms leading to decadal internal variability. The main focus is on the two leading decadal modes of variability, namely the Interdecadal Pacific Variability (IPV) and the Atlantic Multidecadal Variability (AMV). Research will focus on inter-basin connections and processes connecting the subsurface ocean to surface temperature anomalies, which in turn force and respond to large-scale atmospheric changes. The second goal is to extract the fingerprint of the various external forcings on regional climate and to mechanistically understand their contribution to decadal variability. A special focus will be on aerosols and interactions between all external forcings and IPV/AMV internal modes. The third goal is to quantify the respective influence of IPV/AMV internal modes and anthropogenic factors on both observed (1920-2010) atmospheric and continental changes and near-future change estimates (2010-2035). A specific focus will be put on the hydrological cycle and extreme events, namely droughts and tropical cyclones. MORDICUS is a process-oriented project motivated by the first key lessons drawn from the analysis of simulations from the 5th Coupled Model Intercomparison Project (CMIP5). It follows the recommendations and needs for mechanistic approaches to go beyond the initial CMIP5 decadal initiative, which was primarily focused on predictability issues. MORDICUS is an ambitious modeling-oriented project proposed by a national consortium of the teams involved in CMIP5. MORDICUS is based on a suite of innovative model experiments and configurations designed to tackle the above key scientific questions that will contribute to anticipate the next CMIP6 exercise. Together with multi-model climate prediction exercises that aim to exploit the full predictive potential of forced and free climate variations, MORDICUS results will form the raw material for the decision making needed to enable societies to adapt to the climate changes of the next decades.