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Abstract:

The achievement of several sustainable development goals and the Paris Climate Agreement depends on rapid progress towards sustainable food and land systems in all countries. We have built a flexible, collaborative modeling framework to foster the development of national pathways by local research teams and their integration up to global scale. Local researchers independently customize national models to explore mid-century pathways of the food and land use system transformation in collaboration with stakeholders. An online platform connects the national models, iteratively balances global exports and imports, and aggregates results to the global level. Our results show that actions toward greater sustainability in countries could sum up to 1 Mha net forest gain per year, 950 Mha net gain in the land where natural processes predominate, and an increased CO₂ sink of 3.7 GtCO₂e yr^−1 over the period 2020–2050 compared to current trends, while average food consumption per capita remains above the adequate food requirements in all countries. We show examples of how the global linkage impacts national results and how different assumptions in national pathways impact global results. This modeling setup acknowledges the broad heterogeneity of socio-ecological contexts and the fact that people who live in these different contexts should be empowered to design the future they want. But it also demonstrates to local decision-makers the interconnectedness of our food and land use system and the urgent need for more collaboration to converge local and global priorities.

Abstract:

It has long been suggested that the mid-latitude atmospheric circulation possesses what has come to be known as ‘weather regimes’, loosely categorised as regions of phase space with above-average density and/or extended persistence. Their existence and behaviour has been extensively studied in meteorology and climate science, due to their potential for drastically simplifying the complex and chaotic mid-latitude dynamics. Several well-known, simple non-linear dynamical systems have been used as toy-models of the atmosphere in order to understand and exemplify such regime behaviour. Nevertheless, no agreed-upon and clear-cut definition of a ‘regime’ exists in the literature, and unambiguously detecting their existence in the atmospheric circulation is stymied by the high dimensionality of the system. We argue here for an approach which equates the existence of regimes in a dynamical system with the existence of non-trivial topological structure of the system’s attractor. We show using persistent homology, an algorithmic tool in topological data analysis, that this approach is computationally tractable, practically informative, and identifies the relevant regime structure across a range of examples.

Abstract:

Floods usually threaten human lives and cause serious economic losses, which can be more severe with global warming. Therefore, it is a salient challenge to find out how global flood characteristic changes and whether current flood protection standards will face more pressures. This study aims to characterize changes in global floods and explicit flood defense pressures in warming climates of 1.5–3.0 °C above pre-industrial levels by running four well-calibrated lumped hydrological models using bias-corrected Global Climate Model (GCM) simulations for 9045 watersheds worldwide. The results show that global warming from 1.5 to 3.0 °C has increasingly dominated all continents, with amplification effects on changes of flood frequency and magnitude. Southeast Eurasia, Africa, and South America are hotspots of changes for significant proportions of watersheds with larger flood patterns and greater changing extents than others. For example, for the 3.0 °C warming period under the combination of shared socioeconomic pathway 2 and representative concentration pathway 4.5 (SSP245) scenario, the regionally averaged 50-year flood magnitude will increase by 25.6 %, 30.6 %, and 16.4 % for these regions, respectively. The increases in occurrence and magnitude indicate that current flood protection standards will face increasing pressures in future warming climates. The design-level flood frequency is projected to increase for about 47 %, 55 %, 70 %, and 74 % of watersheds in 1.5, 2.0, 2.5, and 3.0 °C warming periods under the SSP245 scenario. However, large uncertainty are observed for the change of flood characteristics dominated by GCMs and their interactions with SSP scenarios and hydrological models. This study implies that the current flood defense standards should be enhanced and climate adaptation and mitigation strategies should be proposed to cope the change of future flood. Plain language summary: Floods usually threaten human lives and cause serious economic losses, which can be more severe in the context of global warming. It is a salient challenge to find out how global flood risk changes and whether current flood protection standards will face more pressures. This study aims to characterize changes in global floods and explicit flood defense pressures in warming climates of 1.5, 2.0, 2.5, and 3.0 °C above pre-industrial levels. Here we show that amplification effects of higher air temperature on the range of changes in flood frequency and magnitude are projected. Southeast Eurasia, Africa, and South America are hotspots of changes for significant proportions of watersheds with larger flood patterns and greater changing extents than others. Most watersheds worldwide is likely to face increasing flood defense pressures in warming climates. Our findings could improve the understanding of future flood conditions under the warming climates and provide information to mitigation and adaptation policymaking.