Being aware of the interconnectedness and mutual causality of the climate system and land surface processes, and the overall relevance for the Water-Food-Energy Nexus, the Climate Hub Unit will consider in high detail the relevance of land use dynamics and changes in land use and water management on various scales, i.e., ranging from continental to the river basin scale.

1.On a continental scale, the impact of land use changes on the climate system requires a direct consideration in the land use scheme of regional climate models. Since it is known that Land Use and Land Cover Changes (LULCC) can have significant impacts on the evolution of climate, it is important to incorporate these changes into climate models for improving simulations of historical and future climate change. On the one hand, while IPCC’s state-of-the-art Global Climate Models (GCMs) account for LULCC through dynamic vegetation components that interact with the atmosphere, their spatial resolution –generally coarser than 100km between two grid points– is insufficient to represent the details of surface heterogeneities for regional applications. On the other hand, although Regional Climate Models (RCMs) do provide suitably high spatial resolutions (~12 km) to represent the main features at regional scales, LULCC are paradoxically not accounted for e.g., in the widely used regional climate projections from the Coordinated Regional Climate Downscaling Experiment (CORDEX). LULCC also impacts climate via changes in the surface albedo, roughness and evapotranspiration (Perugini et al., 2017). These so-called ‘biophysical effects’ can impact climate as much as GHG forcing locally, but they are comparatively much less studied and understood. Although these biophysical impacts resulting from LULCC likely affect the local climate to a similar extent than the global GHG forcing (Noblet-Ducoudré et al., 2012), they are comparatively much less studied and understood. For this reason, the Land Use and Climate Across Scales (LUCAS) project has investigated the biophysical effects of land cover changes on the regional climate through an idealized experiment comparing worlds where Europe would be fully covered by either forest or grass (Davin et al. 2020; Asselin et al. 2022). While such approaches can show the substantial contribution of LULCC on climate response and its uncertainties, the Climate Hub will advance existing knowledge by implementing realistic change rates of LULCC into RCMs to obtain high-quality climate information that will better serve governments, stakeholders and the public in the development of innovating adaptation strategies to climate change. Consequently, the activities of the Climate Hub with regard to land use on continental scales will focus on the following aspects:

• Implement realistic land use changes based on preindustrial and future projected LULC maps into regional climate models.
• Identify the driving mechanisms through which LULCC affect climate and assess their impact on climate variability and extreme events.

2. On the river basin scale, an accurate assessment of LULCC is crucial for the management of natural resources, particularly soil and water. It is however particularly complex to attribute the role of changes in climate versus socioeconomic forcing in on land use changes and land use management. It requires high-resolution, spatially explicit land use change models, such as e.g., iCLUE (Verweij et al., 2018; Huber Garcia et al., 2018), which use a combination of i) empirically quantified relations between a land use class and its drivers, and ii) dynamic modelling of the competition between different land use classes to simulate land use change. Appropriate models relate the driving factors to each land use by means of a statistical relationship providing the suitability of a location for a specific land use. iCLUE, e.g., supports multiple statistical methods to describe a relationship between predictive variables and a land use class, e.g., stepwise regression as a statistical method.

Such models form the baseline to assess land use changes for future scenarios, converting socioeconomic drivers and climate change into land use change at the river basin scale. Changes in land use, and in particular in scenario dependent implications on land use management, can have severe implications on water demand and water supply, thus forming a major building block for the understanding of the causal connection between land use change and the functionality of the nexus. Fig. Xx illustrates the implications of various scenarios on land use change and water use change in the Ebro River basin, Spain, under conditions of scenario dependent socioeconomic and climatic changes (Sustainable vs. Myopic):

Furthermore, the Climate Hub Unit focusing on the Water-Energy-Food-Ecosystems-Land Use-Climate Nexus modeling will also employ Nexus Informatics that will identify and quantify interlinkages among sectors and resource use. Achieving the SDGs, along with the long-term sustainability of these systems requires holistic and systems solutions that integrate across scales and disciplines.

This Unit will combine knowledge of this complex and interdisciplinary system of systems with relevant technological, policy, and human behavioral modeling and datasets.

• System Dynamics Modelling is used to map detailed and extensive sector-specific data from major databases and scenario models (e.g., E3ME-FTT, OSeMOSYS, MAGNET, SWIM) for national level analysis of interlinkages and for the identification of Nexus hotspots, where resource interdependencies create vulnerabilities and threaten resource security.
• Trade-off analysis is conducted to identify best possible options. Economic sectors such as agriculture, industry, services, and households are modeled, and calculations include GDP, government, governance, finance and investments taking into account state of the art economic modeling. Environmental sectors include land, soil, water, energy, consumption and production patterns, waste and biodiversity, while GHG emissions are calculated from all sectors.

In collaboration with the Energy and Climate Unit, the effect of these emissions on Climate Change is done by translating them to an associated temperature increase. The incorporation of a wide range of Social Sectors in the analysis including population, education, health, infrastructure, employment, transportation, income, etc. is a great strength.

The flexible and comprehensive System Dynamics platform that is produced can be adjusted to go in great detail in a sector of interest and possibly remain at a more aggregate level in other sectors, if needed.

Proposed solutions and technological / financial pathways are tested holistically and their effects are examined across a variety of economic, social and environmental sectors, incorporating a unique System of Systems modeling approach that integrates socio-economic science, engineering and policy and that incorporates informed action, new technologies, planning, policy, behavioral adaptation, and financial pathways.