Key Earth System processes to understand Arctic Climate Warming and Northern Latitude Hydrological Cycle Changes

The KeyCLIM project has created a wealth of new knowledge on the importance of coupled processes for the understanding of climate evolution, with some emphasis on a key region of interest, the Arctic and northern high latitudes. KeyCLIM has resulted already in several important publications comprising those on the aerosol historical forcing (Quaas et al. 2022, Smith et al. 2021), an analysis of biogeochemical feedbacks (Thornhill et al. 2021a), ocean circulation impact on climate sensitivity (Gjermundsen et al. 2021), interactions between North Atlantic Ocean circulation and climate change (Madan et al. 2023), atmospheric rivers and extreme precipitation (Michel et al. 2021), reconstruction of daily flows in Norwegian streams from large scale models (Hagen et al. 2023), transport through oceanic Arctic gateways (Shu et al. 2023), maintenance of energy consistency and energy conservation in CMIP-class models of the atmosphere (Lauritzen et al. 2022), an extensive analysis of NorESMs cloud phase representation in the Arctic (Shaw et al. 2022) and an analysis of air-sea-exchange of DMS in CMIP6 models (Bock et al. 2021). Several other major findings are derived from the KeyCLIM NorESM simulations. Among these a) the quantification and localization of heat and moisture transport into the Arctic; b) an analysis of regional circulation patterns; c) an analysis of the Greenland surface mass balance complemented by a study of the interaction of a dynamic Greenland ice sheet with climate; d) a joint study on the sensitivity of winter Arctic amplification on improved NorESM component parameterisations (see below); e) the impact of marine BrF emissions on atmospheric chemistry, including ozone burdens, accounting for a future ice-free Arctic ocean; f) a study on possibly more frequent abrupt marine environmental changes in the future; g) as well as several planned papers to document and evaluate the KeyCLIM NorESM model improvements. Among the new and most prominent NorESM2 developments within KeyCLIM are an interactive land ice component for Greenland, a high-resolution ocean version, the possibility to switch between hybrid (pressure-density) and pure density ocean vertical coordinate, a more realistic mixed phase cloud parameterisation, a revised heat conductivity of snow on sea ice, a rather complex gas-phase chemistry in both the troposphere and stratosphere, an improved coupling between the ocean and atmosphere, including bromoform sea-to-air-fluxes, and an enhanced parameterisation of mesoscale eddies in the ocean. Based on these developments, the KeyCLIM sensitivity experiments have been completed for joint analysis. Experiments were dedicated to study an improved heat flux for snow covered sea ice areas, a revised ocean eddy diffusivity, an interactive Greenland ice sheet, the revised ice-cloud microphysics, a complex chemistry including bromoform, volcanic aerosols and ozone and a much reduced anthropogenic forcing by aerosols. The sensitivity experiments all demonstrate enhanced future Arctic warming compared to the CMIP6 reference experiment. The amplitude of the additional warming moreover varies considerably, with the difference between the experiment with the strongest and weakest warming reaching 8.6 K by the end of the 21st Century. Surface temperature decomposition reveals that the warming is dominated by changes in clouds, longwave clear-sky fluxes, surface heat fluxes, and ocean heat storage. We also identify an emergent constraint, linking changes in Arctic surface temperatures to changes in ocean heat fluxes and to changes in sea-ice area. The new development will contribute to a new updated version NorESM2.3 to be released in early 2024, a version which is important to prepare for a comprehensive upgrade of NorESM for CMIP7. Model simulation results are being archived utilising the NIRD research data archive. Along the project timeline KeyCLIM organised several workshops to disseminate progress on model development and major results, among these one workshop on ”Constraining model uncertainty in climate sensitivity” and one on ”Precipitation trends in Nordic regions”.

KeyCLIM has resulted in a major scientific co-operation among the community using the NorESM model in Norway with important interdisciplinary exchanges that were achieved in the field of climate sciences. KeyCLIM has contributed and initiated a range of more in-depth projects utilising NorESM on Nordic and EU level. A large number of significant contributions to IPCC AR6 and CMIP6 analysis through peer-reviewed publications have been a major outcome of KeyCLIM (roughly 30 of the 66 peer reviewed publications originating from KeyCLIM so far). KeyCLIMs has developed new components and functionalities of the NorESM model which will be integrated in a new NorESM model version in 2024 (eg land-ice, revised snow hydrology, higher resolution ocean, optional hybrid ocean coordinates, revised eddy mixing in the ocean, climate driven emissions, complex atmospheric chemistry and air-sea-exchange of additional trace gas components, volcanic aerosol treatment ) that benefits Earth System understanding and also facilitate usage of the NorESM model for a wider range of climate change problems. A series of sensitivity NorESM coupled model simulations has been produced and analysed. It provides a basis for further understanding of Arctic Amplification. The simulation results will be shortly archived for further use by the climate science community. KeyCLIM scientists have contributed to relevant and scientific fact-based communication on climate mitigation and adaptation, and thus have enabled a societal discussion needed for sustainable development. KeyCLIM organised two broader science workshops, one on methodologies to constrain climate sensitivity, and on on trends in precipitation in Nordic countries.

The projected enhanced warming of the northern latitudes in the forthcoming decades will impact the region’s hydrological cycle, the cryosphere and biogeochemical cycles, impacts which are associated with forecasted, significant changes of the Earth System functioning. It is not clear how the northern world will look like with a summer ice-free Arctic Ocean, changed atmospheric and oceanic circulation patterns and the many specific responses of the Northern Earth system. It will require a powerful Earth System model to generate new understanding of the possible future climate evolution. Scandinavian and Norwegian climate will be in particular affected by Earth System changes in the North Atlantic, Arctic and adjacent land masses, thus this region is chosen to be key for this project. The core research community utilizing the Norwegian Earth System Model (NorESM) is assembled for this project proposal to improve our understanding on three levels: 1) to analyze bias and deficiencies in NorESM, engaging into complementary analysis of the CMIP6 model experiment archive and re-analysis data, from this to recommend on model development needs; 2) to study those key Earth System processes in the key region which are important for the northern climate evolution for the next 50-300 years, improving NorESM where needed; 3) isolate the role of sphere couplings and feedbacks for future projection of climate with a suite of specifically designed NorESM coupled simulations, supplementing CMIP6. Ultimately KeyCLIM shall achieve a much improved picture of the near and long term future climate for northern latitudes and Norway, providing open data to the climate mitigation and impact community. KeyCLIM studies of the interaction between the spheres and important feedbacks will allow the research community to assess the possibility of irreversible changes in the climate system.