Baltic Earth Working Group on Future projections of the Baltic Sea region

• How will the climate of the Baltic Sea region change in future and what are the causes?
• What climate data are needed by society and how can the continuous need for climate data for climate
  adaptation be met, taking into account the uncertainties due to model shortcomings, future greenhouse
  gas as well as aerosol emissions, land-use changes, nutrient loads and natural variability?  

In Baltic Earth, regional Earth system models for the Baltic Sea region consisting of compartments for the atmosphere, ocean, sea ice, land surface and the terrestrial and marine ecosystem have been developed in the past, but also today, and used to downscale available projections of future climate, e.g. from CMIP5. Global climate models are still too coarse to reflect the details of the Baltic Sea region (orography, land-sea mask and relevant regional processes). Hence, the increased horizontal resolution and better process descriptions, e.g. mixing in the ocean, by regional climate models are needed. The added value of the latter was proven by many studies. Because projections of future climate are needed for climate adaptation, regular updates of scenario simulations are required, following the CMIP protocol. While the production of regional projections for the atmosphere and data provision are internationally coordinated within CORDEX, there is not yet a comparable project for regional oceans, especially for coastal seas such as the Baltic Sea. In cooperation with a number of recently approved national and international (EU) projects, the existing regional climate models will be used to create new scenario simulations for the Baltic Sea region under CMIP6 and CMIP7 and in upcoming phases, internationally coordinated by CORDEX and CLIVAR. A key objective of RT8 is to advance the development and evaluation of high-resolution regional Earth system models and to generate large multi-model ensemble simulations that allow for more robust quantification of uncertainties.

Potential activities under RT8 include performing multi-model ensemble scenario simulations using regional coupled Earth system models (RESMs) with the highest available horizontal and vertical resolutions, driven by the latest CMIP data and following international standards coordinated by CORDEX and CLIVAR, including common simulation protocols and standardised datasets. Work will also focus on calibrating new generations of RESMs to reduce model uncertainties, as well as evaluating climate models over the historical period to identify robust biases and the underlying processes that require improvement. Natural variability will be assessed through large ensemble simulations, complemented by the development of model substitutes (emulators) using machine-learning methods to expand ensemble size with reduced computational resources. Further activities include exploring the predictive capacity of regional and global climate model simulations with the aim of producing decadal climate predictions for the Baltic Sea region, and contributing to the international community of regional climate modellers and model users by supporting knowledge exchange across model domains. The research topic will also support climate-impact and adaptation needs by engaging with relevant users and stakeholders, such as HELCOM, and by providing data and expertise to climate services, including contributions to the Baltic Earth–HELCOM Climate Change Fact Sheet. Finally, the RT8 will contribute to expert assessment reports, including BEAR II and IPCC AR7.