Abstract

Climate re-analyses are generated by combining Earth system models with meteorological observations, using methods that are similar to those used for numerical weather prediction (NWP). Reanalysis datasets contain a wealth of information about past weather and the recent climate, in the form of multi-decadal time series for many geophysical variables on global grids. Several major NWP centres intermittently conduct reanalysis projects as part of their research and development activities. Reanalysis data have been widely used by the scientific community, especially in the Earth sciences, as evidenced by the very high number of citations of reanalysis products in the published literature. The quality and utility of reanalysis products have improved greatly over the years, mainly due to steady progress in modelling, Earth observation and data assimilation. At the same time, as society awakens to the real consequences of climate change, demand for reliable information about weather and climate has increased rapidly. Reanalysis data, together with other types of climate data, are now routinely used to assess past, present and future impacts of climate change in agriculture, water resources, energy, health, urban planning, transport and other sectors. Consequently, the community of reanalysis users is growing and becoming much more diverse, with experts in different domains, technical consultants, data scientists and many others who are climate-literate but may not be specialized in climate science. Having the needs and requirements of this new user community in mind, the Copernicus Climate Change Service (C3S; Buontempo et al., 2022) was designed to facilitate and support development of effective climate services based on high-quality, consistent, scientific data. The service focuses on simplifying access to data and enabling new applications for planners, policy makers and technical experts in the private and public spheres. The backbone of C3S is the Climate Data Store (CDS), which provides open and free access to a catalogue of more than 150 quality-controlled climate datasets, including observations, reanalysis products, climate predictions and climate projections. The CDS has currently more than 325,000 registered users from around the world, who collectively download and process more than 1 Petabyte of data every day. The most-used CDS dataset by far is the ERA5 reanalysis (Hersbach et al., 2020), produced and maintained by the European Centre for Medium-Range Weather Forecasts (ECMWF). ERA5 provides hourly estimates for a wide range of atmospheric, land and oceanic climate variables on a 31-km global grid, for the period from 1940 to present. The reanalysis continues to be extended forward in time, with daily updates made available to users within 5 days of real time. ERA5 is used by C3S to monitor essential climate variables such as air temperature, precipitation and sea ice and serves as a primary data source for numerous international media reports on climate. Users everywhere rely on ERA5 data as essential input for local adaptation and risk assessment activities. This special issue highlights the role of reanalysis in climate services. The papers selected provide a few notable examples of the wide variety of applications needed to address the impacts of climate change in key sectors of society. Three of the papers concern the energy sector. The topic of the paper by Fallon et al. (2023) is the transformation to sustainable energy systems, and how reanalysis data are used in risk assessments for planning and operation of reserve power infrastructure. Hunt and Bloomfield (2024) describe the use of hydrometeorological data from ERA5 to explore the potential of wind and solar energy across India. This application represents a major use case for atmospheric reanalyses, which provide the best available source of global information about wind and solar radiation. The Dubus et al. (2023) paper describes a service for the energy sector designed to provide climate impact indicators for electricity demand and supply from wind, solar and hydropower. The service is used for trend analysis, seasonal outlooks and future projections based on different energy mix scenarios in the long term. The Di Napoli et al. (2023) paper highlights the crucial role of reanalysis data in the health sector, in this case for monitoring and assessing the impacts of climate-related hazards on human mortality, labour capacity, physical activity, well-being, infectious disease transmission and food security and undernutrition. The paper describes the Lancet Countdown monitoring system for health indicators, which relies mainly on global reanalysis data for detailed information about heatwaves, precipitation extremes, wildfires, droughts, warming and impacts on ecosystems. Rapid changes in the Greenland cryosphere have greatly affected the local environment and the communities that depend on it. Melting of the Greenland ice sheet has major consequences for global sea level rise. The ability to monitor and understand those changes is paramount. The study by Box et al. (2023) provides a thorough analysis of the quality of available data on the Greenland icesheet and precipitation patterns from global and regional reanalysis data. The paper by Prudhomme et al. (2024) describes the development of global hydrological reanalyses, which are generated by combining physically based hydrological models with meteorological data from atmospheric reanalyses. Hydrological reanalyses are used to monitor land water resources and ocean dynamics, to improve our understanding of large-scale hydrological extreme fluctuations and for the development of early warning systems. They also provide long-term context for identification of extreme hydrological events, such as droughts and floods. Reanalysis has become an indispensable resource for climate monitoring, risk assessment and local adaptation activities, as illustrated with the papers in this special issue. In recognition of the crucial role of reanalysis for climate services, the World Meteorological Organization (WMO) has established a Lead Centre for coordination of assessment of multi climate reanalysis (LC-GCR), to be led by ECMWF. The result, we expect, will be further improvements in reanalysis quality, access and utility to the benefits of users around the world. Carlo Buontempo: Conceptualization; writing – review and editing. Chiara Cagnazzo: Conceptualization; writing – review and editing. Dick Dee: Conceptualization; writing – review and editing. The Copernicus Climate Change Service is funded by the European Union and implemented by ECMWF together with numerous public and private entities in Europe and elsewhere.