OBSERVER: Copernicus data for climate-informed decisions

As global warming accelerates, decisions on infrastructure, energy, mobility, water and food security or urban planning need to be informed by the potential impacts of climate change. The Copernicus Climate Change Service (C3S) ERA5 reanalysis dataset provides key insights about our past and present climate, helping to foresee the risks, and assisting policymakers in making informed decisions. 
The consequences of climate change, anticipated by scientists many years ago, have become a part of our everyday lives. Throughout 2023, almost every corner of the world was affected, and nearly every individual has witnessed, to varying degrees, an increase in the frequency or intensity of extreme weather occurrences such as heatwaves, wildfires, droughts, or heavy rainfall. This transformation is occurring rapidly, and so we need swift adjustments in policies.

Reliable and high-quality data plays a crucial role in decision-making, as understanding the past establishes the baseline risk profile needed for effective future planning. 

Agricultural areas around Thessaly, Greece, flooded on 20 September 2023, two weeks after storm Daniel caused record-breaking rainfall.  Credit: European Union, Copernicus Sentinel-2 imagery.

The ERA5 reanalysis from C3S provides reliable quality-controlled data on the state of Earth’s climate. It presents a thorough account of climate conditions on an hourly basis spanning from 1940 to a few days before the present, generating maps-without-gaps with a resolution of about 30 km.

When it comes to tracking global temperatures, particularly temperature anomalies, which involve comparing the temperature for a specific period with the average values over a historical period taken as a reference (e.g. 1991-2020), ERA5 stands out as one of the most widely used datasets globally. 

For example, using ERA5 data, C3S determined that the five months of June, July, August, September, and October 2023 were the warmest on record compared to the 1991-2020 averages for the same months. As a result, there is a high level of certainty that 2023 will be the warmest year on record.

Globally averaged surface air temperature anomalies relative to 1991–2020 for each October from 1940 to 2023. Data source: ERA5. Credit: C3S.

One ERA5, a family of datasets

Users have open access to the ERA5 dataset in the Climate Data Store (CDS) where, in addition to the main dataset, they can also benefit from other versions tailored to their needs. Let’s take a closer look at the ERA5 family of products. 

ERA5 

ERA5 provides detailed, hourly data on atmospheric, sea surface, and land surface conditions from 1940 up to five days before the present. By integrating billions of observations with cutting-edge simulations, the dataset stands as one of the most precise reconstructions of past global weather. With a horizontal resolution of 30 km, ERA5 is able to retrieve comprehensive global data on atmospheric conditions, ocean waves, and various land surface variables.

After ERA5 datasets are generated, they undergo a two-month period of quality control, during which C3S reviews and corrects any errors identified. Such instances have occurred only once since operations began, and users are promptly notified of any such corrections. 

ERA5-T 

ERA5-T (‘T’ standing for temporal) is a product in which the data from the core ERA5 dataset is pre-released with a latency of just 5 days, to meet the demands of users who require more recent data. In most cases, ERA5-t and ERA5 are identical.

ERA5-Land 

ERA5-Land is a high-resolution version of the land-related variables from the ERA5 dataset, providing a consistent view of the evolution of the land surface through land variables (lake water temperature, river discharge, soil moisture and others). It does this at an improved spatial resolution of 9 km to provide hourly data on a global scale for 50 land variables from January 1950 to three months before the present day. 

Comparison of soil temperatures graphs with different products, namely ERA-Interim, ERA5 and ERA5-Land. The charts show soil temperature of the top 7 cm of soil at 12 UTC on 15 March 2010 according to ERA-Interim (79 km grid spacing, left), ERA5 (31 km grid spacing, middle), and ERA5-Land (9 km grid spacing, right). The temperature values over the Mediterranean Sea in ERA-Interim and ERA5 are sea-surface temperature values. Credit: C3S.

ERA5-Land-T

ERA5-Land-T combines the improved spatial resolution of ERA5-Land with the reduced waiting time of ERA5-T, reducing the delay for ERA5-Land data to just five days before the present, meeting the needs of users requiring data on recent land conditions. 

ERA5 – a truly unique dataset 

Drawing on the extensive data resources of the European Centre for Medium-Range Weather Forecasts (ECMWF) and its Member States, ERA5 stands out as one of – if not - the most used reanalysis datasets globally. Trusted by influential organisations such as the Intergovernmental Panel on Climate Change (IPCC) and the World Meteorological Organization (WMO), ERA5 data is also cited in numerous research papers. 

The teams dedicated to ERA5 continually improve the dataset through diligent efforts in data rescue and ongoing verification operations. Apart from verifications conducted by C3S, scientists worldwide also engage in regular assessments, comparing ERA5 results with observations and other reanalysis datasets to ensure reliability and accuracy. 

Currently, all the ERA5 teams are working on ERA6, the upcoming version expected to feature an improved horizontal resolution of at least 18 km. ERA6 will also improve the realism of interactions between the atmosphere, land, and ocean through additional and more efficient observations. Anticipated to be accessible around 2027, ERA6 will represent a significant advancement in reanalysis capabilities. 

Annual average surface air temperature anomaly (°C) for 2022, relative to the average for the 1991–2020 reference period. Produced for C3S’ European State of the Climate 2022. Data source: ERA5. Credit: C3S.

But why do we need a reanalysis in the first place?

Reanalysis activities originated from the need to have comprehensive information about current and past conditions to accurately predict future weather. To fill the gaps in observations, numerical models are used to provide the most realistic estimate of the values of all variables, including for locations where direct observations are not available, essentially reforecasting past conditions.

And so, reanalyses originally emerged as a secondary element of numerical weather prediction. However, what was once a byproduct has now become a distinct product in its own right. Since the introduction of ERA5 in 2017, the reanalysis has generated a coherent "map without gaps" of the climate, filling in areas or timeframes where observations were lacking.

Despite the complexity of creating, maintaining, and validating a reanalysis dataset, the advantages significantly outweigh the challenges, particularly at a time when comprehending the trajectory of our climate has become necessary. C3S has consistently improved this formidable climate-monitoring tool by incorporating new observations and refining simulations, in tandem with improvements to the ECMWF Integrated Forecasting System. 

ERA5 in support of decision-making

The range of possibilities with ERA5 is almost boundless, as long as we pose the right questions and possess the basic skills to process the data. C3S, along with platforms like WEkEO, provides tools that facilitate both access to and processing of the data. 

Evolution of the maximum temperatures for Brussels during summer from the dataset "Climate Variables for Cities in Europe from 2008 to 2017." (illustration). Credit: C3S/ VITO.

For example, ERA5 enables local authorities to assess temperature and precipitation trends, helping them to anticipate infrastructure investments and formulate agricultural strategies. Mountain communities can use ERA5 data to gain crucial insights into changes in snowfall, while coastal regions benefit from critical information on ocean waves and temperatures, which are important for industries such as fishing and tourism.

An increasing array of decision-ready applications and publications grounded in ERA5 is used to inform global policies. Notable examples include the C3S flagship ‘European State of the Climate’ report and the monthly, yearly and seasonal updates available on the C3S website. ERA5-derived applications, like the "Extreme precipitation statistics for Europe," present data on extreme precipitation from 1979 to 2019, enabling analysis and comparison of indicators. Furthermore, C3S, in collaboration with the Union for the Mediterranean, has developed two applications: one assesses the impact of climate change on the suitability of Mediterranean habitats for vector-borne diseases, while the other explores the risk to UNESCO World Heritage Sites from sea level rise in the Mediterranean Basin.

View of the application “Extreme sea levels on Mediterranean UNESCO World Heritage Sites” developed by C3S in partnership with the Union for the Mediterranean. Credit: C3S and the Union for the Mediterranean.

Policymakers and the public are already using ERA5 data without being aware of it. For example, ERA5 played a key role in the training of GraphCast, the artificial intelligence weather prediction model created by Google, which has gained attention for its exceptional accuracy. 

“The ERA5 reanalysis dataset is very well established as a scientific tool, used by organisations worldwide as a trustworthy source of information about our changing climate,” says C3S Director Carlo Buontempo. “As global warming accelerates at alarming levels, we are putting all our efforts into developing new tools that will make our data more accessible and meaningful to decision-makers that need to make fast, informed decisions affecting the lives of millions of people,” he concluded.