Global and European temperatures

Methodology for indicator calculation

The following global meteorological datasets have been used to compute the time series of global mean temperature and European land temperature:

· HadCRUT5: This dataset is a collaborative product of the Met Office Hadley Centre and the Climatic Research Unit (CRU) of the University of East Anglia. HadCRUT5 is a combination of sea-surface temperature (SST) measurements over the ocean from ships and buoys and near-surface air temperature measurements from weather stations over the land surface.

· NOAA Global Temp v5 : This dataset is a product of the National Centre for Environmental Information of the U.S. National Oceanic and Atmospheric Administration (NOAA).

· GISTEMP v4: This dataset is a product of the NASA Goddard Institute for Space Studies (GISS).

The temperature anomalies from the original datasets were adjusted here to the ‘pre-industrial’ period between 1850 and 1899.

· Berkeley Earth: temperature dataset produced by Berkeley Earth; an independent U.S. non-profit organization focused on environmental data science.

· ERA5 Dataset produced by ECMWF and available in the Climate Data Store. ERA5
is the fifth generation ECMWF reanalysis for the global climate and weather for the past 4 to 7 decades. Currently data is available from 1950. Reanalysis combines model data with observations from across the world into a globally complete and consistent dataset.

· JRA-55: The Japan Meteorological Agency (JMA) conducted the second Japanese global atmospheric reanalysis, called the Japanese 55-year Reanalysis or JRA-55. It covers the period from 1958, when regular radiosonde observations began on a global basis.

These datasets were provided by the Copernicus Climate Change Service (C3S), following the developed methodology to relate recent (1991-2020) global temperature to 1850-1900, a period taken to represent the pre-industrial level. New and updated global temperature datasets have been recently published, which has resulted in a new estimate for the latest 30-year reference period 1991-2020, defined by the World Meteorological Organization (WMO). The new approach for monitoring global temperature change since the 1850-1900 period is being used in the WMO statements on 'The state of the global climate' from the Preliminary Statement for 2021 onwards (see details there, under 'Datasets and methods – Global temperature data'). This approach results in a best estimate of 0.68°C with an uncertainty range (0.54 to 0.78°C) to relate the standard WMO reference period 1981-2010 to 1850-1900. Extending this approach to the WMO reference period 1991-2010 gives a best estimate of 0.88°C with an uncertainty range (0.72-0.99°C), which sums the 0.68ºC of additional warming from pre-industrial to the 1981-2010 period, the 0.19ºC difference from 1981-2010 to 1991-2020, according to ERA5, and an adjustment of 0.01ºC for consistency with new estimates documented by the Sixth IPCC Assessment Report. In the datasets here provided by C3S this method has been used, by calculating the anomalies of each dataset relative to its own average for 1981-2010 and adding the offset of 0.88°C, to then relate it to 1850-1900.

IPCC estimates are not available for the all-land, European and Arctic regions for which C3S provides temperature indicators. For these cases, the changes from 1850-1900 are based on estimates of the differences between 1991-2020 and 1850-1900 averages derived from the Berkeley Earth, GISTEMPv4, HadCRUT5 and NOAAGlobalTempv5 datasets. 

Spatially explicit temperature trends in Europe are derived from E-OBS v25.0e (Cornes et al., 2018). E-OBS is a daily gridded observational data set for precipitation, temperature and sea level pressure in Europe based on ECA&D information. The ECA&D project maintained by KNMI has collected homogeneous, long-term daily climate information from about 200 meteorological stations in most countries of Europe and parts of the Middle East. The dataset covers the period from 1950 on. Trends are calculated using the annual mean and then applying an ordinary least squares regression using the model y=a+b*x+e, where e is assumed to be a first order autoregressive process. The significance is computed from a two-sided Student’s test for the null hypothesis of no trend. The code used to extract the trend and significance results is the same as in the IPCC Atlas. The underlying stations timeseries used as input to the interpolation of E-OBS are usually not corrected for inhomogeneities (except for E-OBSv19.0eHOM). Also, the station density is not constant through time. Trend analyses should therefore be treated with caution.

The projected changes in European near-surface air temperature (°C) are based on the multi-model ensemble average of GCM simulations from the Coupled Model Intercomparison Project Phase 6 (CMIP6) initiative . CMIP6 is a project coordinated by the Working Group on Coupled Modelling (WGCM) as part of the World Climate Research Programme (WCRP).

Further information on all these datasets is available from the cited publications.