Coverage bias in the HadCRUT4 temperature record

Kevin Cowtan and Robert Way

Temperature series

Note: Maintaining a scientific data set to professional levels of quality assurance is beyond our resources. These results are therefore presented as a best effort. Scientific results should ideally be based on data from a range of sources.

There are two versions of the temperature series. The version 1.0 series are monthly updates of the calculation as presented in Cowtan and Way (2014). These will be updated to include December 2013 and then discontinued.

The version 2.0 series are based on a more rigorous treatment in which the land and ocean data are reconstructed separately and then blended. Version 2.0 data are also based on the full HadCRUT4 ensemble, and include ensemble as well as coverage uncertainty estimates. The data are described in this update.

All except the long (1850-present) reconstruction are rebaselined on 1981-2010. Rebaselining the data on the period 1981-2010 enables the satellite data to be used for infilling, and in the v1.0 calculation increases the reliability of recent trends, but also discards some data which may impact the reliability of long term trends.

Update 2014-10-7. CRU have updated their station data to version 4.3. Observations have been introduced for a number of important high latitude stations for the period from early 2012 to mid 2014. Monthly averages post 2012 have changed by up to 0.04C. Monthly averages prior to 2012 show much smaller changes. Annual averages and trends are largely unaffected.

Version 2.0

Long reconstruction (1850-present):
  • HadCRUT4 infilled by kriging
    1850-present, original baseline (i.e. 1961-1990), ensemble medians
    Global mean/uncertainty: monthly, annual. Gridded data: ascii, netcdf
Short reconstructions (1979-present):
  • HadCRUT4 hybrid with UAH
    1979-present, baselined on 1981-2010, ensemble medians
    Global mean/uncertainty: monthly, annual. Gridded data: ascii, netcdf
  • HadCRUT4 hybrid with MERRA
    1979-present, baselined on 1981-2010, ensemble medians
    Global mean/uncertainty: monthly, annual. Gridded data: ascii, netcdf
  • HadCRUT4 infilled by kriging
    1979-present, baselined on 1981-2010, ensemble medians
    Global mean/uncertainty: monthly, annual. Gridded data: ascii, netcdf
  • GHCN/HadSST3 hybrid with UAH
    1979-present, baselined on 1981-2010
    Global mean/uncertainty: monthly, annual. Gridded data: ascii, netcdf
  • GHCN/HadSST3 hybrid with MERRA
    1979-present, baselined on 1981-2010
    Global mean/uncertainty: monthly, annual. Gridded data: ascii, netcdf
  • GHCN/HadSST3 infilled by kriging
    1979-present, baselined on 1981-2010
    Global mean/uncertainty: monthly, annual. Gridded data: ascii, netcdf

Temperature files contain 5 columns: date, temperature, total uncertainty (1σ), coverage uncertainty (1σ), ensemble uncertainty (1σ). Map data are in a similar format to the ascii files provided by Hadley or NCDC, i.e. 1 line with year and month followed by 36 lines of 72 values.

The land temperature reconstruction is used for air temperatures over land and sea ice, and the sea surface temperature reconstruction for open sea. Sea ice coverage is fixed for each month of the year, and is determined from HadISST using the period 1981-2010. For the long reconstruction (i.e. 1850-present) the median ice coverage in the given month of the year is used for each cell; for the short reconstructions (i.e. 1979-present) the minimum ice is used. Hybrid data use the hybrid approach for air temperatures over land or sea ice, and kriging for sea surface temperatures.

Version 1.0

  • HadCRUT4 hybrid with UAH
    1979-2013, baselined on 1981-2010
    Global mean/uncertainty: monthly. Gridded data: ascii, netcdf
  • HadCRUT4 infilled by kriging
    1979-2013, baselined on 1981-2010
    Global mean/uncertainty: monthly. Gridded data ascii, netcdf
  • HadCRUT4 infilled by kriging
    1850-2013, original baseline (i.e. 1961-1990)
    Global mean: monthly. Gridded data ascii, netcdf

Temperature files contain 3 columns: date, temperature, standard deviation (1σ). Map data are in a similar format to the ascii files provided by Hadley or NCDC, i.e. 1 line with year and month followed by 36 lines of 72 values.