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CRUTEM4 time series

Calculating error bars on a time series average of CRUTEM4 is not a trivial process. Each component of the uncertainty on the gridded values has a different correlation in space and/or time, and these correlations have to be allowed for when computing uncertainties in regional or temporal averages of the gridded data.

The station errors have large autocorrelations in time, but no correlation in space, and the grid-box sampling errors have little correlation in either space or time, so these two error components are small for global and large-scale averages.
The biases (urbanisation and exposure) have strong correlations in space and time, so they are just as large for decadal global averages as for monthly grid-point values.
Spatial averages contain an additional source of sampling error as there are regions of the world where there are no observations. We calculate the size of the error bars due to the non-complete global coverage by looking at the effect of reducing coverage in a globally-complete reanalysis dataset.

To try and make these different effects clear we present these uncertainty components separately in the plots and data files for each time-series. The data files contain 12 columns:

Column 1 is the date.
Column 2 is the best estimate anomaly. (For the current year this will be the average for the year so far. The latest month that has been processed will appear in the monthly files).
Columns 3 and 4 are the upper and lower bounds of the 95% confidence intervals from the station and grid-box sampling uncertainties only.
Columns 5 and 6 are the upper and lower bounds of the 95% confidence intervals from the coverage uncertainties only.
Columns 7 and 8 are the upper and lower bounds of the 95% confidence intervals from the bias uncertainties only.
Columns 9 and 10 are the upper and lower bounds of the 95% confidence intervals from the combined station and grid-box sampling, and coverage uncertainties.
Columns 11 and 12 are the upper and lower bounds of the 95% confidence intervals from the combined effects of all the uncertainty sources represented in CRUTEM4 (i.e. the range within which the anomalies are highly likely to occur according the the CRUTEM4 uncertainty model). These uncertainty sources are: station and grid-box sampling, bias uncertainties and coverage uncertainties.

More details are given in Brohan et al (2006).

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