From the 1990s onwards the unit received requests for raw data made by those critical of the scientific understanding of global warming, and after the UK Freedom of Information Act (FOIA) came into effect in 2005, there were Freedom of Information requests to the Climatic Research Unit for this raw data.
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In a decision announced on 27 July 2011 the Information Commissioner's Office (ICO) required release of raw data even though permissions had not been obtained or in one instance had been refused, and on 27 July 2011 CRU announced release of the raw instrumental data not already in the public domain, with the exception of Poland which was outside the area covered by the FOIA request.
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HadCRUT is the dataset of monthly instrumental temperature records formed by combining the sea surface temperature records compiled by the Hadley Centre of the UK Met Office and the land surface air temperature records compiled by the Climatic Research Unit (CRU) of the University of East Anglia.