Effective October 1, 2013, the calibration of the Landsat 7 Enhanced Thematic Mapper Plus (ETM+) Band 6 will be updated to correct a bias error that has been present in all thermal data since the last calibration update in 2010. Vicarious calibration teams detected a bias error of 0.036 watts per square meter steradian micron (W/(m2 sr µm)) that causes overestimation of top-of-atmosphere temperatures by approximately 0.26 Kelvin (K) at 300K. Calibration parameters within the Calibration Parameter File (CPF) have been adjusted to correct for this bias error.
All data processed by USGS/EROS from October 1, 2013 forward will be correctly calibrated within 0.4K at 300K using updated CPF coefficients. Users can correct for the bias error themselves by subtracting 0.036 W/(m2 sr µm) from the top-of-atmosphere radiance product for data products processed between January 1, 2010 and September 30, 2013.
More details about this update can be found at https://www.usgs.gov/landsat-missions/science_L7_Cal_Notices.php.
Source: USGS Landsat Program