Arctic ice pack
Records of Arctic Sea ice from the United Kingdom's Hadley Centre for Climate Prediction and Research go back to the turn of the 20th century. Before 1950, the data is not as good as after 1950.  Reliable measurements of sea ice edge begin within the satellite era. From the  late 1970s, there were different satellites that could provide data which was independent of solar illumination or weather conditions. The frequency and accuracy of passive microwave measurements improved with the launch of the DMSP F8 Special Sensor Microwave/Imager (SSMI) in 1987. Both the sea ice area and extent are estimated. The latter is lager, as it is defined as the area of ocean with at least 15% sea ice.