High Quality Content by WIKIPEDIA articles! Measurement of sea ice is important for safety of navigation and for monitoring the environment, particularly the climate. Record keeping of direct observations began over a thousand years ago, but was sparse until the 1950s. Truly comprehensive records began with the satellite era in the late 1970s. Modern records include data about ice extent, ice area, concentration, thickness and the age of the ice. The trends in the records show a significant decline in Northern hemisphere sea ice and a very small, statistically insignificant increase in the winter Southern hemisphere sea ice.