Study 'examines historical length changes of glaciers in W Greenland from 1800-2010 & finds 'avg. rate of retreat was largest in first half of 20th century.' Data from the paper shows avg. rate of retreat peaked at 25 meters per year during 1930's & has decelerated to about half that rate over past 70 years. Note glaciers have been generally retreating for 20,000 years since peak of last major ice age, & since end of Little Ice Age in 1850'