The vast amount of sea ice covering the Arctic fluctuates on a seasonal basis, and the winter peak marks a turning point before a melting period during the warmer spring and summer months. Arctic sea ice typically expands to a maximum in March and shrinks to a minimum in September each year. The National Snow & Ice Data Center said on Thursday that this year’s maximum occurred on Feb. 25, about two weeks earlier than the average, barring any unlikely additional growth of ice late in the season.