The winter ice covering the Arctic Ocean has reached its annual peak, but the extent of sea ice cover this winter is smaller than it has been at the end of any winter since 1978, when scientists began keeping consistent satellite records.

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.

Average maximum extent of Arctic sea ice, 1979 to 2014 Russia NORTH POLE Norway Extent of Arctic sea ice as of Feb. 25 Alaska Greenland ICELAND Pacific Ocean Atlantic Ocean Canada UNITED STATES Russia Extent of Arctic sea ice as of Feb. 25 Pacific Ocean Atlantic Ocean Canada Avg. maximum extent of Arctic sea ice, 1979 to 2014 Avg. maximum extent of sea ice, 1979 to 2014 Russia NORTH POLE Norway Extent of Arctic sea ice as of Feb. 25 Alaska Greenland ICELAND Pacific Ocean Atlantic Ocean Canada UNITED STATES Russia Extent of Arctic sea ice as of Feb. 25 Pacific Ocean Atlantic Ocean Canada Avg. maximum extent of Arctic sea ice, 1979 to 2014

The center said that recent weather patterns partly explain why the maximum this year is smaller than in previous winters. The North Pacific was warm this year because the atmospheric jet stream of cold air looped farther north in that region than is typical. The jet stream also plunged farther south than usual near the United States, bringing cooling temperatures and triggering heavy snows in much of the country.

Summer minimums in the Arctic’s ice cover can have a greater effect on the global climate than winter maximums, according to Walt Meier, a NASA scientist and an expert on sea ice. During the relatively sunny summers, the dark ocean surface of ice-free parts of the Arctic absorbs much more solar energy than highly reflective sea ice. This can create a warming feedback loop when the ocean absorbs sunlight and heats the air above it.

Scientists believe that the sharp long-term decline in Arctic sea ice is primarily driven by planetary warming that is occurring because of human emissions of greenhouse gases. The trend shows up most strongly in the summer readings.

18 million sq. km Each line represents one yearly cycle of sea ice fluctuations Yearly fluctuations in area of Arctic covered by ice Millions of sq. km Feb. 25 2015 maximum 1979 16 The dotted line represents the average ice extent from 1981 to 2010 14 2015 Arctic ice cover usually reaches an annual maximum around March Ice cover usually shrinks to its minimum in September 12 Size of Canada Ice extent for 2010 to 2014 is shown with green lines 1980 8 6 2012 Arctic ice extent reached the lowest point on record 4 Size of Alaska SUMMER WINTER Size of California JAN. FEB. MARCH APRIL MAY JUNE JULY AUG. SEPT. OCT. NOV. DEC. 18 million sq. km Feb. 25 2015 maximum Yearly fluctuations in area of Arctic covered by ice Millions of sq. km Each line represents one yearly cycle of sea ice fluctuations. The dotted line represents the average ice extent from 1981 to 2010 1979 16 14 Ice cover usually shrinks to its minimum in September 2015 Arctic ice cover usually reaches an annual maximum around March 12 Size of Canada Ice extent for 2010 to 2014 is shown with green lines 1980 8 6 2012 Arctic ice extent reached the lowest point on record 4 Size of Alaska SUMMER WINTER Size of California JAN. FEB. MARCH APRIL MAY JUNE JULY AUG. SEPT. OCT. NOV. DEC. Area of Arctic sea ice Millions of sq. km, 1981 to 2010 18 million sq. km 12 Feb. 25 2015 maximum Ice extent for 2010 to 2014 is shown with green lines 6 2012 0 FEB. APRIL JUNE AUG. OCT. DEC.

Dr. Meier said that during the winter much of the ice near the edges of the ice sheet covering the Arctic is thin and seasonal, whereas some of the older and thicker ice melts during the summer. The differences between these seasonal processes mean winter ice cover does not help predict how much ice there will be by the end of summer. “When you lose summer ice you aren't really just losing it for that year, you're also losing some ice from many years ago,” he said. “That makes it harder for things to go back towards normal.”