LONDON: Arctic winters may have got shorter by a month. Arctic lakes have been found to be freezing up later in the year and thawing earlier, creating a winter ice season about 24 days shorter than it was in 1950.
Satellite imagery has also confirmed that climate change has dramatically affected the thickness of lake ice at the coldest point in the season: In 2011, Arctic lake ice was up to 38 centimeters thinner than it was in 1950.
The study of more than 400 lakes of the North Slope of Alaska is the first time researchers have been able to document the magnitude of lake-ice changes in the region over such a long period of time.
The research team used satellite radar imagery from the European Space Agency to determine that 62% of the lakes in the region froze to the bottom in 1992. By 2011, only 26% of lakes froze down to the bed, or bottom of the lake.
Overall, there was a 22% reduction in what the researchers call "grounded ice" from 1992 to 2011.
Twenty years of satellite radar imagery say that changes in air temperature and winter precipitation over the last five decades have affected the timing, duration and thickness of the ice cover on lakes in the Arctic.
In this region, warmer climate conditions result in thinner ice cover on shallow lakes and, consequently, a smaller fraction of lakes freezing all the way through during winter months.
"We've found that the thickness of the ice has decreased tremendously in response to climate warming in the region," said lead author Cristina Surdu from the University of Waterloo. "When we saw the actual numbers we were shocked at how dramatic the change has been. It's basically more than a foot of ice by the end of winter".
Researchers were able to tell the difference between a fully frozen lake and one that had not completely frozen to the bottom, because satellite radar signals behave very differently, depending on presence or absence of water underneath the ice.
Radar signals are absorbed into the sediment under the lake when it is frozen to the bottom. However, when there is water under the ice with bubbles, the beam bounces back strongly towards the radar system. Therefore, lakes that are completely frozen show up on satellite images as very dark while those that are not frozen to the lake bed are bright.
