Dramatic new evidence of the effect of warming on the world's polar regions -- especially the Arctic -- has been presented in a time-lapse movie of 27 years of melting of polar ice.
Released by the National Oceanic and Atmospheric Administration, the video consists of an animation of data gathered from almost three decades of Arctic sea ice observations.
Rising ocean temperatures in that time have caused a increased amount of perennial ice -- sea ice more than a year old -- to melt, worrying scientists.
"We've come to an era where the older ice is getting to be less observed and the coverage of the ice pack now is a lot more first-year ice and second-year ice," says Mark Tschudi, a research associate at the University of Colorado-Boulder.
"In general, older ice tends to be thicker ice, so the fact that the pack has transformed into a pack of younger ice means it's more susceptible to melt out in the summer."
There's a feedback process at work here, scientists say.
Sea ice is strongly reflective, bouncing around 80 percent of the sunlight hitting it back out into space.
However, when the ice melts and exposes the darker ocean surface that absorbs around 90 percent of the sunlight, the result is elevated ocean temperatures and, in turn, more melting of ice.
Tschudi has been using Arctic Ocean buoys and satellite data to track variations in sea ice from 1985 through 2014, and NOAA used his data to create its time-lapse movie.
In 2014, ice that managed to remain frozen through at least four seasons made up 10.1 percent of the ice pack, a worrying decrease from the 26 percent average seen in the 1980s, he says.
Arctic sea ice coverage usually peaks in March, starts to retreat in spring and summer with rising temperatures, and historically reaches a coverage minimum in September.
An all-time low in extent was recorded in the summer of 2012, just 1.3 million square miles as compared to an average from 1979 to 2000 of 2.57 million square miles.
In addition, the six smallest seasonal minimums in the entire period of satellite observation all have happened since 2007, NOAA says.
NOAA scientists say a completely ice-free summer in the Arctic is "very likely" by mid-century, although some other researchers have suggested it could occur as early as 2020.
