The Arctic has ended the winter with one of the lowest ever recorded levels of sea ice cover, US scientists said.
Arctic sea ice melts and regrows over the year, freezing throughout the winter months to reach a maximum extent in late February or March, then melting through the summer to hit a low point in early or mid-September.
Maximum sea ice cover in mid March, before the summer melt began, was the second lowest in the 39-year satellite record, just behind last year's record lows, scientists from the National Snow and Ice Data Centre (NSIDC) said.
At its maximum extent, on March 17, Arctic sea ice cover was 14.48 million square kilometres (5.59 million square miles) this year, which is 60,000 square kilometres (23,200 square miles) more than in 2017.
The experts said the low levels of ice again this year were due to a late autumn freeze-up and persistent high air temperatures throughout the winter.
The four lowest levels of maximum sea ice cover have all been seen in the last four years.
In February, for the fourth winter in a row, an extreme heatwave was seen over the Arctic Ocean, the researchers said.
Rod Downie, head of polar programmes at WWF, said: "This announcement confirms the downward spiral of Arctic sea ice.
"The Polar Regions are our planet's air conditioning unit - and it's breaking down during a heatwave.
"On the day before Earth Hour, it shows we must take urgent action to reduce greenhouse gas emissions and tackle climate change head-on before it is too late."
Landmarks, businesses and members of the public around the world are taking part in Earth Hour at 8.30pm local time on Saturday, turning off the lights to mark the need to protect the environment and take action on climate change.