Greta Thunberg has deleted a tweet warning climate change could "wipe out humanity" unless there is a halt on fossil fuel usage by 2023.
The tweet referred to a warning from James Anderson, a Harvard University professor of atmospheric chemistry, that pollution is pushing the climate back 33 million years when there was no ice on either pole.
Speaking at the University of Chicago, he said: "The chance there will be any permanent ice left in the Arctic after 2022 is essentially zero," with 75-to-80 percent of permanent ice having melted already in the last 35 years.
