"The estimate is low - we know it's low," Craig Hilton-Tailor, the manager of the list, said. More than 41,000 species were surveyed.
Three of the species added to the list are Galapagos corals, which are critically endangered by the warm-water Pacific Ocean pattern El Nino and climate change.
Hilton-Tailor said: "It's really hard to identify whether it's climate change or not that's driving some of these species to extinction.
"Climate change doesn't operate by itself - it's operating in tandem with other threats and it's usually the combination of climate change and possibly the threat of a new disease … it's different combinations that can push species over the brink."
Raising awareness
Hilton Tailor said the aim of the Red List is to encourage a change of attitudes among policy makers and ordinary people alike.
"If everybody on the planet co-operated and adopted a sustainable way of living, a lot of these problems would go away," he said.
He said the plight of the western lowland gorilla, which moves from endangered to critically endangered on the latest list, was of particular concern.
The decline of the western lowland gorilla is largely due to Ebola, a fatal haemorrhagic virus, and commercial hunting.
Dolphin threatened
The Yangtze river dolphin, also known as the baiji, is also critically endangered and possibly extinct, with perhaps one or two individual creatures remaining, the report says.
Changes in river flows due to dams, pollution, over-fishing and the use of electric shocks to fish in the Yangtze system are all factors in the dolphin's disappearance.
The species is also endangered by an increase of the amount of commercial marine traffic in China, which is the world's fastest growing economy.
"Any poor dolphin would really have to do amazing acrobatics to avoid being hit by one of those ships," Hilton-Tailor said.