Earth`s biodiversity continues to dwindle due to deforestation, climate change, over-exploitation and chemical runoff into rivers and oceans, said the study, published in Nature.
"The evidence is in -- humans really are causing extreme extinction rates," said co-author Stephen Hubbell, a professor of ecology and evolutionary biology at the University of California at Los Angeles.
But key measures of species loss in the 2005 UN Millennium Ecosystem Assessment and the 2007 Intergovernmental Panel on Climate Change (IPCC) report are based on "fundamentally flawed" methods that exaggerate the threat of extinction, the researchers said.
The International Union for the Conservation of Nature (IUCN) "Red List" of endangered species -- likewise a benchmark for policy makers -- is now also subject to review, they said.
"Based on a mathematical proof and empirical data, we show that previous estimates should be divided roughly by 2.5," Hubbell told journalists by phone.
"This is welcome news in that we have bought a little time for saving species. But it is unwelcome news because we have to redo a whole lot of research that was done incorrectly."
Up to now, scientists have asserted that species are currently dying out at 100 to 1,000 times the so-called "background rate," the average pace of extinctions over the history of life on Earth.
UN reports have predicted these rates will accelerate tenfold in the coming centuries.
The new study challenges these estimates. "The method has got to be revised. It is not right," said Hubbell.