Surging demand for rhino horn to decorate daggers and treat everything from hangovers to cancer is driving the iconic animals to the brink of extinction. The only way to save them is to humanely harvest rhino horn and sell it legally, scientists argue in a controversial new paper.

Only 5,000 black rhinos and 20,000 white rhinos remain, mostly in South Africa and Namibia, the scientists note. The western black rhino was declared extinct in 2011.

The paper, published Thursday in the journal Science, is a bid to spark “serious discussions around establishing a legal trade” at an international conference on the trade in endangered species that starts Sunday in Bangkok, lead author Duan Biggs told NBC News.