The announcement of the winners of this year's Goldman Environmental Prize is an opportunity to celebrate activist leaders. But it is also a moment to recognize just how much courage their efforts (and those of a great many others) can demand.

When my dear friend Berta Caceres and I won the prize in 2015, Berta said in her acceptance speech, "I have given my life for the service of Mother Earth." Not long after, Berta was assassinated in Honduras. Her story is tragic, but not unique. Indeed, just months later, Isidro Baldenegro Lopez, another Goldman Environmental Prize recipient, was shot dead.

There has never been a more dangerous time to be an environmental activist. Consider the violence unleashed against the environmental defenders protesting the Dakota Access Pipeline in the United States. Police were accused of using excessive force to try to disperse members of the Standing Rock Sioux Tribe and their supporters, who argued that the project would contaminate water and damage sacred burial sites.