Fears of a sixth mass extinction have been stoked by reports that we have reached a dreaded milestone: the loss of the first mammal to climate change. Last month, the Bramble Cay melomy, a rodent native to an Australian island, was proclaimed extinct. Decades of seawater inundation on Bramble Cay, where sea level has been rising at twice the average global rate, cost the melomies 97 percent of their habitat. The rodent was small, but its loss, a harbinger of what could come, carries big implications. It’s too late for the melomy, but the rest of us on this fragile planet urgently need a new global green deal.