UNITED NATIONS — The last time Amina J. Mohammed went home to see her extended family in northern Nigeria, her cousins asked her to skip her usual gifts of clothes and sweets and bring them something that they really needed: body scanners for the gates of the local mosque, to guard against suicide bombers sent by Boko Haram.

Their request summed up for her what has happened to the place she calls home, and how years of dysfunction and destitution had turned the region into a battleground. “Terrorists are not born,” is how she put it the other day. “What was it that birthed Boko Haram?”

The answer, in her view, can be found in the problems she has spent the last three and a half years trying to get world leaders to agree to address: corrupt government leaders, crumbling schools, and the effects of climate change. In northern Nigeria and the surrounding Sahel region, that has meant the aggressive advance of the desert, swallowing what were once farms, and the shrinking of Lake Chad, which once seemed so vast that when she crossed it in a hovercraft, she said it was “like I was flying to England.”