As the climate has
warmed since the 19th
century—and espe-
cially in the past few
decades—the Alps have
lost two-thirds of the
ice from their roughly
4,000 glaciers. On the
Pers Glacier in eastern
Switzerland, geophys-
icist Christine Seupel,
engineer Dieter Müller,
and glaciologist Andri
Moll measure the ice
thickness with ground-
penetrating radar.60 NATIONAL GEOGRAPHIC