I just looked through everyone's picks. There are some "deviations" from the main theme, so that leaves some my choices safe from criticism!
VioletLightning...great picks! "On The Beach" was one of the first ones I thought of! Anyway, here goes....
ESCAPE FROM NEW YORK
ON THE BEACH (1959)
MAD MAX
ROAD WARRIOR
SOYLENT GREEN
OMEGA MAN
PLANET OF THE APES (1968)
THE MATRIX
WATERWORLD
TERMINATOR 1,2,3 (have not seen the new one yet)
SILENT RUNNING
RADIOACTIVE DREAMS
There's probably more, but after more than 50 years of watching movies, the filing cabinet in my head is jammed!