Nuclear winter
From Tardis Wiki, the free Doctor Who reference
Nuclear winters were longterm lowerings of a planet's temperature caused by the debris from nuclear explosions blocking the planet's access to sunlight. (PROSE: Cold Fusion [+]Loading...["Cold Fusion (novel)"])
Circa the 511th century, Earth's frozen period was caused by "a couple of nuclear winters", as well as a large comet. (PROSE: The Coming of the Terraphiles [+]Loading...["The Coming of the Terraphiles (novel)"])