Here's another post I dug up on Goddard's site, claiming a February 20, 1969 NY Times article said "Arctic Soon to Be Ice Free". That's a twist of the actual headline that said a single "Expert Says Arctic Ocean Will Soon Be an Open Ocean". Watts has the article.
Until recently there was a suspicion that the warming trend of the century before 1940 was a byproduct of the industrial revolution. Carbon dioxide, produced by combustion, makes the atmosphere less transparent to infra-red radiation, thus trapping heat like the roof of a greenhouse.
There is evidence that the carbon dioxide content of the world's air has risen from 10 to 15 per cent during the last century. However, the cooling trend of recent years indicates that other factors are at work, including perhaps the volume of dust and smog in the air. This tends to reduce the solar heat reaching the surface.