Which scientists?
Your god: Carl Sagan.
He's not the only one.
https://www.nationalgeographic.com › culture › article
Oooops....
What now? Are you going to scream,
"Kock Brothers!....Right-wing blog!.....Big Oil....Blue Meanies!"
'Cause, that;s your other god.....
National Geographic.
Atmospheric tests are different than surface and sub-surface bursts, because atmospheric tests eject un-fissioned Pu239/240, and un-fissioned U235/238 (when used as a tamper, a reflector or both), daughter products (93% of Pu239 fissions create Zirconium and Xenon), the Aluminum weapons casing and the electronics package into the stratosphere.
The effect is the same as a catastrophic volcanic eruption like Mt St Helens, Mt Pinatubo or Krakatoa which ejected particulate matter into the stratosphere.
The particulate matter reflects UV-, UV-B and UV-C radiation back into Space, which results in cooling. That is, in fact, the theory behind "nuclear winter" notwithstanding Dr Carl Sagan's misunderstandings and outright lies.
That there was a slight temperature decrease is well-documented.
NASA is comparing temperatures today against a period of time when there was a known artificial decrease in temperatures and then claiming temperatures are rising when in fact they are not.