Probably very slight after the first couple of decades during which surface testing was common practice. For many years there were rumours among the laity that the tests caused radical climate change, and people blamed every wet summer and dry winter on nuclear tests. Then the media got bored with the theme, and the bombs stopped changing the weather.

There is a suggestion that a really large exchange of nuclear bombs in the event that WWIII were to break out, could cause such dramatic climate cooling that we might go into a new ice age. I am not sure how compelling the reasoning is and I am not urgently in favour of trying it out in any case, but it is not an unreasonable idea. I cannot help thinking though, that survivors would have enough other concerns to keep their minds off the weather, at first anyway.

While bombs still used to be tested in the air, their main effect on climate presumably would have been the injection of aerosols into the stratosphere, and possibly some destruction of the ozone layer. The net effect should have been a cooling of the climate, though I am not aware that anything measurable ever was detected. In contrast, we find that contrails of commercial jets have clear and significant effects on the climate round the busier major airports in some countries.

Go well,

Jon