The point non-programmers tend to miss here is that lack of testing doesn't just mean the model is a a little off. It means the model has no connection at all to reality, and either outputs garbage or echoes whatever result the programmer told it to give. Any programmer who claims such a model means something is committing fraud, plain and simple.
This really is a pretty un-bayesian way of thinking - the idea that we should totally ignore incomplete evidence. And by extension that we should chose to believe an alternative hypothesis (''no nuclear winter') with even less evidence merely because it is assumed for unstated reasons to be the 'default belief'.
An uncalibrated sim will typically give crazy results like 'increasing atmospheric CO2 by 1% raises surface temperatures by 300 degrees' or 'one large forest fire will trigger a permanent ice age'. If you see an uncalibrated sim giving results that seem even vaguely plausible, this means the programmer has tinkered with its internal mechanisms to make it give those results. Doing that is basically equivalent to just typing up the desired output by hand - it provides evidence about the beliefs of the programmer, but nothing else.
The FHI's mini advent calendar: counting down through the big five existential risks. The first one is an old favourite, forgotten but not gone: nuclear war.
Nuclear War
Current understanding: medium-high
Most worrying aspect: the missiles and bombs are already out there
It was a great fear during the fifties and sixties; but the weapons that could destroy our species lie dormant, not destroyed.
But nuclear weapons still remain the easiest method for our species to destroy itself. Recent modelling have confirmed the old idea of nuclear winter: soot rising from burning human cities destroyed by nuclear weapons could envelop the world in a dark cloud, disrupting agriculture and the food supplies, and causing mass starvation and death far beyond the areas directly hit. And a creeping proliferation has spread these weapons to smaller states in unstable areas of the world, increasing the probability that nuclear weapons could get used, leading to potential escalation. The risks are not new, and several times (the Cuban missile crisis, the Petrov incident) our species has been saved from annihilation by the slimmest of margins. And yet the risk seems to have slipped off the radar for many governments: emergency food and fuel reserves are diminishing, and we have few “refuges” designed to ensure that the human species could endure a major nuclear conflict.