Different components in the model can be tested separately. How stratospheric gases disperse can be tested. How black soot rises in the atmosphere, in a variety of heat conditions, can be tested. How black soot affects absorption of the solar radiation can be simulated in laboratory, and tested in indirect ways (as Nornagest mentioned, by comparing with volcanic eruptions).
Yes, and that's why you can even attempt to build a computer model. But you seem to be assuming that a climate model can actually simulate all those processes on a relatively fundamental level, and that isn't the case.
When you set out to build a model of a large, non-linear system you're confronted with a list of tens of thousands of known processes that might be important. Adding them all to your model would take millions of man-hours, and make it so big no computer could possibly run it. But you can't just take the most important-looking processes and ig...
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.