in my line of work what you speak of refers to projection not assumption...educated and calculated forecast based on history with formulated adjustments to account to the demographics and needs of a population...in layman's term supply and demand.
the problem with your theory is you base everything on a linear model where patterns allow for natural occurrences, but you fail to incorporate occurrences created by humans...variances that get thrown into the mix and have created havoc in our overall climate scheme... chernobyl, hiroshima, nagasaki, 9/ll, gulf war and the burning of 600 kuwait oil wells, exxon valdez spill, fukushima,...etc, etc... and we're not even talking the many world wide incinerators and fossil fuel burning factories and vehicle emissions that create gases that get into the atmosphere...forget the conspiracy theory and just open your eyes.
Yes, I make a projection of what will occur. Built into that project are layers and layers and layers of assumptions.
What they do to project the climate forward is very similar. And there is nothing linear about my models. There are extreme shock scenarios that are built in. Are there extreme shock scenarios built into their models? I don't know, I would assume so. If you are going to build a proper model, you build in a certain percent chance of an event occurring and let stochastic scenario generators generate the random numbers that then fit into the probability distribution of what occurs.
I remember many stories and seen many pictures from back in the day where acid rain killed entire forests. That might still happen in China but it no longer happens here. Our vehicle emissions are mostly CO2 and water. We have put scrubbers on power plants so that they are very clean. Yes, accidents happen. Take the deep water gulf leak. How horrible did they predict that was going to be? Thousands of miles of beaches were going to be covered in oil. What happened? Pretty much nothing because deep sea oil leaks happen all the time without us doing anything and the Earth is built to handle those. Exxon Valdez, bunch of animals died but the area is recovering much better than all of the dire warnings predicted.
I'm not saying we shouldn't try to take care of our planet and yes, disasters happen and can hurt the environment. Those you listed have very little effect on the climate. Yes, atomic explosions can throw particles into the air that shield some sunlight and cool the Earth. A single volcano eruption can do much worse. I think you are confusing ruining the environment and climate change. They are not the same. I'm not saying we should all go dump all our trash into our rivers because it won't have an effect. I'm not going to stop driving my truck because I think it is going to cause Florida to be under water.