Great points. To add a little more detail, predicting the results of greenhouse gasses in the atmosephere is science. Predicting the amount of gasses in the atmosephere requires predicting human economic activity in detail (how much, in what form, etc.), which is impossible, especially on longer timescales (imagine how many investors would love to know how to do that!)
The predictions I've seen, at least in the IPCC reports,[1] show not lines but confidence intervals that widen over time.
The predictions I've seen, at least in the IPCC reports,[1] show not lines but confidence intervals that widen over time.
[1] https://news.ycombinator.com/item?id=9772353