The temperature increase when the CO2 content of the atmosphere doubles is called climate sensitivity. This doubling compared to 150 years ago (280 ppm) is likely to come because China, India, and other countries continue to expand their use of fossil fuels, and cooling over the next few decades will not increase ocean CO2 uptake at the same rate.
A doubling of the CO2 content leads to a temperature increase of 1 °C to 6 °C with 85 % probability. Was this really "calculated" or rolled the dice on a few bottles of wine? This automatically comes to mind with the numbers from 1 to 6 and especially when you read the small print footnote about it: "No best estimate for equilibrium climate sensitivity can now be be given because of a lack of agreement on values across assessed lines of evidence and studies."
Considering the billions of research money that have been spent, we certainly expected something more precise. In any case, it is not a consistent theory, if after 20 years of intensive calculations the uncertainty is bigger than before. Maybe some professors just let students turn the screws for 20 years and then present "new findings" at world climate conferences in Buenos Aires, Marrakesh, Montreal, Bali, Cancun, Paris etc.? More than 20,000 people come to these climate conferences, most of them by plane.