If you double the amount of CO2 you don't double the warming.

Correct. If warming doubled, we'd all be dead. Global average temperature is 15°C (288 Kelvin), and doubling it would be 303°C (576 Kelvin), far above boiling. We'd all be dead and the oceans would evaporate. The planet would be like Venus.

But nobody says that's what's going to happen from doubling CO2.

We're looking at a much more modest temperature increase of a few degrees, probably about 4°C if we do nothing to control it. That's about the same difference in global temperatures between the last glaciation period, when New York was buried under a mile-high mountain of ice, and now. That's enough to radically shift climate zones and cause geo-political chaos but not enough to boil the oceans.

We have literally no grasp on what causes global climate.

You have literally no grasp of what causes global climate.

The rest of us do. At the simplest level, it's just basic physics.

The sun provides energy. Some of that energy is reflected by clouds, or from the land and ocean. Some is absorbed by the atmosphere, and some by the ocean and land surfaces. The energy absorbed heats the planet, causing it to radiate more heat back out into space, until the planet is at thermal equilibrium -- energy coming in and energy going out are in balance. (Technically, the planet may never quite reach perfect equilibrium due to small fluctuations in solar output, but close enough.)

The major drivers of climate include

• the sun's output and the distance of the earth from the sun;
• the albedo of the surface (how much light is reflected back into space);
• the number and type of clouds, and how high they are in the atmosphere;
• and the chemical composition of the atmosphere.

Everything else is fine details. The fine details control whether (say) Idaho will be desert or rain forest, but not the over all global temperature.