BTW - No one brought up the spectre of Trump, you invoked it. People are, rightly, criticizing your gut feeling that flies in the face of our best understanding of multiple hard scientific fields and the tons of works and dedication by tens of thousands of researchers around the globe.
CO2 levels have been this high, and much higher, in the distant past. But the climate has also varied enormously in the distant past as well. Roughly 55 million years ago, the planet had nearly no ice. A bit over two billion years ago, it's believed that the planet was almost entirely covered in ice.
So no, it's not really a reasonable issue. CO2 levels have varied a great deal, as has the climate. Nothing in the Earth's climate past suggests that the current dramatic rise in CO2 levels wouldn't also result in dramatic changes to the climate.
This is, of course, a very simplified explanation. As one goes back hundreds of millions to billions of years, other large scale changes like plate tectonics, atmospheric composition/mass, sun brightness, continental weathering, etc. will cause the average surface temperature to vary considerably.