First, a little theory. The molecular structure of CO2 makes it transparent to high frequency radiation (which derives from a HOT source, such as the sun) and opaque to LOW frequency radiation (which derives from a COOL source such as the warm Earth's surface). High frequency radiation from the sun (heat and light) easily penetrates the Earth's atmosphere since its source is at 6,000C at which temperature CO2 is transparent to radiation. It strikes the surface of the Earth where it warms it to about 295K (20-25C). Low frequency radiation from the cool Earth cannot penetrate CO2 which is, as I said, opaque to low frequency radiation. So the Earth's atmosphere gets warmer since the sun's high frequency energy is converted into low frequency energy (ie heat) which is trapped by CO2. The more CO2 the warmer it gets.
Physical experimental evidence? Easy.
Place a variable temp heat source on one side of a large glass flask filled with CO2. Put a thermometer inside the flask and another thermometer outside the flask on the side away from the heat source. Start heating the heat source. As the heater heats up the temperature registered by the thermometer inside the flask steadily increases and the temperature registered by the thermometer outside the flask, on the side away from the heater registers no temperature change. This is because CO2 is opaque to radiation from a relatively cool source of heat (up to about 5-600C) and so the heat energy is trapped in the flask, heating it up, and doesn't pass thru to the thermometer on the far side of the flask. As the heater increases in temp (to 1,000C or so), meaning the frequency of the radiation from the heater gets higher, the thermometer inside the flask ceases to increase in temp and the thermometer on the outside of the flask starts to register an increasing temp. This is because CO2 is transparent to high frequency (ie high temp) radiation and so the radiation simply passes thru the CO2 in the flask and impinges on the thermometer on the outside of the flask causing it to register a steadily increasing temp while having no effect on the thermometer inside the flask.
Thus, CO2 allows high temp radiation from the sun to reach the Earth's surface and traps the low temp radiation from the relatively cool Earth's surface.
Any high school science lab can demonstrate this for you.