The balance
The Earth is warmed by one thing, sunlight, and cooled by one thing, its own glow. Everything warm gives off invisible infrared light, and the ground and oceans shine it back toward space day and night. When the energy going out matches the energy coming in, the temperature holds steady. Tip the balance either way and the planet warms or cools until the two match again.
The trap
Most of the air, nitrogen and oxygen, is transparent to both sunlight and infrared and plays no part. But a few gases, water vapour, carbon dioxide, methane, are transparent to sunlight and not to infrared. They let the Sun’s energy down to the ground, then catch a share of the infrared heading back up and re-radiate it in every direction, including down. Some heat that would have left is returned. The ground has to warm until it glows hard enough to push the same total out through the partly closed door.
That is the greenhouse effect, and without it the Earth would be a frozen ball about thirty degrees colder than it is. Set the gas to None in the scene and watch the ground go cold.
Try it: switch to Pre-industrial, then Today, then Doubled, and watch how many red packets turn back.
A thin trace that matters
Carbon dioxide is a trace gas: about 420 parts in every million, less than a twentieth of a per cent of the air. It matters out of all proportion to its amount because it does a job nothing else in the air is doing. Adding a little more is like adding another blanket to a bed: the blanket is thin, but there was nothing else holding the heat in.
Two centuries of burning coal, oil and gas have raised the amount from about 280 parts per million to over 420, faster than at any time in the geological record we can read. The extra is enough to hold back a small fraction more of the outgoing heat, and that small fraction has warmed the planet by more than a degree.
Feedbacks
Warming sets off changes that warm things further. Warmer air holds more water vapour, itself a greenhouse gas. Melting ice exposes dark sea and soil that soak up sunlight instead of reflecting it. Thawing ground releases methane. Clouds cut both ways, shading by day and blanketing by night, and are the largest uncertainty in how far the warming goes.
What the model leaves out
The scene shows the trap and nothing else. It has no oceans, which store heat for centuries; no clouds; no ice; no seasons; and no time lag, though the real system takes decades to settle. What it shows correctly is the core: sunlight in, infrared out, and a thin layer of gas deciding how much of the infrared makes it.