The idea is simple: the sun radiates heat. That heat warms the earth. Some of that warmth is radiated back into space. Part of that radiated warmth is absorbed by CO2 and other greenhouse gases and radiated right back to the earth.
All pretty straightforward.
Over 150 years ago (in 1858) a scientist named John Tyndall proved it through experimentation, with an ingenious mechanism (that I think looks a bit like a Rube Goldberg device):
Since then, there have been thousands more observations and experiments, all confirming and refining the initial discovery.
For a pretty amazing demonstration of this, watch this video:
The point of all this is that the greenhouse effect was discovered 186 years ago, and has existed pretty much unchanged since then.
The idea behind global warming is older than certain other ideas we have:
Germs Make You Sick - We knew CO2 warmed up the planet before we knew that you should wash your hands after you take a crap. When we discovered the greenhouse effect, doctors were still operating on patients with dirt-encrusted fingernails, and thought you got sick because you had too much of the wrong kind of blood.
Slavery Is Wrong - Back when we first discovered the greenhouse effect, and indeed even when it was first proven in a lab, slavery was still legal in the United States, and the Civil War had yet to be fought.
Mormons - Joseph Smith had not yet published the Book of Mormon when Fourier published his ideas about greenhouse gasses.
Republicans - That's right. I find it ironic that global warming due to greenhouse gases was discovered 30 years before the Republican party was formed. You'd think if anything the greenhouse effect wouldn't believe in the Republican party, not vice-versa.
So please, as we enter a new round of denying reality, let us remember that global warming due to the greenhouse effect has been a known fact of life for 186 years.
186 YEARS, for dog's sake! It's not going anywhere.