John Gribbin, a professional science writer, provides a very accessable complete coverage of the issues here: (Emphasis added)
Living with the greenhouse effect
"A great deal of what you read and hear about the so-called "greenhouse effect" is either exaggerated, or misrepresented, or both. But the basis for concern about uncomfortably rapid global warming occurring within our own lifetimes and those of our children rests on just three facts, and a reasonable inference.
The first fact is that there is an atmospheric greenhouse effect, which keeps the Earth warmer than it would otherwise be. The simplest way to get an idea of how important this is is to compare temperatures at the surface of the Earth with those on the airless Moon. There is no significant difference between the distance of the Moon from the Sun and the distance of the Earth from the Sun, so both receive the same amount of heat on each square metre of the surface that faces the Sun, and, other things being equal (which they are not) should reach the same equilbrium temperature. The average temperature at the surface of the Moon (averaging over the whole surface, including day and night sides) is actually -18 degC, while the average temperature on the surface of the Earth is 15 degC. The blanket of our atmosphere keeps the surface of the planet a full 33 degC warmer than it would otherwise be, and crucially (as far as life forms like us are concerned) raises the temperature above the freezing point of water. "
LG - Note that this analysis eliminates any possible argument from anti's that solar variation is the only factor.
"There is no mystery about how it does this. Some gases in the air (chiefly carbon dioxide and water vapour) absorb infrared radiation (the same kind of heat radiation you can feel if you hold your hand near a warm radiator). Sunlight passes through the atmosphere essentially unaffected, and warms the surface of the Earth. The warm surface radiates infrared, not light (because it is cooler than the surface of the Sun), and some of this outgoing infrared radiation is absorbed in the atmosphere and re- radiated in all directions. Some of the re-radiated infrared radiation goes back down and increases the temperature at the surface. This is the atmospheric greenhouse effect -- and nothing to do, incidentally, with the way a greenhouse keeps warm, which is by letting sunlight in and stopping convection, trapping hot air that tries to rise under a roof of glass.
There are complications. Any increase in surface temperature increases the amount of evaporation from the oceans, which puts more water vapour in the air and increases the greenhouse effect, in a feedback process. As a result, adding a relatively <b>small amount of carbon dioxide can produce a disproportionate warming, once the feedback is allowed for. "
At this point, Gribbin goes into a discussion of diminishing returns of CO2 levels in atmosphere, which is in error. Replace it with this more scientific analysis of the issue.
Basic Radiation Calculations - Spencer Weart
In part, Weart's paper includes the following, which clearly scientifically demolishes the anti's common argument that "adding more CO2 cannot trap more heat above a certain limit."
[B]Neither Kaplan nor anyone else of the time was thinking clearly enough about the greenhouse effect to point out that it will operate regardless of the details of the absorption. The trick, again, was to follow how the radiation passed up layer by layer. Consider a layer of the atmosphere so high and thin that heat radiation from lower down would slip through. Add more gas, and the layer would absorb some of the rays. Therefore the place from which heat energy finally left the Earth would shift to a higher layer. That would be a colder layer, unable to radiate heat so efficiently. The imbalance would cause all the lower levels to get warmer, until the high levels became hot enough to radiate as much energy back out as the planet received. (For additional explanation of the "greenhouse effect," follow the link at right to the essay on Simple Models.) Adding carbon dioxide will make for a stronger greenhouse effect regardless of saturation in the lower atmosphere.
(And actually, there is no saturation. With the primitive infrared measuring techniques of his day, Ångström’s assistant had gotten a midleading result. He could only measure the heat transferred across . Measurements from the 1940s on have shown that there is not nearly enough CO2 in the atmosphere to block most of the infrared radiation in the bands of the spectrum where the gas absorbs it. That’s even the case for water vapor in deserts where the air is extremely dry.)
Back to Gribbin
" All of these complications (and others) are taken account of in computer models of the greenhouse effect. These models accurately describe the differences in temperature between the airless Moon and the Earth, and the pattern of temperatures seen on Mars."
"The second fact is that the amount of greenhouse gases in the atmosphere has increased over the past hundred years or so. Since the early 19th century, the amount of carbon dioxide in the air has increased from below 280 parts per million to above 350 parts per million. The absolute quantities are small, which only goes to show just how powerful the greenhouse effect is -- that 280 ppm plays a considerable part in keeping us 33 degrees warmer than the Moon. But the proportional increase is more than 25 per cent, clearly a dramatic change in any natural system. There is no doubt that this increase comes mainly from burning fossil fuel -- coal, oil and gas. First, not only the increase itself but the rate of increase matches the rate at which such fuel has been burnt, including "blips" caused by two World Wars and the Arab oil crisis. Secondly, analysis of air bubbles trapped in the icecaps of Greenland and Antarctica show that the natural concentration of carbon dioxide in the air has been below 280 ppm for hundreds of thousands of years. Other greenhouse gases, including methane (which occurs naturally and is a byproduct of agricultural activities) and CFCs (which are entirely synthetic, and are also implicated in the destruction of the ozone layer) are also building up in the air."
"The third fact is that the Earth has got warmer over the past century, by about half a degree, Celsius. The evidence comes from meteorological stations scattered about the surface of the planet, and chiefly located on land masses. But the accuracy of the measurements was dramatically confirmed by satellite observations in the 1980s and 1990s."
Note that the third fact is much more strongly confirmed since Gribbin wrote his article in the 1990's
The reasonable inference is that the global warming (which is real) is related to the buildup of greenhouse gases in the atmosphere (which is real), but that the computer models may slightly overestimate the strength of the additional greenhouse effect.
No responses without authoritative science-based references please.