Here's Wikipedia's simplified but canonical description of the greenhouse effect:
The greenhouse effect is a process by which thermal radiation from a planetary surface is absorbed by atmospheric greenhouse gases, and is re-radiated in all directions. Since part of this re-radiation is back towards the surface and the lower atmosphere, it results in an elevation of the average surface temperature above what it would be in the absence of the gases.
Solar radiation at the frequencies of visible light largely passes through the atmosphere to warm the planetary surface, which then emits this energy at the lower frequencies of infrared thermal radiation. Infrared radiation is absorbed by greenhouse gases, which in turn re-radiate much of the energy to the surface and lower atmosphere.
This greenhouse effect forms the basis for warmist doctrine: that human CO2 emissions are causing a catastrophic increase in the global average temperature.
Although people like Gaia theorist James Lovelock have predicted that global warming will kill billions of humans the belief that warming would be catastrophic for life on earth is largely unresearched and probably indefensible. There have been many extended warm periods in the earth's history and the fossil records we have for them suggest that each produced more life, and a greater diversity of life, than the cooler periods preceeding them. We have reasonable information, furthermore, on the Roman and Medieval warm periods and not only did each of these kick off significant civilizational development, but the polar bear made it through them embarrassingly undead and not a single estuarial or river basin culture, whether in Asia, Eygpt, or Europe, is known to have drowned.
The Wikipedia article quoted above gives the two main facts warmism depends on: gas molecules in the atmosphere tend to re-emit energy obtained from the sun; and, measured net solar radiation does not fully account for near surface air temperatures - but doesn't directly raise the problem that the measured effect is roughly an order of magnitude too large to be accounted for by the known interactions between thermal radiation and atmospheric greenhouse gases other than water vapor.
This problem has produced a widespread search for a forcing multiplier - something which reacts to a small increase in atmospheric CO2 to force a big increase in atmospheric warming. So far, however, none of the candidates for this have withstood even friendly critical review - meaning that those who argue for CO2 as a primary source of a significant atmospheric greenhouse effect are committing themselves to the biggest magical hand wave since crystalline epicycles brought concensus to the Ptolemaic Universe.
The processes modelled in IPCC and related warmist calculations as this unknown climate sensitivity factor may exist in the real world - but every experimental effort to demonstrate that minor increases in atmospheric CO2 lead to major changes in surface temperature has failed to show a repeatable effect in anything near the right range:
- The biggest "experiment" on this is, of course, reality: CO2 concentrations appear to have gone up by more than 20% since 1958, but we have neither a clear definition of the average global surface temperature nor data to support the belief that any of the proxies we have for it have shown significant change over the period.
- The smallest and most often repeated experimental demonstration, the Al Gore tabletop special in which one jar contains a bit more CO2 than the other, produces essentially the same result if the same weight of an inert gas like argon is used in place of the CO2.
- It is easy to demonstrate the greenhouse effect by pointing a thermal imaging camera at the sky, but within the limits of google and bing searches it appears that no one mapping variations in the effect to variations in local concentrations of greenhouse gases other than water vapor and urban smog has demonstrated effects beyond the levels (roughly one tenth of the IPCC climate sensitivity assumptions) predicted by the theoretical calculation with no multiplier.
What we know of the earth's climate history does not support the warmist indictment against anthropogenic CO2 emissions either: none of the warming periods recorded in human history can have been triggered by human CO2 production - and the longer term geological record seems directly contradictory too. Essentially all of the earth's surface has been tropical at one time and glaciated at another, but nearly all of the information we have about the atmosphere during those periods suggests that CO2 concentrations rose during, but not before, tropical periods and fell significantly during, but not before, glaciations.
What we can say therefore about the belief that atmospheric CO2 increases are causing significant global warming is that it has no theoretical support, no experimental support, cannot be seen operating in the real world, and is contradicted by what we know of global climate history.
Where neither science nor history can explain warmism, politics can. Basically, if you're someone like Al Gore whose political career is based on railing against American republicans, then an obvious reason for singling out CO2 as a threat to humanity's future is that this is an easy sell: there is a grain of truth in the science, people can see smokestacks, the SUV is a widely approved target for angry rhetoric, and the political audience is generally eager to accept the burdens a demonstrated need to control national energy use would put on them.
Similarly, what we can say about the idea that global warming would necessarily prove net negative for life on earth is simply that this idea has not been extensively studied but seems to have neither theoretical nor experimental support and is contradicted by what we know about the history of life on earth, but meets the political need for players like Gore et al because people cannot be held hostage to the threat of a good thing happening if they don't knuckle under.
It is important in forming personal beliefs about the relative roles of science and politics in warmism that we separate belief from reality: most of the alarmists seem to be true believers, most of the deniers merely Missouri skeptics, but there seems to be no objective evidence to suggest that either side genuinely knows whether the global climate is changing or not. Thus we can probably agree that the publicity now generally given a hot day in Death Valley and denied an extended cold emergency in Peru reflects an editorial agenda more than it does climate, but the combination of theory, data, and definitions we have is not sufficient to let us know whether either fell outside longer term climate norms.
The deeper issue here is not that the political action now strangling western economies is politically motivated, but that accepting the arguments for seeing warmism as sheer political fraud means accepting that the talking heads citing science to sell it to the masses are either deluded or dishonest - but because no wolf today doesn't mean no wolf tomorrow, it also means that warmist politicization of the research process has to be seen as having destroyed the credibility of all involved, and thus as having greatly weakened the world's ability to recognize and respond to a real threat should one now materialize.