Malcolm and others have a refrain of "show me the evidence", so I thought it was worth setting this out as clearly as possible. If you want more detail, the Garnaut Report [1] chapters 3-5 are a good summary.
What is undisputed
1. The so-called greenhouse gases (GHGs)* do what they say on the tin. Specifically, they absorb certain wavelengths of infrared radiation. The sun pours in 342 watts per square metre of energy into the planet every day, and some of it is not reradiated back into space because of greenhouse gases. If they didn't do that at all, it would be too cold for human or much other life. There is no qualified scientist on the planet that disputes this basic science.
- * for ease of reference, added from a later comment: the GHGs specified in Kyoto , and thus the ones controlled by the National Greenhouse and Energy Reporting System and the Carbon Pollution Reduction Scheme, are: carbon dioxide, methane, nitrous oxide, sulphur hexafluoride, hydrofluorocarbons and perfluorocarbons. There is an argument in Garnaut (p59) about also including chlorofluorocarbons and hydrochlorofluorocarbons, but these are covered by the Montreal Protocol, and therefore should be phased out by 2030 anyway. Water vapour and ozone also have greenhouse effects, but aren't primarily human in origin, and water vapour at least is complicated, since much of it forms clouds, which reflect energy from the sun as well as retain heat below them.
2. There is more greenhouse gas in the atmosphere than at any time over the last at least tens of thousands of years, and the amount has been climbing steadily year on year at an accelerating rate for at least the last fifty years. These are direct observations, for recent years by direct measurement, and for the history from concentrations of CO2 in deep ice cores.
3. The great majority of the growth in atmospheric CO2 is anthropogenic – ie we did it, by burning fossil fuels and clearing land and spreading fertiliser and raising cows (cow and sheep farts contribute 11% of all Australia's GHG emissions, and there wouldn't be any of them here at all if humans weren't raising them). The more powerful greenhouse gases don't appear in nature at all, so we can't blame anyone or anything else for them (unless you have evidence that aliens have been secretly importing them from off-planet).
What is only disputed by the willfully blind or deranged
1. The planet has been getting hotter in the last 50 years, and for the last 30 years has been hotter than ever recorded. There are no remaining unexplained anomalies in the observational series whatsoever: regional and other differences eg in the high atmosphere are all accounted for. I have heard people claim that the world has cooled / stopped warming since 1990: since 12 of the last 13 years are the highest on record, they have to resort to three-card tricks or falsified data to attempt to justify this ludicrous claim. It is true that some regions around the north Atlantic were as warm in 1250 as they were in 1975, but a) the sort of fluctuations in the Gulf Stream that caused that are included in the mainstream models, b) this wasn't true of most of the rest of the world, and c) anyway, all but four of the years since 1980 have been warmer than that. There is, of course, no direct observational record of either the medieval anomalies or the regionality. Both the claims and the refutation rely on anecdotal and other contemporary observations without instruments, and on indirect observations such as tree-rings. Insofar as anecdotes and tree rings count for anything, they support the submission that the world wasn't generally warmer / colder at those times as much as or more than they support the claim that it was. We can't claim that European monk's accounts are more reliable than Indian or Chinese ones, only note that they record different trends. NB: if you were relying on The Great Global Warming Swindle for a different view on how warm it was in 1250, note that the UK broadcasting regulator has castigated the program's makers for altering their graphs to mislead viewers [2] (eg, on that one, by re-labelling 1975 as "Now").
2. Getting hotter will be a Bad Thing. Thousands upon thousands of detailed studies have been done over the last twenty years on the impacts of warming. They have indeed identified a few impacts that are positive, but these are enormously outweighed by the negative impacts on everything from crop yields to disease ranges. Even the crop yield gains from CO2 fertilisation in some crops disappear at warming of more than two degrees, and are overwhelmed by the rainfall changes above that.
What can be debated
1. How much warming is due to GHGs? The mainstream science says that doubling GHG concentrations raises the temperature by 2 to 6°C. The sceptics say 1) something else is causing the warming, eg solar cycles, volcanism, or whatever, and therefore 2) since the GHGs definitely do cause warming, they're contributing less than the mainstream models show, and so we don't have to worry yet about further increases in GHGs, because they won't warm us as much as is feared.
2. er – that's it. Since GHGs do cause warming (basic physics) and are increasing (undisputed observations), any other touted cause boils down to that argument.
Note also that alongside the sceptics there are at least as many scientists (eg James Hansen) who think that the models underestimate the GHG effects, and therefore we need more drastic action than the mainstream models suggest.
The problem with all the other alternative explanations for warming is that they either don't change at the right times in the right direction to explain the temperature record, or that there is no observable mechanism for them to put enough energy in. For example, the total energy expended/burnt by all human activities to date is simply insufficient to have had any direct impact on warming: we may be able to keep our cities a degree or so warmer at ground level on a cold day, but in the great energy budget scheme of things it just doesn’t count.
Solar variations obviously do have impact –all of this energy came from the sun originally – but they vary at different times and different directions than the temperature record does. Huge amounts of effort have been put into understanding this, and the chances that there is some missing high-energy source we don't know about and can't detect are very small indeed. Finding one that stands up to scrutiny would be worth huge amounts in support from people like Exxon, so there is plenty of incentive for scientists to publish if they found one. And equal bunce for peer reviewers who got that paper through to publication. And the count of peer-reviewed papers in scientific journals supporting the sceptics' position: none, nada, zero.
What if it's a conspiracy?
What if all these thousands of scientists working in all the countries of the world have a secret conspiracy to defraud us all? Well, if just one of them decamped from the conspiracy with evidence of it, they'd be living in luxury on the rewards from those interested in keeping the gases flowing. What are the chances that a) it exists, and b) none of them have gone public? I leave that exercise to the reader.
What if warming is mostly caused by arbitrarily advanced alien space bats?
If there is some real as-yet-unidentified cause of warming that we don't recognise, or if thousands of scientists have uniformly and systematically got their model coefficients wrong, and some combination of other factors is causing warming, then what should we do that's different to what is currently planned?
Well, this runs into the undisputed territory again. GHGs do cause additional warming, and warming is a Bad Thing. So, if the warming to date is caused by something else, and that is going to carry on having this effect (if we don't know what it is, it would be imprudent to assume anything else), then we should take action to reduce as far as possible our efforts to add to that warming. So, if the mainstream science is wrong, we need to reduce GHG emissions even more than is currently planned. Simple really. Now let's get on with it.