So far I have come up with a preliminary 5 groups:
- Data Based: "I am skeptical because of this simple fact...."
- Ignorance based Arguments: "I am skeptical because we don't know enough"
- Theory Based: "I am skeptical because I know of an equally good/better theory to explain it."
- Political: "I am skeptical because Al Gore flies in a private jet"
- Conspiracy: "I am skeptical because they are in it for the taxes and grants"
- It hasn't warmed since 1998 (literally true, but in context of climate, wrong)
- Human co2 is a tiny % of co2 emissions (literally true but irrelevant)
- Water vapor is the most powerful greenhouse gas (literally true but irrelevant)
- Volcanoes emit more co2 than man (false)
Because these claims are so easy to verify, it is a real red flag when someone who should know better uses them as an argument.
Ignorance based Arguments
- Climate is all chaotic and we don't understand it at all
- Uncertainty mentioned in IPCC reports, some paper, temperature records
- Models are unreliable
- The concept of global temperature is meaningless
Theory based arguments
- It's the Sun
- Imminent global cooling
- The greenhouse effect is a myth
- co2 was higher in the 40s 
- (insert personal climate theory)
These arguments go beyond simple claims based on data. These are alternative theories contradicting the mainstream. They have to explain why they don't accept the mainstream theory. This often occurs in three ways:
- This has simply overturned the mainstream theory.
- An Ignorance based Argument: They claim that their theory is no less likely than the mainstream one because so little is known, etc.
- A Conspiracy Argument: For example Ernst Beck who coined the "co2 was higher in the 40s" theory uses a Conspiracy Argument to explain the existance and acceptance of the mainstream theory.
Note that the first and third cases are making positive assertions about how the climate works (or some aspect of it) and therefore they can no longer make Ignorance based Arguments in these areas. For example if they are going to claim "recent warming on earth is caused by the sun" as a fact they then can't turn around later and claim "The concept of global temperature is meaningless" without contradicting themselves.Political Arguments
- Al Gore's house has a runway of Private Jets
- I am skeptical because environmentalists don't live in mud huts
These arguments have no bearing on the science. I have distain for these arguments. I generally ignore them, but I include them as a group here because often these arguments are made by the same skeptics. As noted above these arguments are often made in conjunction with Ignorance based Arguments.
Conspiracy based arguments
The Journal of Inactivism has an interesting genealogy of climate conspiracy theories which lists the various conspiracies used by skeptics to explain why manmade global warming is such a prominent theory. Highlights include:
- In the 80s we get "Mainstream scientists are soviet stooges", it's a "soviet plot" 
- A "doctrine to replace marxism" 
- Scientists "artifically sustain debate" (ie "commit fraud") for funding and grants 
- Politicians use it to increase taxes 
- I've heard Margret Thatcher get blamed for it somewhere
I will probably revisit this list at some point and update it. Some arguments can be seperated out better and I might be missing some categories. Ideas/corrections are welcome.
 F. Bi. 2008. Towards a genealogy of climate conspiracy theories. Intl. J. Inact., 1:37–42
E. Beck. 2007. 180 years of atmospheric CO2 gas analysis by chemical methods., Energy & Environment, Volume 18 No. 2