Below is the condensed version of the arguments that skeptics make in regard to catastrophic anthropogenic global warming.
Most climate change skeptics actually agree that the earth has warmed the last 50 years and that humans are probably responsible for at least a portion of that warming. Those are not the issues that divides "alarmists" and "deniers".
The main issues that divide the two camps are as follows:
1. Many of the negative effects of a warmer planet are unproven. For instance, there is ZERO observational evidence that there have been more hurricanes, stronger storms, more frequent tornadoes, decreased crop yields or increased droughts as a result of the warming the last 50 years. Contrary to popular belief our climate isn't getting "weirder", just warmer. Whether or not there will be stronger or more frequent storms in the future as a result of a warmer planet is also unproven since this will depend on where the warming occurs and because climate models have been inaccurate so far in this regard. Attempts to show otherwise are based on cherry picked data from specific regions of the globe or short time periods.
2. Even sea level increases are controversial. For instance, the amount of sea level rise per decade since 1960 is not much higher than the rate that was observed in the 1800s and early 1900s before we began emitting so much CO2. In fact, sea levels have been rising steadily for the last 20,000 years. So there's clearly other factors present, possibly involving tectonics or changes in the sea floor. The human contribution to rising sea levels so far has been minimal.
3. Other extreme predictions such as 40% extinction of species by 2050 or New York City to be partly under water by 2020 and coastal cities to be uninhabitable in the next few decades are also unproven and typically made by scientists who have never had their prior predictions verified before.
4. Climate forecast models made without the benefit of hindcasting have consistently over-predicted the amount of observed warming (by roughly three times) when applied to independent data. Climate models have also incorrectly predicted where the most warming will occur. Even on historical simulations that accurately reproduce past global average temperatures, climate models do not replicate the observed temperature & precipitation at individual stations. Additionally, EVERY decent operational forecast meteorologist knows that raw 2-meter temperature forecasts from a short-term (1-7 day) numerical weather prediction model are garbage. That's why the NWS statistically post-processes the model forecasts and turns them into something useful. However NO such statistical post-processing is applied to climate models. Climate models are also coupled with ocean models, since oceans play an important role in modifying our climate. However, our knowledge of oceanic processes pales in comparison to our knowledge of the atmosphere and is also likely a source of error. As a result of all this, it is highly likely that climate scientists give too much confidence in their model simulations and as a result the genuine uncertainty going forward is actually much greater.
5. Despite the increase in CO2 the last 18 years or so, there has been no corresponding increase in global temperatures. Nobody knows why but there have been over 3 dozen possible reasons provided. Bottom line is there are clearly other natural forcing mechanisms at work which are largely unknown. However, this is a double-edged sword for the pro AGW camp because if there are unknown mechanisms which can prevent warming on a short time scale then it demonstrates that natural mechanisms and variability play a more important role than previously thought and could have also contributed a bigger portion of the warming the last half-century. [Note: in all fairness this could be the result of cherry picking. Move the start time to 1995 and there is slight warming trend but move the start point to 1999 and it looks like temps level off. Time will tell. If the temps in the next five years begin to uptick then this argument used by skeptics will be invalidated. However if they remain level by 2020 then this argument is still in play and will require an explanation of the pause, since by then the temp will be outside the forecasted envelope of possibilities].
6. Nobody has defined the optimal climate. Why should today's climate be automatically defined as the best one possible? A climate that's several degrees warmer offers possible benefits largely dismissed by the AGW community: longer growing seasons, more usable farm land, increased vegetation growth, fewer deaths in winter, and people in general prefer warmer climates.
7. There is no way of knowing how the future climate will change as a result of both human and natural influences. All efforts made to curb global warming today could ultimately be a waste of time and money.