Global temperatures are decreasing
As the skeptic's argument goes, global warming is not occurring because global temperatures have actually been decreasing. Often the decrease is described as being over the last ten years or so. For example: "Manmade global warming is a hoax created with a political agenda to reduce our use of fossil fuel. Even natural global warming (caused by sun activity) doesn’t exist, as evidenced by the seven to 10 years of cooling." link
Especially when the ten year time scale is given, this is most likely a reference to the fact that temperatures have in general been lower than they were in 1998. But this is not evidence of a global cooling trend. Instead, it is due to the fact that 1998 was an abnormally warm year, due to a strong El Nino. So, the "Global temperatures are decreasing" claim depends on a very select data cherry-picking.