Here is a graph of temperature over the past century, taken from here:
Let's say that you were a complete hack, and you were out to deny the existence of global warming. What bogus arguments could you present that might sound convincing to someone who hasn't seen the chart?
Here are two such arguments that are recycled over and over again:
1. There's been no global warming since 1998. See the huge spike slightly before 2000? Thats 1998. If you recall - and I assume anyone who is reading this was alive back then - that was the year of El Nino.
Obviously, by picking an extremely hot, atypical year as a baseline, its not difficult to get the desired conclusion that global warming isn't happening. One uses a moving average - the fat black line - to smooth out natural fluctuations in the data. Looking at the black line, its difficult to avoid the conclusion that the earth is not warming.
2. Most 20th century global warming occured before 1940. There are many factors that affect global warming. Here are three such factors: volcanic activity, brightness of the sun, carbon emissions. According to our understanding of the atmosphere, the first two factors caused the pre-1940 warming. The last factor caused the recent spate of warming. This is particularly troubling, because the last factor - carbon emissions - is in danger of increasing indefinitely (unlike the first two factors). Which is why we need steps like the Kyoto protocol to curb it.