The existence of human-caused global warming is pretty much settled science, but there are powerful interests who have reason to sew doubt. These deniers - they aren't scientific skeptics as they aren't expert in the field and their arguments have never held up - continue to confuse and delay. Some of the key people were active in supporting the tobacco industry and they were very successful for a few decades until the scientific evidence was finally too overwhelming. Their goal is not to disprove, but to cast doubt and stall movement allowing an industry (the fossil fuel industry in this case) to remain profitable.
Here is a nice list of denier statements along with rebuttals.