When you say deny, that implies that global warming is fact.
Here are some reasons.
a) no proof the Earth is warming, any number of years whether it be 100 or 1000, does not predict a trend for the next year or next 100 (200 years ago we were in a "Little Ice Age", 30 years ago there were predictions of another ice age),
b) there is very little proof that CO2, a minute particle in the atmosphere, could change the weather.
c) there are a lot of lies being circulated by the GW nuts, the warmest years on record are in the 1930's, polar bears have 4 times the population of 30 years ago, there is just as much ice, or more, at the poles as there was 20 years ago, and the strength, number, or magnitude of natural disasters (hurricanes in particular) have not increased (ever hear of Galveston or Jamestown, or Hurricanes Gilbert, Hugo or Andrew?)
You can find all the facts, it just takes some work getting through all the environmentalist propaganda.
I would not believe any one who is receiving money from GW groups to prove GW. The guys that are speaking out get ostracized and ridiculed, they have nothing to gain by presenting the facts.
The "deniers" have done their homework.