Claiming global warming doesn't exist is just idiotic. Any 10 year old can look at a 60 year chart of global average temperatures and notice an overall upward trend. What hasn't been proven yet is whether it's being spurred by man, something we can hopefully figure out some day.
It's because a lot of things depend on a lot of things, a lot of things depend on some things, some things depend on a lot of things, a lot of things depend on some things, and some things depend on some things, and vice versa. That's just the way the atmosphere works!