to control items such as smog. they were never granted the authority to control greenhouse gasses. if congress decides that greenhouse gasses are a threat on par wit smog in the 70's and decides to task the EPA with its' regulation , then that's fine. I'll disagree with their finding, but everything at least is Constitutional. For the EPA to decide that it can expand it's own authority and set itself up as a check on Congressional action (or inaction), however, is unconstitutional and sets a dangerous precedent indeed.