Methane gas is being released with the oil. You've seen the latest video, the one that BP sat on for three weeks, presumably with Obama's consent. At the beginning, the pipe is blowing what looks like steam into the water. That's methane. After a few seconds, mostly oil is coming out, but some methane is visible just below it. The "ice crystals" which clogged the first containment structure were methane hydrate, which forms when methane dissolves in cold water. But a lot of methane is making it to the surface. I have looked and haven't found any estimates. They are having enough trouble figuring out how much oil is coming out of that tube.
Methane is a potent greenhouse gas. According to the EPA it is about 20 times more potent than carbon dioxide.
http://www.epa.gov/methane/The Cambridge University Press Blog "This Side Of The Pond" has an alarming post about methane as a greenhouse gas.
"Molecule for molecule, methane is about 25 times more potent a greenhouse gas than carbon dioxide. Today, the amount of methane in our atmosphere is spiking at an alarming rate. Arctic research vessels are discovering “methane chimneys” where CH4 bubbles-up and foams on the ocean’s surface. Scientists call this situation “a ticking time bomb.” They fear that, as the climate warms, vast stores of methane could be released from thawing tundra and melting deposits of methane ice beneath the ocean. If this process gains momentum, it could initiate a self-reinforcing feedback loop that would spiral out of control even if we cut our greenhouse emissions to zero."
http://www.cambridgeblog.org/2010/05/oil-coal-methane-and-climate-change/