It was huge. It also contained lots of methane, they sprayed chemicals all over the Mexican gulf to contain the spill, an explosion, a sunken oil rig, etc. As mentioned, I remember it from the news back then and don't really have a clue about the details. :D

Regardless, the point was that currents can change without Jake Gyllenhall having to chase antibiotics on a derelict frozen tanker in NYC.

The changes will mostly destabilize the weather, and prevent heat from being transferred from A to B. And long those long-term weather effects are scarier than Hollywood blockbusters.