Natural oil seepage in the Gulf of Mexico ranges from 1-5 million barrels a year of petroleum products. The BP spill, at it's highest estimated rate of 5,000 barrels a day, comes to 1.8 million barrels of oil a year. Don't misunderstand me, that's a LOT of oil, but it's well within the span of natural seepage for the gulf. So what made Deepwater Horizon so bad?
- It was a concentrated blowout
- Humans did it
The first part is why we had oil impacting beaches and wildlife; those 1-5 million barrels of natural seepage are spread out throughout the gulf, so the oil is diluted. Deepwater Horizon was a point gusher, so it created a concentrated cone of higher-concentration oil. The good news is that the Gulf has a complex ecosystem that already breaks down oil; the bad news is we tend to create concentrated oil spills.
The second part is what made the gusher look so bad. When a volcano goes off, it's a disaster, but it's not a MAN MADE disaster, so we have nobody to blame but God. Deepwater Horizon, which was a blowout of pressurized methane gas on the level of an "act of God" was considered such a big disaster because it was man made: we had people to blame for it. (I'd be interested to see how environmentalists would handle a natural blowout in the gulf).
Deepwater is not the only large oil blowout in the gulf. We just had better TV cameras and a more vocal environmental community to complain about it. Ixtoc I was a similar disaster, in the latter half of 1979. 3 million barrels from that one. It ruined coastlines and took 9 months to plug, but we were still here to panic about Deepwater Horizon 30 years later.
The Gulf of Mexico is YUUUGE, and so is the world. While I don't mean to minimize the disastrous effects of either of these spills, we need to look at the map. On the grand scheme of the gulf itself, both disasters took place REALLY close to land. Had it happened in the center of the gulf, maybe it'd have completely dispersed before affecting land.
The name "Deepwater Horizon" gives the impression that it was way out in the middle of the gulf, while the horrible images on the news made it seem that the entire Gulf coastline was being decimated. It's all perception, though; the Gulf itself couldn't care less about a few million more barrels of seepage. But there was a more insidious problem.
See, we'd already had Ixtoc I. We already knew what it took to contain a disaster like this. The government already had regulations on the books for how to monitor and manage a spill, and keep it contained away from shore. But BP didn't follow those rules. There weren't enough booms stationed nearby to stop the spill. International cooperation choked causing delays in getting disaster supplies to the Gulf. To add on to that, until Deepwater we didn't realize just how well the ecosystem of the Gulf had evolved to consume oil, so we sprayed the slick with millions of barrels worth of dispersant, which spread the oil out, but also made it impossible for the microbes and other life of the Gulf to manage the oil, which meant is HAD to be scooped out by hand or cleaned off shores.
Okay, so, TL;DR: No. Deepwater Horizon was not a world destroying event. You could have the terrorists put a nuke on the sea floor in hopes that it would shatter so much rock that the seepage would be too much for us to handle, but as mentioned before, Deepwater had to drill DEEP, so you'd need to do enough damage that you could use those resources more profitably elsewhere.
But Terrorists don't go for damage, they go for effect. So as experience with Deepwater Horizon has shown, if terrorists could blow up two or three blowout preventers at once, they could convince the world that they're screwed, even though history and science show that the disasters would likely remain local, and that might be good enough for them.