The magnitude of the catastrophe will depend on the oil's fate: the amount of oil released, how the oil is transformed chemically and physically, and how far and wide it travels. To date, scientists are far from answering any of these questions.
The oft-cited 5000 barrels per day of oil spewing from the leaking well is almost certainly an underestimate. Scientists eyeballing videos of the sea-floor gusher or gauging the extent of the surface slick in satellite images see five or even 10 times as much oil coming out 3.5 weeks into the disaster. Continued analysis will improve those estimates, and ongoing efforts to stanch the flow may be informative, but as one oceanographer puts it, for now, "it is what it is."
Researchers have a better handle on how the oil is "weathering." Samples collected from the sea surface show that, as expected, the oil is tending to lose its more volatile—and more toxic—components as it evolves from a simple liquid to an emulsified "mousse" to tarballs. Although well-aged tarballs are the least damaging form of lingering oil, those starting to appear on beaches are still so sticky that plants and animals could suffer greatly. Detergent-like dispersants applied offshore accelerate both physical and biological weathering, but chemists have yet to see obvious signs that dispersants are helping. Then there are biologists' concerns that dispersants could be affecting marine life in the open gulf directly through their toxicity or indirectly by causing more of the oil to linger far below the surface where fish and bottom-dwellers are.
Researchers could soon get a better idea of what's happening to the oil and dispersants as well as where it's all going as field sampling gets in gear. Reports of large subsurface plumes of oil—perhaps enhanced by dispersants—are beginning to come in as sampling from boats and ships is extended to the subsurface. And two different autonomous underwater vehicles are scheduled to start mapping subsurface oil using optical sensors; one of them can return water samples for detailed analysis. The as-yet-loosely-coordinated effort to characterize the evolving spill is being conducted by the National Oceanic and Atmospheric Administration (NOAA) and the U.S. Geological Survey, while the National Science Foundation is supporting fieldwork through rapid-response grants.