There is no serious opposition to the above observations, at least not in the hypothetical sense. Where differences of opinion arise is over which of these effects will actually come to bear following a given oil spill. From the many published scientific observations of oil spills one might conclude that acute ecological damage was in most cases "light" to "moderate" (Milford Haven, Santa Barbara, San Francisco, and Chedabucto Bay spills), but in a few cases damage was "severe" (Tampico Maru, West Falmouth and, because of the misuse of toxic dispersants, Torrey Canyon).

Of course, spilled oil affects different organisms differently. Oil pollution has consistently done direct and significant damage to diving sea birds to the point of threatening the local survival of several species. This in itself is a most serious consequence of oil spills. Most observations have been confined to or have concentrated on visible intertidal organisms, which while certainly susceptible to heavy dosages of oil, are notoriously hardy marine organisms. It must be kept firmly in mind when judging the severity of oil spills as reported in the literature that the literature usually ignores the less easily visible, less tolerant subtidal organisms ().

The long-term effects of oil spills are too little known to make definitive conclusions. A British pollution biologist, R.B. Clark (), concluded that "even catastrophic coastal pollution, combined with the grossest misuse of toxic dispersants, appears to do no irreparable harm, although recovery of the flora and fauna may take some years." However, his statement applied to those situations where recovery is possible, i.e., where pollution is not chronic, and to temperate and subtropical waters. On the other hand, follow-up studies of the fuel oil spills at West Falmouth () and in Baja California () indicate that recovery can be slow and pollution relatively persistent from a single spill. The observable long-term effects of a single oil spill seem to range from less than one year to more than a decade after the initial accident.

Chronic pollution, via outfall, land runoff, or ship discharge may have consequences of much longer duration. Chronically polluted waters near oil terminals and refineries may be severely perturbed environments, and exclusion of normal plant and animal species may continue for as long as pollution continues. The effects of large, chronic inputs into the marine environment from sewage, runoff, and tanker operation have not been investigated but clearly warrant concern. It is especially important to note here that chronic discharges of oil are not evenly spread throughout the world's oceans, but almost all occur in coastal waters, the most productive part of the sea.