There's no question that BP's Deepwater Horizon oil spill is an unmitigated disaster of yet unknown societal and economic proportions, not to mention the possibly irreparable damage to coastal and marine ecosystems. According to government estimates, as of yesterday anywhere from 39 million to 111 million gallons of crude oil has gushed into the Gulf of Mexico (that excludes captured oil). Most officials lean toward the higher totals while noting up to 2.5 million gallons more continue to spill each day -- that's an Exxon Valdez spill (nearly 11 million gallons total) about every four days.
Yet, while this oil spill and others before it have dominated the news, according to a 2003 National Research Council (NRC) report, at least 375 million gallons of oil end up in the world's oceans virtually unnoticed every year from natural sources and from human activities associated with the extraction, transportation and use of oil. Should the current rate of uncaptured oil discharged from the BP well continue, the spill will equal the yearly amount of oil entering the world's oceans sometime in August. Which is just about the time relief wells will, supposedly, completely plug the Deepwater Horizon gusher.
Unfortunately, no such end is in sight for the apparently massive background level of oil pollution.
Natural seepage generally occurs in regions of concentrated oil (and gas) -- not surprisingly, these are often the same areas where oil wells are drilled -- such as the Gulf of Mexico and off the coast of California, and accounts for 47% of oil pollution worldwide. It follows that anthropogenic sources account for the majority (53%) of yearly oil pollution (almost 200 million gallons) of the world's oceans.
Perhaps surprisingly, a vast majority of this 53% is a consequence of mankind's use of oil rather than accidental spills associated with offshore drilling or tanker transport. Technological advances in offshore drilling and safety measures -- notwithstanding the BP fiasco -- and regulations regarding the design of oil tankers (enacted in response to the Exxon Valdez disaster) have dramatically reduced blow outs, spillage resulting from nominal oil production (offshore and onshore facilities), and oil tanker disasters. As a result, collectively these sources account for only 3% of non-natural oil pollution.
Of special interest and relevance to the U.S., the NRC reports that of the 375 million gallons entering the oceans worldwide every year, at least 75 million originate from North America. The largest component (62%) is seepage from the floor of the Gulf of Mexico and to a lesser extent the ocean bottom off the California coast. While proportionally less than for the globe, anthropogenic sources are not insubstantial (28.5 mil gallons), with the vast majority not from accidents in extraction or transport, but human use, including misuse and waste of oil and oil-based products.
The vast majority of use-related oil pollution (about 87%) in North America is from urban runoff into rivers, discharges from commercial and recreational marine vessels, and air pollution. Among the individual sources are improper disposal of used motor oil, (e.g., from cars, lawn mowers, etc), oily storm-water drainage from city streets, runoff of municipal and industrial petroleum waste products (e.g., from refineries and power plants), leaking from storage facilities, oil released from bilge cleaning and other ship operations, and deposition by rain of atmospheric hydrocarbons. More than half of the oil that originates from land-based sources and ends up in North America's near-shore waters comes from the urbanized areas between Maine and Virginia.
The airborne sources of oil pollution into the ocean nominally include emissions from vehicles, power plants and industrial facilities. Added now to an unknown degree is the hydrocarbon load from purposeful spot burning of surface oil in the Gulf. Speculatively, at least some, if not most of the oil rid from the surface of the Gulf might simply settle back or be washed out by rain and spread as a poisonous residue over a larger area including, possibly, otherwise unaffected regions over land.
Individually, each contributor of oil pollution of our seas may be relatively small and sporadic, but collectively amount to tens of thousands of releases into the world's oceans every year. Notwithstanding the justifiable attention on the Gulf spill and its catastrophic consequences, these sources of oil pollution might very well be more important in both scale and degree to the longer-term health of the marine environment. While many aspects of the chemical composition of these diffuse contaminants are largely unknown, they continuously create relatively low but chronic contamination over huge areas of the world's oceans.
We know that in the short term, even a small amount of oil in the sea can have severe effects on marine life depending on the location and timing of its release. Over the long term, there is possibly a latent (hidden) period before evidence of non-obvious consequences of oil pollution are discovered. And then it might be too late to do anything about it (advocates for climate change action know this feeling all too well).
On the other hand, except for natural seepage, the of oil now hemorrhaging into the Gulf is unique in that it enters the water from unprecedented depths (approx. 5000 feet). Consequently, many of the contaminants are washed off or dissolved as the oil rises with some unknown percentage of the total forming extensive plumes well below the surface. The concentrated and continuing effects of contaminants reaching beaches, wetlands and, especially, ecosystems and food chains within the Gulf (and perhaps beyond) will presumably remain open questions subject to intense research for the foreseeable future. One question I do not have an answer to is what, if any research has been done to date studying the ever-present natural seepage of large volumes of crude oil from the floor of the Gulf.
Whatever the outcome of the Gulf disaster, it's clear that ultimately it will be a matter only of determining the degree and extent of the environmental and human tragedies, and whether lessons learned will be sufficient to avoid comparable catastrophes in the future.
POSTSCRIPT: During World War II, 452 oil tankers were sunk in the North Atlantic, as were an unknown number of the 1080 wrecks lying on the bottom of the Pacific. Little is known about the total amount of oil that spilled directly in to the ocean, but it's likely much went to the bottom in ships that remained virtually intact. It is known that two oil tankers sunk in 1942 lie only a few hundred miles from Australia's Great Barrier Reef and are estimated to contain as much oil as the Exxon Valdez. It's only a matter of time before rotting hulks pose enormous potential to foul fragile environments.