The oil slick in the Gulf of Mexico appears to be dissolving far more rapidly than anyone expected, a piece of good news that raises tricky new questions about how fast the U.S. government should scale back its response to the Deepwater Horizon disaster.
The immense patches of surface oil that covered thousands of square kilometres of the Gulf after the April 20 oil-rig explosion are largely gone, although there continue to be sightings of tar balls and emulsified oil here and there.
Reporters flying over the area on Sunday spotted only a few patches of sheen and an occasional streak of thicker oil, and radar images taken since then suggest that these few remaining patches are quickly breaking down in the warm surface waters of the Gulf.
John Amos, president of SkyTruth, an advocacy group that criticized the early, low estimates of the size of the BP leak, noted that no oil had gushed from the well for nearly two weeks.
"Oil has a finite life span at the surface," Mr. Amos said Tuesday, after examining fresh radar images of the slick. "At this point, that oil slick is really starting to dissipate pretty rapidly."
The dissolution of the slick should reduce the risk of oil killing more animals or hitting shorelines. But it does not end the many problems and scientific uncertainties associated with the spill, and federal leaders emphasized this week that they had no intention of walking away from those problems any time soon.
The effect on sea life of the large amounts of oil that dissolved below the surface is still a mystery. Two preliminary government reports on that issue have found concentrations of toxic compounds in the deep sea to be low, but the reports left many questions, especially about an apparent decline in oxygen levels in the water.
And understanding the effects of the spill on the shorelines that were hit, including Louisiana's coastal marshes, is expected to occupy scientists for years. Fishermen along the coast are skeptical of any declarations of success, expressing concern about the long-term effects of the chemical dispersants used to combat the spill and of the submerged oil, particularly on shrimp and crab larvae that are the foundation of future fishing seasons.
After 86 days of oil gushing into the Gulf, the leak was stopped on July 15, when BP managed to install a tight-fitting cap on the well 1,500 metres beneath the surface, then gradually closed a series of valves. Still, the well has not been permanently sealed. Until that step is completed in several weeks, the risk remains that the leak will resume.
Scientists said the rapid dissipation of the surface oil is probably due to a combination of factors. The Gulf has an immense natural capacity to break down oil, which leaks into it at a steady rate from thousands of natural seeps. Although none of the seeps is anywhere near the size of the Deepwater Horizon leak, they do mean that the Gulf is swarming with bacteria that can eat oil.
The winds from two storms that blew through the Gulf in recent weeks, including a storm over the weekend that disintegrated before making landfall, also appear to have contributed to a rapid dispersion of the oil. Then there was the response mounted by BP and the government, the largest in history, involving more than 4,000 boats attacking the oil with skimming equipment, controlled surface burns and other tactics.
Thad Allen, the retired Coast Guard admiral who leads the government's response, has emphasized that boats are still skimming some oil at the surface. He said the risk of shoreline oiling might continue for at least several more weeks.
Still, it is becoming clear that the Obama administration, in conjunction with BP, will soon have to make decisions about how quickly to begin scaling down the large-scale - and expensive -response effort. That is a touchy issue, and not just for environmental reasons.
The response itself has become the principal livelihood for thousands of fishermen and other workers whose lives were upended by the oil spill. More than 1,400 fishing boats and other vessels have been hired to help deploy coastal barriers and perform other cleanup tasks. Those fishermen are unconvinced that the gradual disappearance of oil on the surface means they will be able to return to work soon.