In a little-noticed provision in a 1990 law passed after the Exxon Valdez spill, Congress capped a spiller’s liability over and above cleanup costs at $75 million for a rig spill. Even if the economic damages — to tourism, fishing and the like — stretch into the billions, the responsible party is on the hook for only $75 million. (In this instance, BP has agreed to waive the cap for claims it deems legitimate.) Michael Greenstone, an M.I.T. economist who runs the Hamilton Project in Washington, says the law fundamentally distorts a company’s decision making. Without the cap, executives would have to weigh the possible revenue from a well against the cost of drilling there and the risk of damage. With the cap, they can largely ignore the potential damage beyond cleanup costs. So they end up drilling wells even in places where the damage can be horrific, like close to a shoreline. To put it another way, human frailty helped BP’s executives underestimate the chance of a low-probability, high-cost event. Federal law helped them underestimate the costs.

In the wake of Deepwater Horizon, Congress and the Obama administration will no doubt be tempted to pass laws meant to reduce the risks of another deep-water disaster. Certainly there are some sensible steps they can take, like lifting the liability cap and freeing regulators from the sway of industry. But it would be foolish to think that the only risks we are still underestimating are the ones that have suddenly become salient.

The big financial risk is no longer a housing bubble. Instead, it may be the huge deficits that the growth of Medicare, Medicaid and Social Security will cause in coming years — and the possibility that lenders will eventually become nervous about extending credit to Washington. True, some economists and policy makers insist the country should not get worked up about this possibility, because lenders have never soured on the United States government before and show no signs of doing so now. But isn’t that reminiscent of the old Bernanke-Greenspan tune about the housing market?

Then, of course, there are the greenhouse gases that oil wells (among other things) send into the atmosphere even when the wells function properly. Scientists say the buildup of these gases is already likely to warm the planet by at least three degrees over the next century and cause droughts, storms and more ice-cap melting. The researchers’ estimates have risen recently, too, and it is also possible the planet could get around 12 degrees hotter. That kind of warming could flood major cities and cause parts of Antarctica to collapse.

Nothing like that has ever happened before. Even imagining it is difficult. It is much easier to hope that the odds of such an outcome are vanishingly small. In fact, it’s only natural to have this hope. But that doesn’t make it wise.