Now what happens if we are at an initial equilibrium, and then all of a sudden the US government relaxes the prohibitions on ANWR drilling? If oil traders really believe the policy shift is permanent, and that up to a million extra barrels will be hitting the market in a decade, then this will obviously reduce the expected world price of oil starting at that time. Consequently, any oil producers who had previously settled on a production rate with "excess capacity" — i.e., where they could have produced and sold more barrels today, but decided not to for reasons of profit — will re-evaluate their decision.
Without specifics we have no idea how much the new information will change their output plans, but surely they will pump more in the present than they had previously decided.
If we step back and survey the big picture, what would happen is that the market in a sense would be transferring some of those future ANWR barrels to the present. It's true, the market doesn't have recourse to time machines. But physical barrels of oil that would have otherwise sat underground in 2008, 2009, and so on, will now be brought to the surface and sold, because they have been displaced by the barrels currently buried in Alaska that will be brought to the surface and sold in 2018, 2019, and so on.
It is even possible that the mere threat of opening these areas to drilling has an effect on today's prices:
If this seems too theoretical and farfetched, consider this: In May, the Saudis officially rebuffed President Bush's request for them to increase their output. Yet one month later, they reversed their position. What changed in the interim?
Obviously I don't know for sure what motivates oil barons, but the political mood in the United States shifted in between those two announcements. All of a sudden, opening up ANWR and offshore areas for drilling was "on the table." The mere possibility of an extra million or more competing barrels per day may have been enough to reverse the Saudis' stance.
Markets are wonderful things when they are allowed to work.