Meanwhile, it’s crucial to understand — although the Bush administration would apparently prefer not to — that uncertainty cuts both ways. As the administration likes to point out, the U.S. spends about $2 billion a year on climate-change research. It’s possible that as scientists learn more about how the climate works, they will discover that the threshold of dangerous change lies further away than is estimated, and Washington’s do-nothing policy will come to seem justified. But the reverse is just as likely. In fact, nearly everything that has been discovered about the climate system recently has tended to suggest that the threshold is closer than suspected.