Dow Jones: U.S. President Barack Obama acknowledged the “doubts and difficulties” clouding progress on climate change in Congress, but said the U.S. is determined to tackle global warming at a year-end summit in Copenhagen. “We understand the gravity of the climate threat. We are determined to act,” Obama told a U.N. meeting on climate change in New York on Tuesday. “And we will meet our responsibility to future generations.” Tuesday’s meeting at the U.N. General Assembly is designed to …