President Obama announced on Friday that he had rejected the request from a Canadian company to build the Keystone XL oil pipeline, ending a seven-year review that had become a symbol of the debate over his climate policies.  Mr. Obama’s denial of the proposed 1,179-mile pipeline, which would have carried 800,000 barrels a day of carbon-heavy petroleum from the Canadian oil sands to the Gulf Coast, comes as he seeks to build an ambitious legacy on climate change...The once-obscure Keystone project became a political symbol amid broader clashes over energy, climate change and the economy. The rejection of a single oil infrastructure project will have little impact on efforts to reduce greenhouse gas pollution, but the pipeline plan gained an outsize profile after environmental activists spent four years marching and rallying against it in front of the White House and across the country...“President Obama is the first world leader to reject a project because of its effect on the climate,” said Bill McKibben, founder of the activist group 350.org, which led the campaign against the pipeline. “That gives him new stature as an environmental leader, and it eloquently confirms the five years and millions of hours of work that people of every kind put into this fight.”