Todd Korol/Reuters
Giant dump trucks haul raw oil sands in Alberta, Canada.
President Obama announced his decision Friday from the White House during a televised press conference.
Obama said Keystone XL wouldn't have helped lower gas prices or create that many jobs.
He also said the long-term contribution to climate change - possibly more than 22 billion metric tons of carbon pollution, according to Scientific American - wasn't worth the loss of America's global leadership on fighting emissions that exacerbate global warming.
"If we're going to prevent large parts of this Earth from becoming inhabitable, if not inhospitable ... we have to keep some fossil fuels in the ground," Obama said.
But there's another steep environmental cost Obama didn't mention: Had the White House approved Keystone XL, 54,000 square miles of pristine Alberta wilderness may have been razed to feed it.
"We're not saying the project is good or bad. We're just saying the scale and severity of what's happening in Alberta will make your spine tingle," Robert Johnson, a former Business Insider correspondent, wrote after flying over the Canadian oil sands in May 2012.
Keep scrolling to see Johnson's photo essay, which shows Canadian oil mining - a process in which tar-laden sand is dug from the ground and the oil is separated through a lengthy and messy process.