There's oil in the tar sands of Canada. Lots of it. Oil companies of course want to exploit it, and part of that involved building the Keystone Pipeline.
Extracting oil from tar sands is a lot more energy intensive. You have to heat the ground and burn a lot of fossil fuel just to get the fuel out. That of course generates a lot of CO2 emissions. But the larger issue is just the concentration of CO2 in the atmosphere that will result if we exploit the tar sands like we exploited Saudi Arabia.
NASA's lead climate scientist has run the numbers. What does he say? We go this route and "it's essential game over." The game being human civilization as it exists today. See further discussion here and here.
So this has caused unusual activity in the scientific community. Scientists leaving their offices and protesting in Washington. As Naomi Klein discusses here they really didn't think there was any way to stop the corporate freight train. They've got all the money. What do we have? We have the majority of little people. But if you aren't organized, who cares?
Then came Occupy Wall St. The result? Obama for once did not cave to wealth. He delayed approval, which will scare off investors and stave off the day of reckoning. A huge win.
Of course the corporate lackeys are ticked. Profits above all else. The science and the facts be damned. The Wall St Journal says "But what about jobs!!" Jobs claims are bogus anyway, but does anybody really think the WSJ cares about jobs? They care about profits, which benefit owners. They'll be back, fighting for wealth and ownership and against people. But we've won for once.