As the Alberta Premier the Prime Minister have said, pipeline construction has nothing to do with limiting carbon pollution.  Pipelines move hydrocarbons around.  Whether they are burned depends on other things, like market forces and regulation.

If pipeline companies want to pour money into local economies during the construction phase, bringing jobs and prosperity to construction workers and their communities, we should celebrate that.  If demand for their toxic product slumps, that's good too.

Environmentalists need not oppose pipelines.  Stopping pipelines will not reduce greenhouse gases.  The two are entirely separate.