We look at the environmental and political impact if President Obama greenlights the construction of the pipeline.

Just days after publically reaffirming his belief in the threat posed by climate change, US President Barack Obama is under pressure to prove he means it.

"The first Keystone pipeline, the one before this, managed to leak 12 times in its first year of operation .... Let's assume that it [oil] makes it all that way down the pipeline without spilling, then it spills into the atmosphere. Now all that carbon adds to the load we are already putting on the planet. And coming after the year in which we pretty conclusively melted the Arctic, that would be folly on a whole new scale." - Bill McKibben climate change activist

What is billed as the largest climate rally in history is being planned for Sunday in Washington DC. It is a make-or-break moment for the tar sands industry, and for the fight against global warming.

The specific goal of the rally is to ensure that Obama rejects the construction of the Keystone XL pipeline which would pump tar sands oil from Alberta in Canada to Texas.

The rally is the latest action in a long-running campaign. Just this week, 50 people were arrested while demonstrating against the pipeline outside the White House.

This is an issue that has the potential to alienate a significant section of the president's base. But Obama is also under pressure from Congress and his own party to approve the project.

Last month, a bipartisan majority in the Senate urged approval, after what it described as "an exhaustive environmental review".

The senators concluded it was in the US national interest to build the 2,500km pipeline, arguing it would cut dependence on foreign oil and create jobs.

"Any pipeline can have an accident but if one looks at all the pipelines in the country that cross the Ogallala reservoir for many, many years - both oil and natural gas pipelines - I think without great environmental havoc occurring, on balance I think it makes sense to approve the pipeline and get that oil into the market. " -Charles Ebinger, Energy Security Initiative, Brookings Institution

Opponents, however, point out that the extraction of Alberta tar sands oil will mean irrevocable damage to the environment - "game over for the planet", in NASA scientist James Hansen's words.

In addition, they point to the likelihood of the pipeline leaking its particularly toxic combination of chemicals in areas of huge significance for the environment and the US's water supply.

In fact both environmental campaigners and the energy industry say Obama's decision is a crucial moment for the future of tar sands exploitation. A 'yes' will entrench the fuel as a major source of energy, while a 'no' would render it too unprofitable to be worthwhile to big oil.

A report last year by the nonpartisan US Congressional Research Service found that Canadian tar sands oil emits more greenhouse gases than other crude imports. The report said:

Overall emissions - including during extraction, processing and consumption - are 14 to 20 percent higher for tar sands crude, compared to other transportation fuels in the US

Emissions just from extracting and refining the oil from the tar sands are 72 to 111 percent higher than for other fuels

Tar sands are heavier and require more energy and resource-intensive activities to produce the oil

The pipeline is estimated to increase the US greenhouse gas emission footprint by 3 million to 21 million tonnes

So, is President Obama's environmental legacy at stake?

To discuss this, Inside Story Americas with presenter Shihab Rattansi is joined by guests: climate change activist, Bill McKibben, who was among those arrested outside the White House on Wednesday; climate scientist, Alan Robock; and Charles Ebinger, the director of the Energy Security Initiative at the Brookings Institution.

"What the US needs is energy; it doesn't need oil. You can get energy from other sources. For an oil company to say we need oil makes me a little curious about the reason that they are saying that. I agree with Bill [McKibben] that the main damage from this is symbolic, it's symbolic that we are going to continue to burn fossil fuels and use the atmosphere as a sewer with no charge to put more carbon dioxide in there which will last for thousands of years." Professor Alan Robock, climate scientist

Source: Al Jazeera