Like so many Americans, we are inspired by President Obama’s commitment in his inaugural speech to respond forcefully to the growing climate crisis. The president declared: “We, the people, still believe that our obligations as Americans are not just to ourselves, but to all posterity. We will respond to the threat of climate change, knowing that the failure to do so would betray our children and future generations.”

Now, the test stands before him to act on his words.

The president must demonstrate his leadership by saying no to any new tar sands pipelines. Expanding the market for the most carbon-intensive fuel on the planet is simply incompatible with addressing the threat of climate change and firing up the clean energy sources we need.

Many New Englanders have probably heard of the ongoing struggle against the controversial and dangerous TransCanada Keystone XL tar sands pipeline from Alberta, Canada, to Texas.

But northern New England citizens, especially Mainers, are also directly threatened by ExxonMobil’s plan to pump tar sands oil through Vermont, New Hampshire and Maine, for export out of Casco Bay.

ExxonMobil’s proposal to repurpose a 62-year-old oil pipeline to run highly corrosive tar sands would put the Androscoggin and Crooked rivers, Sebago Lake — the drinking water source for Greater Portland — and Casco Bay at enormous risk from leaks and spills.

Tragically, we know that the risks and enormous costs of tar sands oil are very real.

On July 25, 2010, a pipeline near Marshall, Mich., burst open and spewed more than 1 million gallons of tar sands oil in Talmadge Creek and then into the Kalamazoo River, contaminating more than 30 miles of the river and an adjoining lake.

Tar sands oil is heavier than conventional oil, and it sinks and coats the bottom of the riverbed. As a result, cleaning up such spills is much more difficult and expensive than cleaning up spills involving conventional oil.

More than two years after the Kalamazoo spill, the estimated $725 million cleanup is still not complete, and the U.S. Environmental Protection Agency acknowledges that some parts of the Kalamazoo River may never be fully restored.

Tar sands oil is diluted with a brew of toxic chemicals, and these chemicals were released into the air when the pipeline burst and polluted the river. Shortly after the spill, people in the vicinity reported noxious odors and health symptoms, including nausea and respiratory problems.

In the end, Enbridge, the company responsible for the Michigan pipeline and the Canadian portion of the network that connects to New England, had to purchase at least 130 homes along the river.

ExxonMobil and Enbridge have learned the lesson from the stark public opposition to Keystone XL and the public’s outrage over the Kalamazoo River spill, and they are now trying to sneak their New England project under the radar. By using an existing pipeline, they hope to quietly move ahead and skip permitting and environmental review. We cannot let this happen.

The pipeline they want to repurpose, the Portland-Montreal Pipe Line, crosses an international border, and just like Keystone XL, it must, therefore, be subject to a presidential permit.

The president must act now. 2012 was the hottest year on record.

We cannot afford to wait to take action on climate disruption, and we can’t accept toxic tar sands oil, which moves us backward in our effort to reduce carbon pollution.

The president knows this. Now he must say no to tar sands and the dangerous pipelines that Big Oil wants to run under our farms, rivers and lakes.

On Saturday, thousands of Mainers and New Englanders will rally and march in Portland to say no to ExxonMobil’s tar sands pipeline. We hope you will join us for the 11:30 a.m. rally in Monument Square, and then again in front of the White House on Feb. 17 to say no, once and for all, to tar sands.

Glen Brand is the director of Sierra Club’s Maine Chapter, and Bill McKibben is a founder of the grass-roots climate campaign