An Enbridge pipeline rupture spilled more than 800,000 gallons of crude oil in Calhoun County, Mich., in July 2010. Here, crews cleaned up oil near booms and absorbent materials. (The Minnesota Star Tribune)

Last year, we witnessed a pivotal moment when President Obama rejected the Keystone XL pipeline, making good on his repeated observations that tar sands projects are not in our national interest because of their significant contribution to carbon pollution and climate change.

Not only that, but Secretary of State John Kerry said the State Department's analysis does not show the need for additional pipelines to transport tar sands oil across the Canadian border.

Scientists have made it clear that if we are to achieve a safe, sustainable climate future, we need to keep most fossil fuels in the ground. Because oil from tar sands is one of the dirtiest sources of energy produced, with environmental and climate impacts far more serious than conventional oil, this is the first source of fossil fuels we should reject altogether.

Permitting the Canadian oil giant Enbridge to construct a series of pipelines to carry this dirty fuel through our communities will only invite disaster. (It was reported last week that one Enbridge project, the Sandpiper pipeline in northern Minnesota, has been taken off the table for now, although routes elsewhere are being developed.) As too many communities are already aware, it's not a question of if a pipeline spill would occur, but when.

In July 2010, an Enbridge pipeline ruptured in Calhoun County in Michigan, dumping more than 1 million gallons of tar sands crude oil into Talmadge Creek, a tributary of the Kalamazoo River. The oil spread downstream, leaving 38 miles of contamination in its wake. After five years and an astounding $1.2 billion in cleanup costs, much of the mess in the Kalamazoo appears clean, but we still don't know the long-term harm from Enbridge's spill.

In order to transport tar sands crude, Enbridge and Canadian companies dilute the thick, tarlike substance, resulting in what is called diluted bitumen. This product is highly dangerous and poses a significant and severe threat to our public waters, our health and our climate.

Late last year, a National Academy of Sciences report described cleanup attempts of the diluted bitumen as "highly problematic" and said that the technologies available have only variable effectiveness. Unfortunately, this news doesn't help the residents of the more than 800 communities that already have experienced oil spills from Enbridge's operations, including the disaster near Kalamazoo.

The National Academy of Sciences study underlines the inherent danger surrounding the transportation of crude oil and lends further proof that transporting it by any means is a disaster waiting to happen.

Oil companies have argued that transporting oil by train is an even-riskier mode of transport, yet they are not interested in using expensive pipelines to replace the rail shipments. Rather, they want pipelines to increase their ability to transport more crude. As a result, putting in new pipelines simply increases the health, safety and environmental risks faced by towns and cities where either oil trains or pipelines cut through their communities. Oil trains and pipelines cut through towns and cities across the country, leaving us all to sit in fear of when the next catastrophe may occur.

Perhaps no state knows this better than Wisconsin, the superhighway of tar sands, which already has the world's largest tar sands pipeline outside of Russia.

Wisconsin and its most important waterways — including the St. Croix, the Wisconsin and the Rock rivers — now face the threat of a tar sands spill. It's the same danger that brought pollution, environmental harm and billion-dollar cleanup costs to communities along the Kalamazoo River and hundreds of other waterways and towns around the country.

The president was right to reject the Keystone XL pipeline, and Kerry was correct when he noted that the need for pipeline expansion between the U.S. and Canada doesn't exist.

Tar sands pipelines contribute to further climate disruption, and permitting any pipeline or pipeline expansion would be inconsistent with America's climate goals, the global pact set in Paris, and the commitment Obama and Canadian Prime Minister Justin Trudeau made to transition our countries off fossil fuels.

If we are to permit further expansion of pipelines, we will lock America into dirty fuels rather than continuing the transition to clean, renewable energy. As our energy portfolio changes with rapidly falling prices for wind, solar and other renewable-energy technologies, and with growing awareness of the urgency of addressing climate change, building new pipelines for tar sands would be a costly, long-lasting mistake for both environmental and economic reasons.

After strong commitments were made by the U.S. and virtually every other country during the Paris climate talks, it is important that the Obama administration hold these pipeline projects to the same standards it held Keystone XL, which will lead to rejection of every one.

Doing so is the only way to protect our climate, safeguard our waterways and communities, and ensure the health of our families and future generations.

John Marty is a member of the Minnesota Senate. Mark Miller is a member of the Wisconsin Senate. Both are Democrats.