One month ago, more than 100 North American climate scientists and I warned President Obama and Secretary Kerry that they should reject the proposed Keystone XL pipeline – indeed, that it would greaten the risk of dangerous and potentially irreversible climate changes.



Soon thereafter, the administration delayed its long awaited decision on the pipeline – and its insurance of decades of dirty tar-sands extraction, further rises in greenhouse gas levels, and greater warming of the planet – to review the mounting evidence of environmental impacts ... and, perhaps, to hold off until after the mid-term elections.

So why on earth is a group of US Senators – mostly Republicans, but a handful of Democrats, too – still trying to circumvent the approval process and double down on climate change-generating fossil fuels?

The measure on standalone Congressional approval – a last-ditch effort by Senators beholden to fossil-fuel interests and the Koch Brothers, or simply afraid of being targeted by them during their re-election bids – now looks doomed to fail by a couple of votes, but the effort remains mystifying: "Some of us who support it have a little trouble understanding why it's such a big deal," Democratic Senator Mary Landrieu of Lousiana said the other day.

So allow me to clarify, since the answers still haven't gotten through, no matter how many times we scientists repeat them.

Burning fossil fuels for energy over the past two centuries has now warmed the planet about 1 degree Celsius (about 1.5F), with at least another 0.5C of warming likely as global temperatures continue to rise in response to cumulative historical emissions. That leaves little wiggle room (about 0.5C) if we are to avoid crossing the 2-degrees Celsius warming mark deemed "dangerous" by many scientists studying the impacts of human-caused climate change.

Even the White House is rolling out the red carpet for the facts: the National Climate Assessment, prepared by hundreds of my colleagues, was unveiled on Tuesday morning, warning that climate change has moved "firmly into the present", as Obama makes his most ambitious climate push in months. The UN secretary-general, Ban-Ki Moon, has called for a summit to make serious progress: "If we do not take urgent action, all our plans for increased global prosperity and security will be undone," he said this week. "We can avert these risks if we take bold, decisive action now," he writes in another Guardian op-ed today.

Even the very conservative estimate of my climate scientist colleague Andrew Weaver, which by some gentle critiques leaves out extra fossil fuel emissions resulting from tar sands extraction, is dire: extracting and burning all of the Keystone-targeted oil would likely result in approximately 0.4C of additional warming. Add that to the observed 1C warming and the additional 0.5C committed warming, and we've only got about 0.1 degrees Celsius to spare before we hit that dangerous limit.

Indeed, given the underlying uncertainties, those estimates could well lock in 2C warming – if not more. This is why my colleague James Hansen has characterized approval of the pipeline as tantamount to "game over for the climate". This is why the Congressional shell game should end, on the Senate floor, right away.

To those elected officials who believe we should build the Keystone XL pipeline, I ask: Are you committed to keeping global warming below dangerous levels? If so, are you advocating for a moratorium on all other sources of fossil fuel energy? Are you ready for no more coal mining, no more natural gas extraction and no more oil drilling? Because that is what would likely be required if were to avoid truly dangerous changes to our climate and still approve the pipeline.

Keystone is not "a marginal thing", as pundits argued as recently as Tuesday morning in calling for a compromise. This is not a marginal issue, nor one for compromise.



When it comes to US energy policy, there is a worthy debate to be had about how we reduce our fossil fuel emissions while growing our economy and meeting our energy needs. What might be the role of natural gas and/or nuclear energy in the "bridge" we must build to a fossil fuel-free future? What instruments should we employ to price carbon emissions? Cap and trade? So-called "fee and dividend"? Or how about the revenue-neutral carbon tax favored by Republicans like former congressman Bob Inglis, former George W Bush speechwriter David Frum and former Reagan Secretary of State George Schultz?

Let us have that debate.

But building the Keystone XL pipeline simply makes no sense. It represents an investment in infrastructure that will lock in decades of extraction of dirty, expensive fossil fuels at a time when we need to be rapidly pivoting away from a fossil fuel-driven energy economy – as rapidly as possible.

I doubt that any of the remaining Keystone supporters in the Senate, Republican or Democrat, want their legacy to be a planet that they have fundamentally degraded for future generations. But that's what a "yes" vote on this week's vote will mean: the beginning of the end.

