In the ongoing debate over whether the Keystone XL pipeline should be approved, the argument has recently been turning on the axis of safety. Given the accident-prone tendencies of rail shipping, this reasoning goes, we should approve the pipeline to ensure a safer, more reliable method of oil transportation. It "has the potential to take 1 million barrels of crude oil off hazardous rail cars and out of our communities each day," writes Kristen Ferries in the Alabany Times Union. Ed Schulz endorsed similar logic in a recent segment on his MSNBC show.

On the narrow question, they are probably right: Pipelines, while by no means immune to spills, are probably the best way to transport huge quantities of oil. But this completely ignores the logic of climate change mitigation, which demands aggressive action to reduce carbon emissions now. If an argument is built on the premise that "oil is the major driver of our economy," as Schulz suggested in his segment, there is little chance of achieving meaningful action on climate change. In effect, we will have surrendered to catastrophe.

The catalyst to all this was a State Department report earlier this month finding that the approval or rejection of Keystone would not affect carbon emissions that much one way or the other, because the oil would just be transported by rail in the absence of Keystone. (This may or may not be right. Personally, I don't buy it.)

Here's the problem with this logic: The world is dashing headlong toward climate disaster. To stay below the internationally agreed upon 2 degrees Celsius of warming (itself quite a risky goal), we can release something like 1 trillion metric tons of carbon into the atmosphere, total. We've already released something like 550 billion metric tons, leaving roughly 450 billion metric tons as our remaining carbon budget.

Take a look at this post from three and a half years ago outlining decarbonization trajectories depending on when we reach peak emissions (the later the peak, the sharper the decline has to be). Needless to say, we are miles from these paths as a species. The U.S. has done somewhat better than anticipated, thanks largely to accidental factors like the natural gas boom, but we're not at all making the deeper structural reforms that will be required to get emissions down as far as they need to go.

Schulz and others in his camp adopt the label "realist," because it is indeed highly implausible to imagine any sort of climate bill getting through this Congress, let alone the kind of globe-spanning international agreement that would be needed to really tackle what seems to be an intractable problem. Therefore, they conclude, we must continue to operate our society on a carbon basis, with the vague hope that we'll do something about it later once the political climate is more favorable. (Perhaps after Miami is drowned?)

But we should be very clear about what this political "realism" implies. It means abandoning any hope of effective action on climate change. It means a fairly good chance of warming to 4 degrees Celsius and beyond, which, according to the climate scientist Kevin Anderson, "is incompatible with an organized global community, is likely to be beyond 'adaptation,' is devastating to the majority of ecosystems, and has a high probability of not being stable." One could use many words to describe this stance, but "hard-headed" or "pragmatic" would not be among them.

So from a scientifically realistic perspective, political realism is somewhere between cowardly and suicidal. From this perspective, the fact that oil transport by rail is dangerous is just one more reason to leave the rotten stuff in the ground, not an opportunity to find a slightly less dangerous way to shift it around. Because something like four-fifths of currently proven carbon reserves must not be burned if we are to stay under the 2 degree cap.

Of course, stopping Keystone XL is nowhere near enough to stop climate change by itself. It's on the menu, and it might help at the margins, so I say President Obama should stop it. But the real point is that when we consider our political stance, we must keep the iron laws of physics and chemistry in mind. Political realities can change – but science waits for no one.