(Illustration found here.)

Via Climate Progress:

Elizabeth Kolbert is one of the most thoughtful climate journalists.
Her terrific 2006 book, ‘Field Notes from a Catastrophe: Man, Nature, and Climate Change,’ famously ends: “It may seem impossible to imagine that a technologically advanced society could choose, in essence, to destroy itself, but that is what we are now in the process of doing.”

And seven years later, Kolbert sounds another alarm noise in this piece in the current New Yorker on the recent notable landmark of the earth passing 400 ppm for the first time in human recorded history, and the-beyond-folly of okaying the Keystone XL pipeline:

When the milestone was passed, Keeling’s son Ralph, a geochemist at the Scripps Institution of Oceanography, glossed the event as follows: “It means we are quickly losing the possibility of keeping the climate below what people thought were possibly tolerable thresholds.”
Maureen Raymo, a marine geologist at the Lamont-Doherty Earth Observatory, was more blunt.
“It feels like the inevitable march toward disaster,” she told the Times.
…
If the arguments in favor of Keystone are persuasive, those against it are even stronger.
Tar-sands oil is not really oil, at least not in the conventional sense of the word.
It starts out as semi-solid and has to be either mined or literally melted out of the ground.
In either case, the process requires energy, which is provided by burning fossil fuels.
The result is that, for every barrel of tar-sands oil that’s extracted, significantly more carbon dioxide enters the air than for every barrel of ordinary crude—between twelve and twenty-three per cent more.
Alberta’s tar sands contain an estimated 1.7 trillion barrels of oil.
Assuming that only a tenth of that is recoverable, it’s still enough to generate something like twenty-two billion metric tons of carbon.
There are, it should be noted, plenty of other ways to produce twenty-two billion metric tons of carbon.
Consuming about a seventh of the world’s remaining accessible reserves of conventional oil would do it, as would combusting even a small fraction of the world’s remaining coal deposits. Which is just the point.
Were we to burn through all known fossil-fuel reserves, the results would be unimaginably bleak: major cities would be flooded out, a large portion of the world’s arable land would be transformed into deserts, and the oceans would be turned into liquid dead zones.
If we take the future at all seriously, which is to say as a time period that someone is going to have to live in, then we need to leave a big percentage of the planet’s coal and oil and natural gas in the ground.
These basic facts have been established for decades, and every President since George Bush senior has vowed to do something to avert catastrophe.
The numbers from Mauna Loa show that they have failed.
In rejecting Keystone, President Obama would not solve the underlying problem, which, as pipeline proponents correctly point out, is consumption.
Nor would he halt exploitation of the tar sands.
But he would put a brake on the process.
After all, if getting tar-sands oil to China were easy, the Canadians wouldn’t be applying so much pressure on the White House.
Once Keystone is built, there will be no putting the tar back in the sands.
The pipeline isn’t inevitable, and it shouldn’t be treated as such.
It’s just another step on the march to disaster.

Once the steps get quicker, the slope downward more pronounced, there won’t room/time for back-pedaling — boom!