Illustration by Tom Bachtell

A lot of what’s known about carbon dioxide in the atmosphere can be traced back to a chemist named Charles David Keeling, who, in 1958, persuaded the U.S. Weather Bureau to install a set of monitoring devices at its Mauna Loa observatory, on the island of Hawaii. By the nineteen-fifties, it was well understood that, thanks to the burning of fossil fuels, humans were adding vast amounts of carbon to the air. But the prevailing view was that this wouldn’t much matter, since the oceans would suck most of it out again. Keeling thought that it would be prudent to find out if that was, in fact, the case. The setup on Mauna Loa soon showed that it was not.

Carbon-dioxide levels have been monitored at the observatory ever since, and they’ve exhibited a pattern that started out as terrifying and may be now described as terrifyingly predictable. They have increased every year, and earlier this month they reached the milestone of four hundred parts per million. No one knows exactly when CO 2 levels were last this high; the best guess is the mid-Pliocene, about three million years ago. At that point, summertime temperatures in the Arctic were fourteen degrees warmer than they are now and sea levels were some seventy-five feet higher.

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.

President Obama will make a decision in the next few months—unless he puts it off again, as he did in 2011—about whether to approve the Keystone XL pipeline. The question before him is whether it’s in the “national interest” to grant the permits needed for constructing Keystone, which is supposed to dogleg from Alberta to Nebraska, and join a pipeline that will extend to Texas, connecting Canada’s tar-sands deposits with American refineries. The latest figures from Mauna Loa reveal what’s at stake.

Last week, as the President was otherwise engaged—with the uproar over the I.R.S., the Justice Department’s subpoena of phone records from the Associated Press, and the e-mails about the attack on the American consulate in Benghazi—lobbying for the pipeline reached a new level of intensity. At the start of the week, the Canadian government launched an ad campaign to build support for the pipeline in the U.S. One ad, featuring construction workers fitting sections of pipe, says, “America and Canada: Standing together for energy independence.” Although the Canadians have not released the cost of the campaign, the Globe and Mail reported that Canada’s natural-resources department has set aside more than sixteen million dollars for advertising this year. (Canada’s natural-resources minister, Joe Oliver, recently travelled to France and England to push the tar sands; he ended up threatening the European Union, which is considering labelling tar-sands oil as “highly polluting,” with taking the case to the World Trade Organization.) Then, at the end of the week, Canada’s Prime Minister, Stephen Harper, came to New York to make the pitch himself. “All the facts are overwhelmingly on the side of approval,” he said, at the Council on Foreign Relations. With a touch of menace, he added, “I know the Administration will do a thorough analysis before arriving at the right decision.”

The arguments in favor of Keystone run more or less like this: Americans use a lot of oil—more than eighteen million barrels per day. It has to come from somewhere, and Canada is a more reliable trading partner than, say, Iraq. The U.S. already imports roughly a million barrels of Canadian tar-sands oil a day, and if it doesn’t import the rest some other country will. “It’s overwhelmingly likely the oil would find another way to market,” USA Today observed in a recent editorial. For instance, a pipeline could be built to British Columbia, and the oil shipped from there to China, though there are many political and logistic barriers to such a plan—among them the Canadian Rockies.

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. ♦