The Obama administration is unlikely to make a decision on the Canada-to-Nebraska Keystone XL pipeline until late this year as it painstakingly weighs the project's impact on the environment and on energy security, a U.S. official and analysts said on Friday.

The decision may not be made until November, December or even early 2014, said a U.S. official, as President Barack Obama will not rush the process, which still has a number of stages to work through. One of those stages has not even begun yet and will run for months.

"The president has to be able to show that the administration looked under every stone to ensure it knew as much as it possibly could about the impact of Keystone," said the official, who did not want to be named given the sensitive nature of the project.

For the first time in human history, the concentration of climate-warming carbon dioxide in the atmosphere has passed the milestone level of 400 parts per million (ppm). The last time so much greenhouse gas was in the air was several million years ago, when the Arctic was ice-free, savannah spread across the Sahara desert and sea level was up to 40 meters higher than today.

These conditions are expected to return in time, with devastating consequences for civilization, unless emissions of CO2 from the burning of coal, gas and oil are rapidly curtailed. But despite increasingly severe warnings from scientists and a major economic recession, global emissions have continued to soar unchecked.

"It is symbolic, a point to pause and think about where we have been and where we are going," said Professor Ralph Keeling, who oversees the measurements on a Hawaian volcano, which were begun by his father in 1958. "It's like turning 50: it's a wake up to what has been building up in front of us all along."

"The passing of this milestone is a significant reminder of the rapid rate at which – and the extent to which – we have increased the concentration of greenhouse gases in the atmosphere," said Prof Rajendra Pachauri, chair of the Intergovernmental Panel on Climate Change, which serves as science adviser to the world's governments. "At the beginning of industrialization the concentration of CO2 was just 280ppm. We must hope that the world crossing this milestone will bring about awareness of the scientific reality of climate change and how human society should deal with the challenge."

Canada threatened on Wednesday to take the European Union to the World Trade Organisation over its plans to label Canadian oil sands as dirty, but promised not to delay a bilateral trade pact.

The issue has overshadowed relations as Canada and the EU try to deepen economic ties through a trade deal that could generate $28 billion a year in new business and commerce.

Canadian Natural Resources Minister Joe Oliver, on a week-long lobbying trip to Europe, accused the EU of breaking international trade rules and discriminating against Canadian exports.

"We are going to take whatever action we need to, and we may well go to the WTO," Oliver told a news conference. "We will defend our interests vigorously."

Enbridge Inc, Canada's largest pipeline operator, said it shut its 210,000 barrel-per-day North Dakota pipeline for the second time in less than a week after finding contaminated soil during integrity checks.

Graham White, a spokesman for the company, said in an email the line was shut on Monday after the discovery. The company does not yet know when it will restart.

Enbridge estimates 2 barrels of oil leaked from the line.