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.