Carbon Budget

McKibbon explains in Chapter 1 of Eaarth that the level of carbon dioxide in the atmosphere during the period of climate stability that fostered the rise of human civilization was 275 parts per million (ppm) (p.13).   According to James Hansen, the "safe" number, allowing for some warming but not globally devastating, would be 350 ppm.  (For this reason McKibbon and five of his undergraduate students formed 350.org, one of leading environmental groups addressing climate change.)
When McKibbon wrote Eaarth in 2009 the planet was approaching 390.  Now we are over 400. (See the chart from the National Oceanic and Atmospheric Administration.)
 Nature magazine has said that above 350 we "threaten the ecological life-support systems... and severely challenge the viability of contemporary human societies." McKibbon points out that 20 million years ago, when levels were over 350 the oceans were 100 feet higher and temperatures rose 10 degrees C. The Zoological Society of London has said that at 360 coral reefs cease to be viable. (Eaarth, 17).

Taking account of the fact that a portion of the carbon emitted is removed by oceans (becoming more acidic) and forests (that are not cut down or destroyed by fire or pests, the "carbon budget" is how much carbon can be emitted and still have a likelihood of keeping global warming at 2 degrees C.  (Total cumulative emissions from 1870 to 2014 were 400±20 GtC from fossil fuels and cement, and 145±50 from land use change. This total of 545±55 GtC was partitioned among the atmosphere (230±5 GtC), ocean (155±20 GtC), and the land (160±60 GtC). See the Global Carbon Project website.)

The Intergovernmental Panel on Climate Change (IPCC) optimistically sets that amount at 1,000 GT (or, more specifically, one trillion tons of equivalent carbon dioxide which they believe will give the world a 66% chance of staying below 2 degrees C. (Their number does not take into account emissions of methane and other gasses so, in fact, even to stay to their optimistic calculations the "carbon budget" should be corrected to 800 billion tons.)  Humans have already put 545 billion tons of CO2 into the atmosphere, so we have used up well more than half of the total available budget. (These numbers do not include natural feedback loops, such as greater methane (5 times more powerful greenhouse gas than CO2) being released from warming permafrost or seabeds.)  One critical question avoided in the Paris Agreement is who gets to use up the carbon still allowable?  While the numbers combine hard science and guess work, it is clear that there are far greater known reserves of coal, oil, and gas than human beings can use.  As much as possible carbon needs to stay in the ground and we need to transition to a non-carbon based economy as fast as possible.