The math behind the chart starts with the “carbon budget“:
Noting that the relationship between the amount of carbon dioxide we put in the atmosphere and the eventual global temperature is “near linear,” the United Nations’ Intergovernmental Panel on Climate Change calculated the maximum amount the world could emit for a one-third, 50 percent or two-thirds chance of keeping warming below two degrees.
The resulting headline: As of 2011, the world had about 1,000 gigatons, or billion metric tons, of carbon dioxide left to emit in order to have a two-thirds or greater chance of staying below two degrees. After that, net emissions must go to zero.
From here you simply do the math. Energy-related carbon dioxide emissions alone were 32.3 gigatons last year, according to the International Energy Agency, and that does not include other sources, such as deforestation. Based on such numbers, the remaining carbon budget is already under 900 gigatons of carbon dioxide. . .
once you take future deforestation and cement-related emissions between now and 2100 into account, the remaining budget is just 650 gigatons of energy-related emissions. That’s about 20 years at current rates — but emissions are still rising. That trend is currently expected to continue out to 2025 or 2030, despite countries’ recent carbon-cutting pledges, in large part because of growing demand for energy in coming decades.
The UNFCCC recently acknowledged that these pledges, on their own, would hold warming to perhaps only 2.7 degrees Celsius — other analyses are still more pessimistic — and, therefore, that much more must be done in Paris and beyond to ensure attainment of the two-degree goal.
And here’s a link to Charles Ferguson’s new film, Time to Choose, which readers can view for the next 48 hours.