In 2015 Paris Climate Agreement set out a political commitment by 195 members of the United Nations to keep the increase in global surface temperature to “well below 2 °C above pre-industrial levels and to pursue efforts to limit the temperature increase to 1.5 °C above preindustrial levels, recognizing that this would significantly reduce the risks and impacts of climate change”. To successfully restrict warming, to either 1.5 °C or 2 °C above pre-industrial, future emissions of greenhouse gasses such as carbon dioxide must be curbed. Global carbon dioxide emissions total 11 Gigatonnes of carbon per year. The question is: how much carbon dioxide can be emitted in the future and still keep warming within the 1.5 and 2.0 °C targets of the Paris Climate Agreement?
Current state-of-the-art climate models do not constrain the future carbon budget well: estimates for the carbon budget for 2 °C warming range from as little as 90 Giga-tonnes of carbon to as much as 580 Giga-tonnes of carbon, equivalent to between 8 and 50 years worth of carbon emissions at the current annual rate. This talk constrains the future carbon budget compatible with the Paris Climate Agreement, using constraints both from the observed climate changes during the historical period and geological records of climate change over the last 65 Million years. Based on these constraints, it is found that up to 200 Giga-tonnes of carbon can be emitted to remain under 1.5 °C warming, and up to 395 to 455 Gigatonnes of carbon to remain under 2 °C warming. At current emission rates 1.5 °C warming will be exceeded in 17 years time, and 2 °C warming in 35 to 41 years, so there is a limited window to develop a more carbon-efficient future and meet the Paris Climate Agreement.