The History and Importance of 2C and Carbon Budgets

Five years ago two key concepts were adopted that now serve as the framework for efforts to combat climate change. In 2010 following the Durban climate conference in 2009, carbon budgets and the 2 degree Celsius threshold gained widespread acceptance. Historians will look back on this year as a crucial time in the evolution of international efforts to combat climate change.

The notion of an upper threshold limit dates back to at least the 1970s and the work of Yale economist William Nordhaus. One of the first public statements about 2 degree Celsius (3.6 degrees Fahrenheit) came in1990 when the Stockholm Environmental Institute came forward with a statement that 2C should be taken as the level at which dangerous warming would occur.

The first political body to declare that 2C was the goal, was the 1996 European Council of environment ministers. However more than a decade would elapse before the limit was widely embraced. UN Climate Talks began warming up to the idea in 2009 and then at the Cancun climate talks in 2010 countries within the UFCCC formally agreed to, “hold the increase in global average temperatures below 2C above pre-industrial levels.”

As we hover around 400 ppm for the first time in recorded history, we are forced to realize that we are way beyond 350ppm level deemed safe by scientists. We are more than halfway towards our the upper threshold limit of 2C and if we continue along our current trajectory we will see temperature increases of around 6 degrees Celsius (7.2 degrees F). Even if we implement current nationally determined contributions INDCs we will see warming of at least 2.7 C.

A carbon budget represents the amount of CO2 than can be burned to keep temperatures from rising beyond the 2C acceptable limit. The general concept was first introduced in 2009 by a group known as the Nobel Laureates Symposium. This group of scientists, including 20 Nobel Prize winners, signed a memorandum calling for a carbon budget that set limits on global emissions for 2020 and 2050.

We are eating our way through our carbon budget and if we continue with business as usual we will far exceed our carbon budget. According to a University College London’s (UCL) study, 82 percent of all known fossil fuel reserves must be left underground to stay within our carbon budget.

Since the dawn of the industrial revolution we have emitted around 531 billion metric tons of carbon from the burning of fossil fuels, cutting down forests and making cement. A 2013 UN report estimated that we can burn no more than a total of 840 billion tons of carbon.

Failure to reduce emissions and keep temperatures below 2 degrees threaten a wide range of climate impacts including sea level rise, extreme weather, biodiversity loss, deforestation and ocean acidification. 

“This [UN study] gives a clear scientific basis for a carbon budget and an indication of how much carbon we have already used and how much we have left before we start breaking planetary boundaries,” said Samantha Smith, head of the climate and energy program of environmental group WWF. “This has gone from being an interesting piece of modeling to being something that now has solid foundation in the IPCC report.”

The concepts of a carbon budget and the 2C threshold are now instrumental to science based efforts to combat climate change.