Carbon budget over expenditure

The world is on a so-called all-time carbon budget, set at 2900 Gt of CO2. The definition [1] is as follows:

“A carbon budget can be defined as a tolerable quantity of greenhouse gas emissions that can be emitted in total over a specified time. The budget needs to be in line with what is scientifically required to keep global warming and thus climate change tolerable.”

Carbon budget over expenditure
Carbon budget over expenditure

‘Tolerable’ is widely regarded synonymous with a global temperature rise of 2°C and a target of 450 ppm CO2 in the atmosphere. If we stick to this carbon budget, we have a two-thirds chance of staying below 2°C [2].

I was wondering how much is left. How many years till we blow the 2°C Carbon Budget? And then what?

From 2011 onwards, the remaining CO2 to release was set at 565 Gt of CO2 [3]. On current trends [4], we will blow the global carbon budget and lock in more than 2°C of global warming by 2025.

Everyone agrees, the world will have to start following an emission reduction path soon. Question is, what is the most hopeful judgment about this path? Kevin Anderson and Alice Bows from the United Kingdom’s Tyndall Centre for Climate Change Research have set out the task we face [5]:

  1. Deforestation rates peak in 2015 and fall rapidly thereafter to half in 2040 and 0 by 2060.
  2. Emissions intensity of food production is halved with an extra 2 billion people to feed by 2050.
  3. Global emissions peak in 2020 and then decline by 3% each year, with energy emissions in rich countries falling by 6-7%.

If that path is taken by the world we will pump out an extra 3000 Gt of greenhouse gas. And there’s only about 400 Gt left in our carbon budget. Politicians might have thought, carbon budget? Budgets are there to overspend!

Carbon budget: even with the most optimistic assumptions about a global emission reduction path, we will end up overspending it by a whopping 90%.


  1. WWF publication ‘Understanding Carbon Budget’
  2. IPCC AR5 Synthesis Report SPM
  3. Source: Carbon tracker initiative 
  4. Source: Interactive by Duncan Clark for The Guardian (with current human emissions of 37 Gt per year, adjusted by 2% per year since the start of 2014 to account for estimated subsequent increases, based on PBL NEAA 2014)
  5. Kevin Anderson and Alice Bows, ‘Reframing the climate change challenge in light of post-2000 emission trends’, Philosophical Transactions of the Royal Society, Royal Society, 2008. Referred to in ‘Requiem For A Species’ by Clive Hamilton.

More reading about the carbon budget

Confused about the new IPCC’s carbon budget? So am I.

9 Simple Charts That Explain The Global Carbon Budget

Two degrees of warming closer than you may think