What To Know About Carbon Budgets, & How Much Carbon We Can Emit

What To Know About Carbon Budgets, & How Much More Carbon Can We Emit?

Carbon budgets are a way of expressing the remaining carbon dioxide that can be emitted before we reach a certain level of warming. 

There’s both pros and cons to this approach.

In this guide, we look at what a carbon budget is in further depth, and potential limitation on this concept. 


Summary – Carbon Budgets

The amount of carbon dioxide or total greenhouse gases that can be emitted before we think we hit a certain level of climate warming is called the carbon budget

Current carbon budgets are based in part on warming targets such as 1.5 and 2 degrees warming above pre industrial levels

Some figures show we have already exceeded more than 50% of the original carbon budget (which was set at 1 trillion tonnes of CO2)

Some studies show that if we continue at current emission rates without any intervention, we are on track to exhaust the budget by the year 2045, and at that point we will be exceeding 2 degrees warming. To stay within the budget, global emissions have to peak by 2020 and decrease steeply each year after

Other studies show that if we are able to decrease CO2 emissions to zero by the year 2080, we might avoid 2℃ of global temperature increase, but would pass 1.5℃ before the year 2040

The thing to note about a ‘climate budget’ though is that it’s an imperfect concept, developed using data that can change, and carries some degree of uncertainty in various ways

For example, the IPCC changed it’s estimate of the amount of years of current emissions we can continue with in order to avoid 1.5C warming from one report to another from 3 years to 10 years

In reality, there could be a range of different carbon budgets when different variables and considerations are taken into account. The carbon budget can change with new information and assumptions

Some reports indicate that if we want to increase our chances of limiting warming to below 2 degrees celcius, we would have to leave two thirds to 80% of fossil fuel in the ground

Something else that makes the idea of a carbon budget more complex is that different countries have different economies, and have different total, and per capita emission numbers. It may be difficult to assign carbon budgets to different countries, or to convert carbon budgets into individual carbon footprints for individuals, companies, cities and so on


What Is The Carbon Budget?

A carbon budget is the theoretical amount of future carbon emissions can be emitted (or to put another way – that can’t be exceeded) if we want to stay under a certain level of global climate warming (At the moment, it is thought that it’s very unlikely we can still stay under 1.5 degrees total warming, so, a 2 degree carbon budget is often the main discussion point.)


  • The carbon budget is the amount of carbon dioxide emissions we can emit while still having a likely chance of limiting global temperature rise to 2 degrees celcius above pre-industrial levels.
  • The budget [was around] 1 trillion tonnes of CO2 [when it was developed]
  • As of 2011, we’ve already gone through 52% of the budget

– wri.org


So, How Much More Carbon Can We Emit To Limit Global Warming To 2 Degrees Celcius Above Pre Industrial Levels?

  • 48% of 1 trillion tonnes is 485PgC of carbon – this is how much we have left in the budget as of 2011
  • If we continue at current emission rates without any intervention, we are on track to exhaust the budget by the year 2045
  • To stay within the budget, global emissions have to peak by 2020 and decrease steeply each year after


  • If emissions continue at their current rate, the world is on track to exceed the carbon budget (for 2 degrees) by around the year 2045
  • The general consensus is that it’s extremely unlikely we can still limit warming to 1.5 degrees
  • The general consensus is that we can still limit warming to 2 degrees, although it’s going to require very ambitious and immediate action from all countries to reduce emissions. 

– wri.org

Check out the full infographic here – https://www.wri.org/resources/data-visualizations/infographic-global-carbon-budget


  • Based on estimates made in the IPCC’s fifth assessment report (AR5), there would be around 120 gigatonnes of CO2 (GtCO2) remaining from the beginning of 2018 – or around three years of current emissions – for a 66% chance of avoiding 1.5C warming. For a 50/50 chance of exceeding 1.5C, the remaining budget was a modestly larger 268GtCO2 – or around seven years of current emissions.
  • The IPCC’s new SR15 significantly revises these numbers. It raises the budget for a 66% of avoiding 1.5C to 420GtCO2 – or 10 years of current emissions. Similarly, the budget for a 50/50 chance of exceeding 1.5C is increased to 580GtCO2 – 14 years of current emissions.

– carbonbrief.org


  • If we are able to decrease CO2 emissions to zero by the year 2080, we might avoid 2℃ of global temperature increase, but would pass 1.5℃ before the year 2040. 

– theconversation.com


Although very unlikely, some people are still optimistic about 1.5 degrees warming:

  • … global CO2 emissions need to fall to net-zero by mid-century to avoid 1.5C of warming.

– carbonbrief.org


What Is The Expected Impact On The World At 2 Degrees Global Warming?

Read more in this guide:

There’s more information on the impact at 2 degrees in this infographic – https://www.wri.org/resources/data-visualizations/infographic-global-carbon-budget 

It’s also worth noting that the recent Cape Town water shortage caused mainly by a severe drought, was made up to 3 times more likely a risk because of global warming (read more at http://theconversation.com/global-warming-has-already-raised-the-risk-of-more-severe-droughts-in-cape-town-107625). With a further warming up to 2 degrees, this risk increases another 3 fold.


What Are The Emissions Pathways To Other Warming Scenarios Other Than 2 Degrees?

Read more in this guide:


But, … Here Is The Truth To Know About Carbon Budgets, & Figuring Out How Much More Carbon We Can Really Emit In The Future

Coming up with a neat figure about how much more carbon we can emit sounds great – but, we should know that the models and calculations used for these estimations are far from perfect.

If anything, they should be viewed as a very, very rough estimate only, or as a starting point for discussion.

There are still gaps and uncertainties in the data we base the calculations on, and the models we use to forecast future emission and warming scenarios.

Some scientists and sources say we may even be underestimating how much carbon we have left in the budget, and we can emit a lot more before 2 degrees is reached.

There’s several reasons for this, such as:


This is also evidenced by the expanded carbon budget in the newest IPCC report:

  • The newly published Intergovernmental Panel on Climate Change’s special report on 1.5C (SR15) significantly expands the budget for a 66% chance of avoiding 1.5C to the equivalent of 10 years of current emissions.
  • This compares to the IPCC’s fifth assessment report (AR5), which put it at around three years.

– carbonbrief.org


  • It is important to keep in mind that the global ‘carbon budget’ [is really a simplification]. … there is actually a variety of possible carbon budgets, and their size depends on a number of factors such as: the probability of staying below our two-degree warming target, the rates of decarbonization, and the contribution of non-COgreenhouse gases.

– ourworldindata.org


Read more about calculating the carbon budget at https://www.wri.org/blog/2018/10/according-new-ipcc-report-world-track-exceed-its-carbon-budget-12-years 


What’s The Best Approach To Managing Carbon Emissions Then?

We still would want to pay attention to carbon budgets provided to us as a rough indicator.

Paying attention to future emission scenarios and various emission pathways would also be wise.

But, the plan still should be to (ideally):

  • Decrease human caused carbon emissions (mainly from burning fossil fuels) to zero as soon as possible
  • Sequester and absorb existing carbon from the atmosphere to the best of our ability

Uncertainty and rough/over simplified carbon budget estimates don’t remove risk associated with a warming planet.

It’s about covering the potential risk of the consequences/impact that further warming might bring – especially when talking about getting to the point where it doesn’t matter how much we reduce emissions, as natural feedback processes continually amplify the warming already set in motion.


How Much Fossil Fuels Can We Burn To Stay Within Our Carbon Budget?

  • If the world burned all of its currently known reserves [of fossil fuels] (without the use of carbon capture and storage technology), we would emit a total of nearly 750 billion tonnes of carbon.
  • This means that we have to leave around two-thirds of known reserves in the ground if we want to meet our global climate targets
  • … if we wanted to increase the probability of keeping warming below two degrees celsius to 80 percent, we would need stricter carbon limits, and would have to leave 75-80 percent of fossil fuels untouched

– ourworldindata.org


Comprehensive Guide On Climate Change

You can read a more comprehensive and complete guide on climate change here.



1. https://www.carbonbrief.org/analysis-how-much-carbon-budget-is-left-to-limit-global-warming-to-1-5c

2. https://www.carbonbrief.org/analysis-why-the-ipcc-1-5c-report-expanded-the-carbon-budget

3. https://www.carbontracker.org/carbon-budgets-explained/

4. https://www.scientificamerican.com/article/how-the-carbon-budget-is-causing-problems/

5. http://shrinkthatfootprint.com/carbon-targets-for-your-footprint 

6. https://www.weforum.org/agenda/2018/10/according-to-new-ipcc-report-the-world-is-on-track-to-exceed-its-carbon-budget-in-12-years/ 

7. https://www.bettermeetsreality.com/what-is-a-sustainable-carbon-footprint-per-person-to-aim-for/ 

8. https://ourworldindata.org/how-long-before-we-run-out-of-fossil-fuels 

9. https://www.wri.org/resources/data-visualizations/infographic-global-carbon-budget

10. http://theconversation.com/global-warming-has-already-raised-the-risk-of-more-severe-droughts-in-cape-town-107625

11. https://www.wri.org/ipcc-infographics

12. https://skepticalscience.com/how-much-15C-budget-left.html

13. https://e360.yale.edu/features/paris-conundrum-how-to-know-how-much-carbon-is-being-emitted

14. https://www.cicero.oslo.no/no/posts/klima/how-much-carbon-dioxide-can-we-emit (lists the 4 main choices someone must make in choosing a carbon budget, and the importance of negative emissions)

15. https://blog.ucsusa.org/brenda-ekwurzel/what-is-carbon-budget-ipcc2018

16. https://theconversation.com/the-climate-clock-counting-down-to-1-5-107498

17. https://www.wri.org/blog/2018/10/according-new-ipcc-report-world-track-exceed-its-carbon-budget-12-years

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