What’s a carbon budget & how defining carbon footprint?

Like for any budget discussion, the intent of a Carbon Budget could be summarized into a question “Can I afford it ?”. And if the answer is “Well, not really but what if I loan ?”, then comes one another question. What will be the end costs and who will bear them…?

The notion of carbon budget is centric when we are dealing with carbon emissions regulation. It can be defined at Earth level, by industrial sectors or even per habitant.  Let’s explore what’s inside the Earth carbon balance sheet and see what is left….

Carbon concentration follows anthropic activity 

While carbon concentration used to cyclicly vary on Earth, the current value and growth rate has no precedent.

Evolution of Carbon dioxide concentration in the atmosphere over past 800 000 years (Note 1)

Second remark, the correlation with human industrial activities can’t be contested. The step change of the CO2 atmospheric concentration curve clearly coincides with the age of industrial revolution as shown on graph below.

Carbon dioxide concentration in the atmosphere and annual emissions since 1750 (Note 1)

As soon as the potential of fossil reserves has been discovered, they never stopped being extracted. One adding to (and not replacing!) the others, the extraction & use of coal, oil and gas kept increasing to fuel the expansion of human activities. By consequence, the amount of Green House Gases (GHG) released into the atmosphere followed the same trend.

For the following portions of that post, only carbon dioxide will be cited as the GHG contributor given the cited references are translating all GHG mass into equivalent mass of CO2 using its respective GWP (Global Warming Potential) factor.

Like for a bathtub, It’s all about flow !

The carbon dioxide concentration within the atmosphere follows a simple math rule. Anything which is released in excess of what the Earth carbon sinks can absorb stays in the atmosphere. And for a while…. As for any oxide, the CO2 has a strong stability in the atmosphere; it can stay over-there for over 100 years.

Between 2010 & 2019, an average net plus mass of 18,7 Gt of CO2e was released anualy into the atmosphere which account for 47% of total anthropic carbon emissions. In other words, the carbon sinks were able to “flush” only 53% of what the human activity emitted.

Green House Gases flux – Emissions and sinks. (Note 2)

Average temperature on Earth follows the C02 concentration trends

Considering the carbon dioxide gas is a powerful heat trapper, it looks evident that the average temperature on Earth follows its concentration with a direct correlation (as per below curve)

Correlation between average Earth surface temperature and CO2 atmospheric concentration. (Note 1)

Getting Back to budget

As we now understand the trapping performance of the carbon sinks, defining a “Not To Exceed” average temperature on Earth at a given date can be translated into a “Not To Exceed” Carbon emissions budget. That budget can then be declined for different temperature rise objectives compared to pre-industrial levels (commonly defined as 1850s). With that regard, the below chart provides 3 scenarios: +1,5°C / +1,75°C / +2°C along with a probability of success expressed in %.

Carbon Budget applicable from 2018 for 3 different scenario of average temperature rise by 2100 (compared to 1850) (Note 3)

At current anthropic CO2 emissions pace (40 Gt per year), the + 1,75°C state would be achieved within 20 years and the +2°C in about 30 years. As a reminder, the objective of the Paris agreement is to limit by 2100 the temperature increase at +1,5°C compared to 1850 levels. The budget for that objective is below 600 Gt. At current emissions levels, it would be consumed by 2033. World is currently running over its carbon budget….

Rodmap to Net Zero carbon footprint and factor 4.

Net Zero means limiting the amount of annual carbon emissions to strictly what the Earth Carbon sinks can absorb : about 22 Gt. In COPs meetings, the United Nation Climat Change Committee has set the target to achieve Net Zero by 2050. That represents cutting the 2018 world wide carbon emissions level by 2.

European Omission recently (July 14th 2021)committed to reduce by 55% the 1990s GHG emissions level by 2030 (https://ec.europa.eu/clima/policies/eu-climate-action/2030_ctp_en) to ensure Carbon neutrality by 2050.

In 1995, the Club de Rome Report introduced the “Factor 4” notion setting the target for any developed country to cut their use of ressource by 4. Given emergent countries will inevitably increase their emissions through their development, the developed countries shall make an extra effort to reduce their emissions by a factor 4 to ensure an overall world wide reduction of 50% by 2050.

To ensure Carbon neutrality by 2050 (and assuming a 10 billion inhabitants on Earth), the carbon budget per capita shall not exceed 2 tons of CO2 by 2050. To set some orders of magnitude (and an idea of the efforts to accomplish), an European currently emits about 10 tons of CO2 per year (domestic and imported emissions). This goes over 16 tons for an American citizen.

The gap to fill is huge and relying solely on technological efficiency gains won’t clearly suffice. Adapting our modern societies toward more sobriety is quickly required.

The price of the carbon debt goes beyond charts, figures & graphs, it deals with the urgency to shift mindset.


Note 1: Source NOAA (https://www.noaa.gov/)

Note 2: Source Global Carbon Projet (https://www.globalcarbonproject.org/)

Note 3: Source Intergovernmental Panel on Climate Change (https://www.ipcc.ch/)


2 thoughts on “What’s a carbon budget & how defining carbon footprint?”

  1. Carbon budget is a key point to stay below the +1.5°C, maximum 2°C. But it is tricky as well as the atmosphere doesn’t matter where does CO2 come from. Should be we consider an annual budget per capita ? Should we consider a remaining budget per sector ? Should we consider the remaining budget per sector from 2019 considering the percentage of all sector will remain the same? Or from the 1990’s (life was cool without contrails in the sky) Or from the birth of this sector ? Is it reasonnable to keep the same prorata between sectors (we can consider Aviation is mainly for fun while agriculture is mainly to survive) ?
    All these questions must be probably taken into consideration and I want to believe a such smart and advanced sector as Aviation will show the way of the Sobriety.

    1. Those are all good questions as there are many ways to define carbon budgets. Doing it by nation is opening a tricky debate on why countries under development would limit their emissions while developed ones have benefited of no regulated world for decades to build their wealth.
      Defining carbon budget per sector (heavy industry, transport, agriculture, etc) has the advantage to mobilize the whole value chain of an industry into a common goal.
      Cutting the carbon budget cake into sector pieces can be done in different ways but the fairest one appears to divide the remaining amount of carbon emissions to stay below 2°C in 2100 by the respective carbon foot print of each sector. That means spreading the efforts to all actors with the same extent without considering one sector is prevalent to one another…
      The question of how defining and declining a carbon budget for the commercial aviation will be detailed into an Aero post in August!
      More to come…

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