SkS Analogy 24 - Atmospheric Carbon Loans

Building or borrowing from our children’s future?

Borrowing money to finance one’s lifestyle is unsustainable. But debt can be an important component of establishing personal, corporate, or civil foundations. Debt is a temporary financial vehicle to be replaced by sustainable income to either manage the debt at some level, or to pay it back. Continued borrowing to finance an established lifestyle, corporation, or country is unwise and can lead to bankruptcy.

We enjoy a phenomenal, modern lifestyle, because over millions of years Earth prepared for us ample supplies of readily available natural supplies that we’ve used to establish the foundations of modern civilization. But just as we transition from temporary loans to sustainable income to finance our personal, corporate, and civil financial systems, so too, we must transition our energy systems away from fossil fuels to sustainable systems that do not alter the ecosystems on which we depend. Although fossil fuels played a fundamental role in establishing modern society, their use comes with a price.

Let’s illustrate this situation by alluding to a carbon loan, where we define a single carbon loan as the fossil-fuel consumption that leads to a 50 ppm CO2 build-up in the atmosphere.1 Part of the terms of the loan is a promise that we will remove the borrowed 50-ppm CO2 within 30 years of the end of the loan. The end of the loan is defined as the point at which we’ve added 50 ppm CO2 to the atmosphere. Repaying the loan means removing the carbon from the atmosphere using proper agricultural management (such as planting trees that absorb carbon) and Negative Emissions Technologies (NET).

The penalty for defaulting on the loan and not removing the borrowed 50-ppm from the atmosphere is that the temperature of the Earth rises by 0.5°C. Temperature rise triggers additional penalties (i.e., carbon feedbacks), manifested as additional CO2 that we must either remove, or allow to further raise Earth’s temperature.

Temperature rise associated with defaulted carbon loans causes climate change, sea level rise, and other detrimental effects, all of which undo the goal of the loan of building a solid foundation.

The carbon we put into the atmosphere is therefore like a loan with a balloon payment after many years: the costs in the early years of the loan are small, and become large towards the end of the loan. This is a simplification of reality, but conveys the essence of the delay between cause and effect, and emphasizes that the real costs of a carbon loan are not realized until much later than when we receive the benefits.

Consider the following timeline of our carbon loans, with the year and associated atmospheric CO2 concentration. Assuming an Equilibrium Climate Sensitivity (ECS) of 3°C/doubling CO2, and assuming that CO2 continues increasing into the near future at the 2020 rate of about 2.5 ppm/yr:

1820: 280 ppm CO2. Start of the Industrial Revolution.   
1910: 300 ppm: Assume the first 20 ppm CO2 was a gift and not a loan to be repaid.  
1985: 350 ppm: 1st 50 ppm loan received in full; balloon payment due in 2015.
2015: 400 ppm: 2nd 50 ppm loan received in full. First loan in default, Earth warmed to 1°C.
2020: 415 ppm: 1/3 of 3rd 50-ppm loan already received.
2035: 450 ppm: 3rd 50-ppm loan received in full.2
2045: Balloon payment for 2nd loan due. Default means warming to 1.5°C.
2065: Balloon payment for 3rd loan due. Default means warming to 2.0°C.

To keep the warming to 1.5°C or below, in accordance with the 2015 Paris Accord, planners assume we will take out a third carbon loan, that we will default on the second loan, but that using NET we will pay back the third loan before its balloon payment is due. We are therefore accepting our inability to pay back the second loan, and are settling for the best we can do is to limit warming to 1.5°C. Is this realistic?

NET is the technology that we’re relying on to make our balloon payments for the carbon loans. The plan is that we get the benefit of burning fossil fuels now, accept the CO2 buildup from the associated emissions, and use part of the fossil-fuel energy to build systems that we will use to suck carbon out of the atmosphere before the balloon payment is due. This plan assumes ...

Considering that we are making plans now for how we will emit and then remove carbon over the next 50 years or so, we further assume that nature does not emit carbon in response to the warming we’ve caused that overwhelms our efforts to pay back our carbon debt.

A sustainable method for running society is to use fossil fuels only to establish a strong foundation, and then to switch to sustainable, renewable-energy sources. This still leaves us with the environmental penalties of the use of limited fossil fuels. Because a modern society will continue to emit some carbon into the atmosphere, we should only use small carbon loans to build a resilient, financially stable system, and then use part of our wealth to operate NET and other systems to repay the carbon loans.

This is how it should work if we want to maintain a stable climate and sea level.


1. Earth reabsorbs about 50% our carbon emissions. This is why we define the carbon loan as that equivalent to a 50 ppm CO2 buildup in the atmosphere, and why we do not equate the loan directly to CO2 emissions.
2. This is a Business-As-Usual projection, with CO2 accumulating between 2021 and 2040 at about the same rate as from 2015 to 2021. Depending on actions taken by countries to curb CO2 emissions, the date when this loan is received in full could be pushed back. On the other hand, if countries do not follow through on their commitments, the date when this loan is received in full could be brought forward.

Posted by Evan on Tuesday, 26 October, 2021

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