The growing carbon debt

Why the climate change is different from other environmental challenges

We’ve long talked about carbon budgets – how much emissions are allowable before the world passes various temperature targets. But given that the world is on track to pass the 1.5C target in the coming decade and will pass 2C later this century unless we start reducing global emissions soon, the idea of a “carbon budget” is becoming increasingly outdated.

Rather, its time to start talking about the carbon debt, the amount of carbon that will have to be removed by our children and future generations if we ever want to return to the climate of our past.

The pernicious persistence of carbon

We often see climate change through the lens of past challenges of local environmental pollution. But climate as a problem is fundamentally different. Conventional air pollution can be largely solved by stopping emissions. For example, the acid rain resulting from sulfur emissions of coal fired powerplants that degraded forests and rivers in the 1980s was mostly eliminated by using sulfur scrubbers on powerplants in the 1990s and 2000s. The dramatic improvements to air and water quality after the passage of the Clean Air Act and Clean Water Act in the 1970s were largely achieved through reductions in the rate of emissions of conventional pollutants.

These conventional pollutants represent “flow” problems: the severity of the problem is a function of the rate of emissions, and the solution is to reduce emissions to zero – or at least to a low enough level that the costs to society are minimized.

By contrast, climate change is primarily a “stock” problem. Carbon dioxide – the primary driver of climate change – accumulates in the atmosphere where it lasts for an extremely long time. While about half of our emissions are removed by land and ocean carbon sinks over the first century after release, the remainder will remain for many millennia. It takes on the order of 400,000 years for nature to fully remove a ton of CO2 released today.

As Nature aptly put it in a 2008 article, “Carbon is Forever”. But it turns out that the warming from our CO2 emissions is also extremely long lived.

In 2009, Susan Soloman and colleagues published a paper titled Irreversible climate change due to carbon dioxide emissions. They found that even if global CO2 emissions ceased and atmospheric CO2 concentrations began to decline, the warming from those emissions would remain for millennia. This phenomena – a result of continued warming as oceans continue to absorb heat until they reach thermal equilibrium with the atmosphere counterbalancing falling atmospheric CO2 concentrations – also turns up in our most state-of-the-art climate models today.

The implications of this are stark and broadly under-appreciated outside of the climate science community. As Myles Allen and colleagues pointed out in 2009, the warming the world experiences is in essence a largely time-invariant function of our cumulative CO2 emissions. In other words, even if we stopped all emissions today, the world would not cool down substantially for many millennia to come. And if we ever want to bring global temperatures back down from what ever levels they reach when we finally get to (net) zero CO2 emissions, we need to actively reduce the amount of CO2 in the atmosphere by taking the we’ve previously emitted CO2 back out.

This is our carbon debt.

The carbon debt

It is critical for society to get to net-zero CO2 emissions to stop the world from continuing to warm (and to strongly reduce non-CO2 GHGs to counterbalance warming from falling aerosol emissions as we phase out fossil fuels).¹ But by the time we get to that point we will have blown well past 1.5C, and leave a world of higher temperatures, deadly heatwaves, catastrophic wildfires, sea level rise, stronger storms, and a slew of other persistent impacts to our children, their children, and countless others over the next few millennia.

We are passing down our carbon debt to future generations that will have to be paid if they ever want to recover the climate of the past that shaped both the natural world and the development of human civilization. And increasingly, we are normalizing this carbon debt in our models and mitigation strategies.

For example, nearly all integrated assessment models (IAMs) that limit warming to 1.5C or 2C by the end of the century exceed the remaining “carbon budget” by a large amount – around 600 GtCO2 – by relying on carbon dioxide removal (CDR) technologies to remove vast amounts of CO2 from the atmosphere later in the century. This CDR is used both to deal with some residual CO2 emissions that are deemed to difficult or expensive to mitigate, but also to deal with overshoot (particularly in 1.5C scenarios).

The Paris Agreement’s aspirational 1.5C target, in particular, has more or less become defined as an overshoot scenario. Of the 230-odd scenarios in the IPCC AR6 WG3 report that limit warming to at or below 1.5C by 2100, about 220 (or 96%) of them pass that warming level before bringing global temperatures back down with net-negative global CO2 emissions.

But carbon debts turn out to be quite expensive to pay off. For every 0.1C we want to cool the climate after we get to zero emissions, we will have to pay around $22 trillion – assuming we are wildly successful and get the cost of permanent carbon removal down to $100 per ton.²

This means there are real questions about just how far we can (or should) go to reverse the debts carbon we’ve incurred. We may end up in a world where future generations are vastly more prosperous and technically advanced than today, analogous to the difference between the 1920s and 2020s, and these future generations may be able to clean up our mess and restore the global climate to more optimal conditions.

But they also may not; the assumption that future generations will inevitably be richer is at the heart of our choice to discount the future in many policy decisions we make today, but it is fundamentally an assumption. There have long been concerns around intergeneration equity in discounting, and the fundamental unfairness of maximizing benefits for people alive today at the expense of damaging the prospects of future generations. And many impacts of climate chance – such as its devastation of the natural world – likely cannot be undone.

Future generations have no say in our decisions today despite living with their consequences. While some carbon debt is likely inevitable, we should try to minimize the burden we pass down to our children and future generations. As a society we generally shy away from passing the debts of parents down to their children, a rule that we should try and hold to as we consider our growing carbon debt today.

The danger of kicking the can down the road

Unfortunately there is no realistic carbon bankruptcy; barring some far-fetched future of colonizing other planets (which will almost certainly have a climate less habitable than Earth’s no matter how badly we screw up our home), Earth is our home and where we make our stand as a species.

We can minimize our carbon debt by rapidly reducing emissions. But we can only pay down our carbon debt once emissions have occurred by permanently removing carbon from the atmosphere. While there is a real value to temporary carbon removal in the biosphere – such as planting trees or putting more carbon in soils – it does not actually reduce our carbon debt unless we can ensure that the carbon remains in those reservoirs indefinitely.³  Any non-permanent approach will reduce our carbon debt for a time, but will not change the need for future generations to eventually pay it down.

Similarly, since the warming from our CO2 emissions persists for millennia, geoengineering approaches such as solar radiation management do not actually pay off the carbon debt. Rather, they postpone when the debt is due. While there is a case to be made that they could buy us time to develop technologies to pay off the debt, there are also real risks that the ability to kick the can down the road reduces the impetus to either reduce emissions or deploy permanent carbon removal.

Our carbon debt ultimately leaves us with two options: either pay the price of adapting to the ravages of a hotter world – and acknowledge that large parts of the natural world will be lost in the process – or permanently remove enough carbon from the atmosphere to pay down the debt. And the more we emit before we get emissions down to zero, the more costly the carbon debt becomes.

1. Our best estimate is that cooling from aerosols are more or less counterbalanced by warming from shorter-lived non-CO2 greenhouse gases like methane and nitrous oxide. Unlike CO2, these gases have a chemical sink in the atmosphere – they will naturally break down given enough time (~12 years for methane, ~121 years for nitrous oxide) and do not rely on natural sinks for removal. However, aerosol forcing is highly uncertain, and a higher aerosol forcing would lead to a larger carbon debt.

2. Given our best estimate of 220 GtCO2 per 0.1C warming in the recent IPCC 6th Assessment Report, using the transient climate response to cumulative CO2 emissions (TCRE) of 1.65C (1.0C–2.3C) per 1000 GtC.

3. And if we use biosphere carbon removals to avoid increasing our carbon debt, e.g. by neutralizing ongoing fossil CO2 emissions, than we also need to prove that the biosphere carbon removals would never have otherwise occurred in the absence of our intervention. As we discussed a few months back, this is a tall order indeed.