The Carbon Budget for 1.5 Degrees

The carbon budget for 1.5℃ is a key concept in climate science and policy. But what is the carbon budget anyway? The best way to understand this concept is to think about a bathtub. That’s right, a bathtub.

Imagine a bathtub slowly filling with water. 

Inside, an unruly dog is waiting for his bath. Despite your best attempts to keep him still, the dog is wriggling and slipping about, clawing up the sides of the tub, and causing a ruckus. 

At first, when the water level is low, the floor stays dry. But you know that as the tub fills, each slosh of the bathwater will lap at the rim, with the occasional stray splash escaping to wet your floors. You also know that the situation will only grow more dire as the tub fills further. At the midpoint, even the slightest movement from your pup will send water cascading over the brim, and puddles will begin to develop around the base of the tub. Finally, as the tap continues ceaselessly, a heavy stream of water will flow over the tub’s high point and a waterfall will begin cascading down your stairs.

Now, imagine that this bathtub is our planet earth and the running tap is the greenhouse gas emissions being released into our atmosphere. The question you may begin to ask is: how long can we let the tap flow before we reach the point of no return?

What Is a Carbon Budget?

While the opening paragraphs may present a colourful metaphor, this is a very real question that scientists, policy-makers, and industries have been grappling with for years. In 2013, when the Fifth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC) established a near linear relationship between the total amount of CO₂ we put into the atmosphere (the amount of water running out of the tap) and the amount of global warming the earth experiences (the rising bathwater), the idea of a carbon budget was first born:

How much CO₂ can still be released into the atmosphere while maintaining our global temperatures within a liveable range?

While there are varying budgets given to reach varying degrees of global temperatures, the widely accepted stopping point to avoid the worst impacts of the climate crisis and safeguard a liveable planet is between 1.5°C and 2.0°C above pre-industrial levels. Currently, the IPCC’s latest report confirms that global temperatures have risen 1.1°C and are likely to reach 1.5°C by the early 2030s without immediate and deep global emission reductions.

All of us are stakeholders in this planet. This means that the remaining carbon budget (alongside future damage estimates, such as the Social Cost of Carbon) should be one of the most important things on our minds as we make decisions individually and collectively moving forward. 

How Much of the Carbon Budget for 1.5℃ Is Left?

Most estimates for our remaining carbon budget are presented as a specific quantity of emissions. But it’s important to note that behind this given number are a series of judgements and modelling choices (such as how feedbacks in the Earth’s system are accounted for OR how much warming the Earth has experienced to-date) that drive its final result. 

Because of this, many organizations offer differing estimates of the world’s remaining carbon budget. While understandable, these differences unfortunately have downsides within the realm of public opinion and policy. If the target is too relaxed, it may lead some to believe there is no urgency to act. And if the target is too ambitious, it can feed into a sense of paralysis over an “impossible” task. 

We know that neither of these extremes is the case. The truth is, there is an urgency to act and there is still time to do so. Thankfully, as time passes and newer carbon budgets draw from the most recent research and informed observations about the world around us, these variations between estimates become ever more strongly reduced and this picture ever clearer. 

Of these estimates, the following are some of the more recently accepted budgets:

• The 2021 IPPC Sixth Assessment Report estimated that for a 50% likelihood of limiting global warming to 1.5°C there remained a budget of 500 Gt of CO₂ — about 10 years — assuming emissions levels stayed constant.

• The Global Carbon Budget 2022 report estimated a remaining carbon budget of 380 Gt of CO₂ — or about 9 years — assuming 2022 emissions levels.

• Carbon Brief in 2022 combined the latest insights from several reports and estimated the remaining 1.5°C carbon budget could be as low as 260 Gt of CO₂ — or about 6.5 years — assuming 2022 emissions levels.

What Uncertainties Exist in the Carbon Budget for 1.5℃?

As mentioned previously, carbon budgets are not an exact science, but are predicated upon a series of uncertainties that are captured and defined by a particular budget’s modelling choices and value judgements.

As illustrated in the figure below (from CONSTRAIN’s 2019 report: Zero In On), each carbon budget must address a number of different uncertainties before it is possible to present a final budget. 

The climate science modelling choices may change as our understanding of the world around us improves over time through studies and real-world feedback. But the value judgments being made are by their nature predicated on our personal principles and values. As argued in the Zero In On report, the following three subjective choices are the most fundamental to the results of any budget:

1. To what temperature level do we wish to limit global emissions? This can be 1.5°C above pre-industrial levels, 2.0°C, or another temperature.

2. What is considered an acceptable level of likelihood that warming will be kept to the temperature level selected? This second point takes into account the uncertainties involved modelling in global feedback systems due to our emissions. If you’d like a 100% probability of staying below 1.5°C warming, your remaining carbon budget will be lower than if you are willing to gamble on a 50% probability of meeting that target. 

3. How successfully can other factors outside the carbon budget help address rising global temperatures? There exist many non-CO₂ related mitigation options to address climate change. The extent that you believe these other mitigation options are possible or effective may free up more budget or limit you further.

What Is Our Fair Share?

These three value judgements, along with the modelling choices described, lay the groundwork for global carbon budget estimates, but what about national budgets? Once we get into the carbon budgets of individual countries, even more value judgements come into play to decide what amounts to our “fair share” of remaining emissions. 

In Canada, for example, we would need to consider our historic responsibility (how much responsibility we owe at present for the  emissions we’ve produced in the past developing our prosperity), our per capita emissions ratio compared to other countries (how much each individual Canadian emits compared to each individual citizen in another country), and our capacity to reduce emissions. 

Furthermore, if we’re asking these questions about Canada as a whole, then what would be our fair share in Alberta? 

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