How much CO2 can Earth handle? What’s our carbon budget for a safe climate? In today’s blog I will discuss what exactly is carbon budget and what are its types!
What is Carbon Budget?
The term “carbon budget” is used in various contexts. Most commonly, it refers to the total net amount of carbon dioxide (CO2) emissions permissible for humans while keeping global warming within a certain threshold. This concept involves tallying CO2 emissions from industries, households, and other economic activities to figure out the necessary reductions for the future. The ultimate objective is to attain net-zero emissions, establishing a balance between the carbon emitted into the atmosphere and the amount extracted from it.
Types of Carbon Budget
There are several types of carbon budgets. These are:
Remaining Carbon Budget
Historical Carbon Budget
Total Carbon Budget
Remaining Carbon Budget
The “remaining carbon budget” refers to the amount of CO2 that we can still emit without exceeding global warming targets like 1.5°C or 2°C above pre-industrial levels. CO2 emissions from human activities are the primary driver of global warming. We can set a remaining carbon budget because CO2 has a unique effect on the Earth’s system: global warming increases almost directly in line with our total CO2 emissions. Other greenhouse gases have different impacts and must be considered separately.
The remaining carbon budget concept suggests that to stabilize global warming at any particular level, we need to achieve net zero CO2 emissions. This means that the amount of CO2 we emit is completely balanced by the amount we remove from the atmosphere, leading to no overall increase in atmospheric CO2 levels. Once we reach this balance, the CO2 concentration in the atmosphere will slowly start to decrease as it is naturally absorbed by the oceans and land. The idea of a remaining carbon budget also implies that delaying CO2 emissions reductions means we’ll have to cut emissions more drastically and quickly in the future to stay within the budget. Exceeding the budget would lead to either more severe global warming or the need for direct CO2 removal from the atmosphere to bring temperatures back down to the intended level.
Estimating the size of remaining carbon budgets depends on a set of choices. These choices include:
The target global warming level we aim to stay under (like 1.5°C or 2°C compared to pre-industrial times).
The probability we aim for to keep warming below that threshold (such as a 50%, 66%, or higher chance).
Our effectiveness in reducing emissions of other climate-affecting greenhouse gases, like methane or nitrous oxide.
These decisions are guided by science but are ultimately subjective. After making these choices, to calculate the remaining carbon budget for a specific temperature goal, we consider: how much our planet has warmed already; the amount of warming per cumulative tonne of CO2; and the amount of warming that is still expected once global net CO2 emissions are brought down to zero. For example, to limit global warming to 1.5°C above pre-industrial levels with either a 50% or 67% chance, the remaining carbon budgets amount to 500 and 400 billion tonnes of CO2, respectively, from 1 January 2020 onward. Currently, human activities are emitting around 40 billion tonnes of CO2 into the atmosphere in a single year.
Historical Carbon Budget
The remaining carbon budget is affected by the amount of global warming we’ve experienced so far, which comes from historical emissions. These are calculated using the historical carbon budget, a method that tracks all past and current CO2 sources and sinks. It shows how CO2 from human activities has been divided among Earth’s CO2 storage areas: the oceans, land vegetation, and the atmosphere. The portion of CO2 that isn’t absorbed by oceans or land increases atmospheric CO2 levels, leading to global warming. According to the historical carbon budget, out of approximately 2560 billion tonnes of CO2 emitted from 1750 to 2019, about one-fourth was absorbed by the oceans (which also causes ocean acidification), about one-third by land vegetation, and around 45% stayed in the atmosphere.
Total Carbon Budget
When we add up all the CO2 emissions from the past with the remaining carbon budget, we get what’s called the total carbon budget. This means that by combining historical CO2 emissions with the remaining carbon budget, we can estimate the total amount of CO2 that aligns with a certain level of global warming.
Figure: Various types of carbon budgets. Past CO2 emissions largely determine how much the Earth has warmed so far, while the remaining carbon budget shows how much more CO2 we can emit without exceeding certain temperature limits. However, there are several factors that make it hard to estimate the remaining carbon budget accurately. So, when making these estimates, it’s important to state the probability of staying below our warming goal (like having a 67% chance of not surpassing 1.5°C) [Source: IPCC]
In conclusion, our carbon budget is the boundary for a stable climate. Understanding and managing it is crucial for our future. By acting now, we can ensure a healthier planet for ourselves and future generations. Let’s commit to staying within our carbon limits and safeguarding our environment.
Reference
IPCC. Climate Change 2022: Impacts, Adaptation and Vulnerability | Climate Change 2022: Impacts, Adaptation and Vulnerability. Retrieved April 8, 2024, from https://www.ipcc.ch/report/ar6/wg2/
World Economic Forum. (2024, March 6). What are carbon budgets and how can they help us reach net zero? World Economic Forum. https://www.weforum.org/agenda/2024/03/carbon-budget-climate-change-net-zero/
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