Editing IPCC:AR6/WGI/Chapter-5 (section)

=== FAQ 5.4 | What Are Carbon Budgets? ===

<div id="h2-47-siblings" class="h2-siblings"></div>

<div id="faq-5-3"></div>

''There are several types of carbon budgets. Most often, the term refers to the total net amount of carbon dioxide (CO'' 2 '') that can still be emitted by human activities while limiting global warming to a specified level (e.g., 1.5°C or 2°C above pre-industrial levels). This is referred to as the ‘remaining carbon budget’. Several choices and value judgements have to be made before it can be unambiguously estimated. When the remaining carbon budget is combined with all past CO'' 2 ''emissions to date, a ‘total carbon budget’ compatible with a specific global warming limit can also be defined. A third type of carbon budget is the ‘historical carbon budget’, which is a scientific way to describe all past and present sources an'' ''d sinks of CO'' 2 ''.''

The term ''remaining carbon budget'' is used to describe the total net amount of CO <sub>2</sub> that human activities can still release into the atmosphere while keeping global warming to a specified level, like 1.5°C or 2°C relative to pre-industrial temperatures. Emissions of CO <sub>2</sub> from human activities are the main cause of global warming. A remaining carbon budget can be defined because of the specific way CO <sub>2</sub> behaves in the Earth system. That is, global warming is roughly linearly proportional to the total net amount of CO <sub>2</sub> emissions that are released into the atmosphere by human activities – also referred to as cumulative anthropogenic CO <sub>2</sub> emissions. Other greenhouse gases behave differently and have to be accounted for separately.

The concept of a remaining carbon budget implies that, to stabilize global warming at any particular level, global emissions of CO <sub>2</sub> need to be reduced to net zero levels at some point. ‘Net zero CO <sub>2</sub> emissions’ describes a situation where all the anthropogenic emissions of CO <sub>2</sub> are counterbalanced by deliberate anthropogenic removals so that, on average, no CO <sub>2</sub> is added or removed from the atmosphere by human activities. Atmospheric CO <sub>2</sub> concentrations in such a situation would gradually decline to a long-term stable level as excess CO <sub>2</sub> in the atmosphere is taken up by ocean and land sinks (see FAQ 5.1). The concept of a remaining carbon budget also means that, if CO <sub>2</sub> emissions reductions are delayed, deeper and faster reductions are needed later to stay within the same budget. If the remaining carbon budget is exceeded, this will result in either higher global warming or a need to actively remove CO <sub>2</sub> from the atmosphere to reduce global temperatures back down to the desired level (see FAQ 5.3).

Estimating the size of remaining carbon budgets depends on a set of choices. These choices include: (1) the global warming level that is chosen as a limit (for example, 1.5°C or 2°C relative to pre-industrial levels); (2) the probability with which we want to ensure that warming is held below that limit (for example, a one-in-two, two-in-three, or higher chance), and (3) how successful we are in limiting emissions of other greenhouse gases that affect the climate, such as methane or nitrous oxide. These choices can be informed by science, but ultimately represent subjective choices. Once these choices have been made, to estimate the remaining carbon budget for a given temperature goal, we can combine knowledge about: how much our planet has warmed already; the amount of warming per cumulative tonne of CO <sub>2</sub> ; and the amount of warming that is still expected once global net CO <sub>2</sub> emissions are brought down to zero. For example, to limit global warming to 1.5°C above pre-industrial levels with either a one-in-two (50%) or two-in-three (67%) chance, the remaining carbon budgets amount to 500 and 400 billion tonnes of CO <sub>2,</sub> respectively, from 1 January 2020 onward (FAQ 5.4, Figure 1). Currently, human activities are emitting around 40 billion tonnes of CO <sub>2</sub> into the atmosphere in a single year.

The remaining carbon budget depends on how much the world has already warmed to date. This past warming is caused by historical emissions, which are estimated by looking at the ''historical carbon budget'' – a scientific way to describe all past and present sources and sinks of CO <sub>2</sub> . It describes how the CO <sub>2</sub> emissions from human activities have redistributed across the various CO <sub>2</sub> reservoirs of the Earth system. These reservoirs are the ocean, the land vegetation, and the atmosphere (into which CO <sub>2</sub> was emitted). The share of CO <sub>2</sub> that is not taken up by the ocean or the land, and that thus increases the concentration of CO <sub>2</sub> in the atmosphere, causes global warming. The historical carbon budget tells us that, of the about 2560 billion tonnes of CO <sub>2</sub> that were released into the atmosphere by human activities between the years 1750 and 2019, about a quarter were absorbed by the ocean (causing ocean acidification) and about a third by the land vegetation. About 45% of these emissions remain in the atmosphere (see FAQ 5.1). Adding these historical CO <sub>2</sub> emissions to estimates of remaining carbon budgets allows an estimate of the ''total carbon budget'' consistent with a specific global warming level.

In summary, determining a remaining carbon budget – that is, how much CO <sub>2</sub> can be released into the atmosphere while stabilizing global temperature below a chosen level – is well understood but relies on a set of choices. However, it is clear that, for limiting warming below 1.5°C or 2°C, the remaining carbon budget from 2020 onwards is much smaller than the total CO <sub>2</sub> emissions released to date.

[[File:7261161f2caead1242276678333c4098 IPCC_AR6_WGI_FAQ_5_4_Figure_1.png]]
'''FAQ 5.4, Figure 1 |''' '''Various types of carbon budgets.''' Historical cumulative carbon dioxide (CO <sub>2</sub> ) emissions determine to a large degree how much the world has warmed to date, while the remaining carbon budget indicates how much CO <sub>2</sub> could still be emitted while keeping warming below specific temperature thresholds. Several factors limit the precision with which the remaining carbon budget can be estimated. Therefore, estimates need to specify the probability with which they aim at limiting warming to the intended target level (e.g., limiting warming to 1.5°C with a 67% probability).

<div id="references" class="h1-container"></div>