Carbon budget

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For the assessment of the biogeochemical cycle by which carbon is exchanged at the global level between sources and sinks in the biosphere, pedosphere, geosphere, hydrosphere, and atmosphere, also referred to as the Global Carbon Budget, see Carbon cycle.
Estimating the remaining carbon budget at the global level depends on climate science and value judgments or choices. To translate a global budget to the national level, further value judgments and choices have to be made. Figure from the CONSTRAIN Zero In On Report.[1]

A carbon budget is “the maximum amount of cumulative net global anthropogenic carbon dioxide (CO2) emissions that would result in limiting global warming to a given level with a given probability, taking into account the effect of other anthropogenic climate forcers”.[2] When expressed relative to the pre-industrial period it is referred to as the Total Carbon Budget, and when expressed from a recent specified date it is referred to as the Remaining Carbon Budget.[2]

A carbon budget consistent with keeping warming below a specified limit is also referred to as an emissions budget, an emissions quota, or allowable emissions.[3][4][5] An emissions budget may also be associated with objectives for other related climate variables, such as radiative forcing[6] or sea level rise.[7]

Total or remaining carbon budgets are calculated by combining estimates of various contributing factors, including scientific evidence and value judgments or choices.[1][8][9][10][11]

Global carbon budgets can be further divided into national emissions budgets, so that countries can set specific climate mitigation goals. Emissions budgets are relevant to climate change mitigation because they indicate a finite amount of carbon dioxide that can be emitted over time, before resulting in dangerous levels of global warming. Change in global temperature is independent from the geographic location of these emissions, and is largely independent of the timing of these emissions.[12][13]

Translating a global remaining carbon budget in line with a specific climate target to national carbon budgets involves the consideration of aspects of equity and fairness between countries[11][14] as well as other methodological choices.[14]

An emissions budget may be distinguished from an emissions target, as an emissions target may be internationally or nationally set in accordance with objectives other than a specific global temperature and are commonly applied to the annual emissions in a single year as well.

Estimations[]

The finding of an almost linear relationship between global temperature rise and cumulative carbon dioxide emissions[13] has encouraged the estimation of global emissions budgets in order to remain below dangerous levels of warming. Since the pre-industrial period to 2019, approximately 2390 Gigatonnes of CO2 (Gt CO2) has already been emitted globally.[10]

Scientific estimations of the remaining global emissions budgets/quotas differ due to varied methodological approaches, and considerations of thresholds.[15] Estimations might not include all amplifying climate change feedbacks,[16][17][18][19] although the most authoritative carbon budget assessments by the IPCC do account explicitly for these.[8][10] The IPCC assesses the size of remaining carbon budgets using estimates of past warming caused by human activities, the amount of warming per cumulative unit of CO2 emissions (also known as the Transient Climate Response to cumulative Emissions of carbon dioxide, or TCRE), the amount of warming that could still occur once all emissions of CO2 are halted (known as the Zero Emissions Commitment[20]), and the impact of Earth system feedbacks that would otherwise not be covered; and vary according to the global temperature target that is chosen, the probability of staying below that target, and the emission of other non-CO2 greenhouse gases (GHGs).[8][9][10][21][22] This approach was first applied in the 2018 Special report on Global Warming of 1.5°C by the IPCC,[8] and was also used in its 2021 Working Group I Contribution to the Sixth Assessment Report.[10]

Carbon budget estimates depend on the likelihood or probability of avoiding a temperature limit, and the assumed warming that is projected to be caused by non-CO2 emissions.[8][9][10] The values for the carbon budget estimates in the following table are drawn from the latest assessment of the Physical Science Basis of climate change by the Working Group I Contribution to the IPCC Sixth Assessment Report.[10] These estimates assume non-CO2 emissions are also reduced in line with deep decarbonisation scenarios that reach global net zero CO2 emissions.[8][9][10] Carbon budget estimates thus depend on how successful society is in reducing non-CO2 emissions together with carbon dioxide emissions. The IPCC Sixth Assessment Report estimated that remaining carbon budgets can be 220 Gt CO2 higher or lower depending on how successful non-CO2 emissions are reduced.[10]

Emissions budget estimations
Target for average

global temperature rise

Likelihood

of staying below target

Carbon budget

in Gt CO2

Date range Source Location in source Potential variation due

to level of non-CO2 emissions in Gt CO2

2 °C 83% 900 2020-net zero IPCC Sixth Assessment Report[10] Table 5.8 Carbon budget estimates

can be 220 Gt CO2 higher or lowerdepending on how successful non-CO2 emissions are reduced.

1.5 °C 83% 300 2020-net zero IPCC Sixth Assessment Report[10] Table 5.8
2 °C 67% 1150 2020-net zero IPCC Sixth Assessment Report[10] Table 5.8
1.5 °C 67% 400 2020-net zero IPCC Sixth Assessment Report[10] Table 5.8
2 °C 50% 1350 2020-net zero IPCC Sixth Assessment Report[10] Table 5.8
1.5 °C 50% 500 2020-net zero IPCC Sixth Assessment Report[10] Table 5.8

1 GtC (carbon) = 3.66 GtCO2 [10]

Several organisations provide annual updates to the remaining carbon budget, including the Global Carbon Project and the CONSTRAIN project.

Carbon capture[]

Main articles: Carbon dioxide removal, Carbon sequestration, and Carbon capture and storage

Researchers expect emissions will exceed any of these remaining budgets. In order to comply with the budget limits, they expect CO2 will need to be captured from the atmosphere and stored in products, the environment or underground. A 2015 study calculated that carbon budgets can only be met by capturing CO2, "in all but the most optimistic cases, we also find negative emission requirements that have not yet been shown to be achievable".[23]

Scientists widely agree this research is needed. IPCC says, "All pathways that limit global warming to 1.5 °C (2.7 °F) with limited or no overshoot project the use of carbon dioxide removal (CDR) on the order of 100-1000 GtCO2 over the 21st century. CDR would be used to compensate for residual emissions and, in most cases, achieve net negative emissions to return global warming to 1.5°C following a peak (high confidence)."[24]

Even for the less strict goal of 2 °C (3.6 °F) warming, carbon capture is needed. IPCC has only one scenario (they call it a "Representative Concentration Pathway" RCP) which limits warming to 3.6 °F: "RCP2.6 is representative of a scenario that aims to keep global warming likely below 2 °C above pre-industrial temperatures. The majority of models indicate that scenarios meeting forcing levels similar to RCP2.6 are characterized by substantial net negative emissions by 2100, on average around 2 GtCO2/yr."[25]: 57 

National emissions budgets[]

In light of the many differences between nations, including but not limited to population, level of industrialization, national emissions histories, and mitigation capabilities, scientists have made attempts to allocate global carbon budgets among countries using methods that follow various principles of equity.[26] Allocating national emissions budgets is comparable to sharing the effort to reduce global emissions, underlined by some assumptions of state-level responsibility of climate change. Many authors have conducted quantitative analyses which allocate emissions budgets,[27][28][29][5] often simultaneously addressing disparities in historical GHG emissions between nations. National 'Paris-compliant' emissions budgets have also been calculated that quantify the discrepancy between the emissions reductions resulting from current national mitigation pathways and those needed to live up to the temperature and equity commitments enshrined in the Paris Agreement.[27]

One common principle that has been used to allocate global emissions budgets to nations is the "common but differentiated responsibilities".[26] This principle recognizes nations' cumulative historical contributions to global emissions. So those countries with greater emissions during a set time period (for example, since the pre-industrial era to the present) are the most responsible for addressing excess emissions. Thus, their national emissions budgets has to be smaller than those that have polluted less in the past. The concept of national historical responsibility for climate change has prevailed in the literature since the early 1990s[30][31] and has been part of the key international agreements on climate change (UNFCCC, the Kyoto Protocol and the Paris Agreement). Consequently, those countries with quantified cumulative historical emissions of states have the most responsibility to take the strongest actions[32] and help developing countries to mitigate their emissions and adapt to climate change. This principle is recognized in international treaties and has been part of the diplomatic strategies by developing countries, that need larger emissions budgets[33] to reduce inequity and achieve sustainable development.

Another common equity principle for calculating national emissions budgets is the "egalitarian" principle. This principle stipulates individuals should have equal rights to pollute, and therefore emissions budgets should be distributed proportionally according to state populations.[26] Some scientists have thus reasoned the use of national per-capita emissions in national emissions budget calculations.[28][29][34] This principle may be favoured by nations with larger or rapidly growing populations.[33]

A third equity principle that has been employed in national budget calculations considers national sovereignty.[26] The "sovereignty" principle highlights the equal right of nations to pollute.[26] The grandfathering method for calculating national emissions budgets uses this principle. Grandfathering allocates these budgets proportionally according to emissions at a particular base year,[34] and has been used under international regimes such as the Kyoto Protocol[35] and the early phase of the European Union Emissions Trading Scheme (EU ETS)[36] This principle is often favoured by developed countries, as it allocates larger emissions budgets to them.[33]

See also[]

References[]

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