Defining the carbon budget
The concept of a "Carbon Budget" has progressively emerged from the work of the Intergovernmental Panel on Climate Change (IPCC), which highlighted the relationship between global warming and cumulative CO2 emissions since the industrial era. Climate science defines a carbon budget as the number of greenhouse gases that can be spent (emitted) for a given level of global warming. A carbon budget is a maximum amount of cumulative net global anthropogenic carbon dioxide (CO2) emissions that would, with some probability, limit global warming to a given level. The total carbon budget is expressed relative to the pre-industrial period.
It can be referred to as the "remaining carbon budget" defined by the IPCC in its 2018 report when expressed from a specified recent date. It represents our room for maneuver in CO2 emissions to limit global warming below a given threshold of warming relative to the pre-industrial level - limit estimated by the IPCC at 1170 billion tons of CO2 for two °C as of January 1, 2018. It is an estimation of cumulative net global anthropogenic CO2 emissions from a given start date to the point where anthropogenic CO2 emissions reach a net zero level. This would likely limit global warming to a given class, taking into account the impact of other anthropogenic emissions.
The Kyoto Protocol also implemented the United Nations Framework Convention on Climate Change (UNFCCC), calling on industrialized countries and economies to transition to limit and reduce greenhouse gas (GHG) emissions with agreed individual targets. Seven greenhouse gases are addressed explicitly by this Protocol: carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) (in the non-fluorinated gas category), and hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), sulfur hexafluoride (SF6) and nitrogen trifluoride (NF3) (in the fluorinated gas category). These gases can be compared by converting them to carbon dioxide (CO2) equivalents to determine their individual and total contribution to global warming.
The "emissions budget" considered́ in the reduction commitments refers to a total for a set of gases. The carbon budget related to warming, as presented́ by the IPCC, concerns only carbon dioxide (CO2), thus setting aside other GHGs such as methane (CH4) and nitrous oxide (N2O). Therefore, a carbon budget that includes the other GHGs will be higher.
An emissions budget can also be linked to targets for other climate variables, such as radiative forcing or sea level rise. It is calculated by combining estimates of various contributing factors, including scientific evidence, value judgments, or choices.
Budgeting for GHG emissions
Carbon budgets help assess the effectiveness of GHG reduction measures. These may include using renewable energy sources or incorporating mitigation factors such as carbon capture initiatives or reforestation programs.
One of the primary sources of human carbon dioxide emissions is burning fossil fuels: coal, gas, gasoline, and oil. In addition, cement manufacturing also causes a significant carbon dioxide released into the atmosphere.
There are three primary characteristics of GHGs that explain global warming:
1. Concentration: Humans can directly influence the amount of gas released into the atmosphere.
2. Strength: the efficiency with which a gas molecule can trap heat.
3. Lifetime: the average time the gas remains in a non-renewed atmosphere.
Furthermore, an emissions budget should not be confused with an emissions target. An emissions budget can be set at the international or national level regarding marks other than a specific global temperature. It is usually applied to emissions over a single year. Moreover, the budget is not an annual, but a cumulative budget studied over time. It is based on the so-called natural functioning of the Earth.
Finally, once the carbon budget is spent, net emissions must be kept at zero from that point on to avoid exceeding the temperature target.
Budgeting for the future
The carbon budget defines the international community's flexibility to implement its global greenhouse gas emission reduction measures. It informs decision-makers about the speed with which emissions need to be reduced to zero. It is also a simple analytical tool for considering the potential implications of a carbon-constrained future. This tool is, therefore, operational. As a result, public decision-makers have vital information about climate change and the carbon cycle. Moreover, various legal mechanisms are available to encourage and frame these carbon budgeting measures.
Most countries have ratified the Paris Agreement, which obliges governments to set nationally defined targets to limit global warming to below 2°C, or as close to 1.5°C as possible, compared to pre-industrial levels.
The United Nations Framework Convention on Climate Change (UNFCCC) established these targets based on projections of future climate impacts associated with different levels of global warming. These projections set the threshold for the number of greenhouse gases - measured in carbon dioxide equivalents - that can be emitted into the atmosphere. Translating a residual global carbon budget in line with a specific climate target into national carbon budgets implies considering aspects of equity and justice between countries and other methodological choices.
We should also mention the Science Based Targets Initiative (SBTi), which proposes a mechanism - a certification based on a standard - that has begun to bear fruit in the implementation of the conclusions of the IPCC Special Report. Indeed, on the one hand, the most recent 'science-based' climate science can be considered necessary to achieve the objectives of the Paris Agreement. But on the other hand, SBTi is one of the key initiatives for the private sector to reduce its GHG emissions and define a net zero target for a +1.5°C future.
Carbon budgeting can also demonstrate the impacts of inaction, using climate projections to determine future scenarios. According to the Contribution of Working Group III to the IPCC (Intergovernmental Panel on Climate Change) Sixth Assessment Report, published in April 2022, there is still time to limit the global temperature increase to below 1.5°C. However, the scientific community points out that the possibilities for action are rapidly diminishing and that the situation is urgent.
Accord of Paris, 12 December 2015: https://unfccc.int/fr/a-propos-des-ndcs/l-accord-de-paris
CCNUCC, Convention-cadre des Nations unies sur les changements climatiques, entrée en vigueur le 21 mars 1994 : https://unfccc.int/fr/processus-et-reunions/qu-est-ce-que-la-ccnucc-la-convention-cadre-des-nations-unies-sur-les-changements-climatiques
IPCC, 2018: Summary for Policymakers. In: Global Warming of 1.5°C. An IPCC Special Report on the impacts of global warming of 1.5°C above pre-industrial levels and related global greenhouse gas emission pathways, in the context of strengthening the global response to the threat of climate change, sustainable development, and efforts to eradicate poverty [Masson-Delmotte, V., P. Zhai, H.-O. Pörtner, D. Roberts, J. Skea, P.R. Shukla, A. Pirani, W. Moufouma-Okia, C. Péan, R. Pidcock, S. Connors, J.B.R. Matthews, Y. Chen, X. Zhou, M.I. Gomis, E. Lonnoy, T. Maycock, M. Tignor, and T. Waterfield (eds.)]. Cambridge University Press, Cambridge, UK and New York, NY, USA, pp. 3-24, doi:10.1017/9781009157940.001.
IPCC, « 2019 Refinement to the 2006 IPCC Guidelines for National Greenhouse Gas Inventories”, https://www.ipcc.ch/report/2019-refinement-to-the-2006-ipcc-guidelines-for-national-greenhouse-gas-inventories/
IPCC, 2022_: Climate Change 2022: Mitigation of Climate Change. Contribution of Working Group III to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change_ [P.R. Shukla, J. Skea, R. Slade, A. Al Khourdajie, R. van Diemen, D. McCollum, M. Pathak, S. Some, P. Vyas, R. Fradera, M. Belkacemi, A. Hasija, G. Lisboa, S. Luz, J. Malley, (eds.)]. Cambridge University Press, Cambridge, UK and New York, NY, USA.
H. Damon Matthews, Katarzyna B. Tokarska, Zebedee R. J. Nicholls, Joeri Rogelj, Josep G. Canadell, Pierre Friedlingstein, Thomas L. Frölicher, Piers M. Forster, Nathan P. Gillett, Tatiana Ilyina, Robert B. Jackson, Chris D. Jones, Charles Koven, Reto Knutti, Andrew H. MacDougall, Malte Meinshausen, Nadine Mengis, Roland Séférian and Kirsten Zickfeld, “Opportunities and challenges in using remaining carbon budgets to guide climate policy”, Nature Geoscience, 30 novembre 2020.
M. Pathak, R. Slade, P.R. Shukla, J. Skea, R. Pichs-Madruga, D. Ürge-Vorsatz,2022: Technical Summary. In: Climate Change 2022: Mitigation of Climate Change. Contribution of Working Group III to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change [P.R. Shukla, J. Skea, R. Slade, A. Al Khourdajie, R. van Diemen, D. McCollum, M. Pathak, S. Some, P. Vyas, R. Fradera, M. Belkacemi, A. Hasija, G. Lisboa, S. Luz, J. Malley, (eds.)]. Cambridge University Press, Cambridge, UK and New York, NY, USA.« Qu'est-ce que le Protocole de Kyoto ? », Fiche thématique, Vie publique, https://www.vie-publique.fr/fiches/274835-quest-ce-que-le-protocole-de-kyoto
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