An emissions budget, carbon budget, emissions quota, or allowable emissions, is an upper limit of total carbon dioxide (CO
2) emissions associated with remaining below a specific global average temperature. An emissions budget may also be associated with objectives for other related climate variables, such as radiative forcing.
Global emissions budgets are calculated according to historical cumulative emissions from fossil fuel combustion, industrial processes, and land-use change, but 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). Global emissions 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.
An "emissions target" may be distinguished from an emissions budget, as an emissions target may be internationally or nationally set in accordance with objectives other than a specific global temperature. This includes targets created for their political palatability, rather than ones focused on climate science warnings.
The finding of an almost linear relationship between global temperature rise and cumulative carbon dioxide emissions has encouraged the estimation of global emissions budgets in order to remain below dangerous levels of warming. Since the pre-industrial period to 2011, approximately 1890 Gigatonnes of CO2 (GtCO2) has already been emitted globally, and 2050 GtCO2 up to 2015.
Scientific estimations of the remaining global emissions budgets/quotas differ widely due to varied methodological approaches, and considerations of thresholds. Most estimations still underestimate the amplifying climate change feedbacks.
Some common budget estimations are those associated with a 1.5 °C and 2 °C global warming. These estimates depend highly on the likelihood or probability of reaching a temperature target.
|Target for average global temperature rise||Likelihood of staying below target||Emissions budget, petagrams (billion tons) of Carbon||Emissions budget, billion tons of CO
|Date range||Source (Rogelj et al. 2016 has another list of estimates)||Page in source|
|1.5 °C = 2.7 °F||66%||220-250||810-920||2015-2100||Millar et al. 2017||4|
|1.5 °C = 2.7 °F||50%||400-570||2011-2100||Rogelj et al. 2015||3|
|1.5 °C = 2.7 °F||50%||370||1400||2015-2100||Millar et al. 2017||4|
|1.5 °C = 2.7 °F||50%||1060||2016-2100||Matthews et al. 2015||subtraction in Table 2|
|2 °C = 3.6 °F||75%||610-830||2011-2100||Rogelj et al. 2015||3|
|2 °C = 3.6 °F||66%||1200||2015-2100||Friedlingstein et al. 2014||710|
|2 °C = 3.6 °F||66%||1000||2020-2100||Friedlingstein et al. 2014||710|
|2 °C = 3.6 °F||66%||270||990||2012-2100||2015 IPCC 2015||1113|
|2 °C = 3.6 °F||66%||940||2011-2100||Rogelj et al. 2015||3|
|2 °C = 3.6 °F||50%||990-1450||2011-2100||Rogelj et al. 2015||3|
|2 °C = 3.6 °F||50%||2085||2016-2100||Matthews et al. 2015||subtraction in Table 2|
|2 °C = 3.6 °F||50%||1500||2015-2100||Friedlingstein et al. 2014||710|
|2 °C = 3.6 °F||50%||1300||2020-2100||Friedlingstein et al. 2014||710|
|3 °C = 5.4 °F||66%||2900||2015-2100||Friedlingstein et al. 2014||710|
|3 °C = 5.4 °F||66%||2700||2020-2100||Friedlingstein et al. 2014||710|
|3 °C = 5.4 °F||50%||3300||2015-2100||Friedlingstein et al. 2014||710|
|3 °C = 5.4 °F||50%||3100||2020-2100||Friedlingstein et al. 2014||710|
Alternative to budgets set explicitly using temperature objectives, emissions budgets have also been estimated using the Representative Concentration Pathways, which are based on radiative forcing values at the end of the century. (Although temperatures may be inferred from radiative forcing). These were presented in the International Panel on Climate Change Fifth Assessment report.
Researchers expect emissions will exceed any of these remaining budgets. In order to comply with the budget limits, they expect CO
2 will need to be captured from the atmosphere and stored in products, the environment or underground. A 2015 study in Nature says carbon budgets can only be met by capturing CO
2, "in all but the most optimistic cases, we also find negative emission requirements that have not yet been shown to be achievable"
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)."
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.":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. Allocating national emissions budgets is comparable to sharing the burdens of climate change, underlined by some assumptions of state-level responsibility of climate change. Many authors have conducted quantitative analyses which allocate emissions budgets, often simultaneously addressing disparities in historical GHG emissions between nations.
One common principle that has been used to allocate global emissions budgets to nations is the "responsibility" or "polluter-pays" principle. 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) would be most responsible for addressing excess emissions. Thus, their national emissions budgets would 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. Consequently, some have quantified cumulative historical emissions of states, to identify who has most responsibility to take the strongest actions. This principle is often favoured by developing countries, as it gives them larger emissions budgets.
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. Some scientists have thus reasoned the use of national per-capita emissions in national emissions budget calculations. This principle may be favoured by nations with larger or rapidly growing populations.
A third equity principle that has been employed in national budget calculations considers national sovereignty. The "sovereignty" principle highlights the equal right of nations to pollute. The grandfathering method for calculating national emissions budgets uses this principle. Grandfathering allocates these budgets proportionally according to emissions at a particular base year, and has been used under international regimes such as the Kyoto Protocol and the early phase of the European Union Emissions Trading Scheme (EU ETS) This principle is often favoured by developed countries, as it allocates larger emissions budgets to them.
- Christiana Figueres u. a. (2017), "Three years to safeguard our climate", Nature (in German), 546, pp. 593–595, doi:10.1038/546593a
- Meinshausen, Malte; Meinshausen, Nicolai; Hare, William; Raper, Sarah C. B.; Frieler, Katja; Knutti, Reto; Frame, David J.; Allen, Myles R. (30 April 2009). "Greenhouse-gas emission targets for limiting global warming to 2 °C". Nature. 458 (7242): 1158–1162. CiteSeerX 10.1.1.337.3632. doi:10.1038/nature08017. ISSN 1476-4687. PMID 19407799.
- Matthews, H. Damon; Zickfeld, Kirsten; Knutti, Reto; Allen, Myles R. (2018). "Focus on cumulative emissions, global carbon budgets and the implications for climate mitigation targets". Environmental Research Letters. 13 (1): 010201. doi:10.1088/1748-9326/aa98c9. ISSN 1748-9326.
- Raupach, Michael R.; Davis, Steven J.; Peters, Glen P.; Andrew, Robbie M.; Canadell, Josep G.; Ciais, Philippe; Friedlingstein, Pierre; Jotzo, Frank; Vuuren, Detlef P. van (21 September 2014). "Sharing a quota on cumulative carbon emissions". Nature Climate Change. 4 (10): 873–879. doi:10.1038/nclimate2384. ISSN 1758-6798.
- IPCC, 2013: Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Stocker, T.F., D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M. Midgley (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, 1535 pp
- Friedlingstein, P., Andrew, R. M., Rogelj, J., Peters, G. P., Canadell, J. G., Knutti, R., ... & Le Quéré, C. (2014). Persistent growth of CO
2 emissions and implications for reaching climate targets. Nature geoscience, 7(10), 709.
- Jackson, Tim. "2050 is too late – we must drastically cut emissions much sooner". The Conversation. Retrieved 2019-09-23.
- Zickfeld, K.; Arora, V. K.; Gillett, N. P. (2012-03-01). "Is the climate response to CO
2 emissions path dependent?". Geophysical Research Letters. 39 (5): L05703. doi:10.1029/2011gl050205. ISSN 1944-8007.
- Matthews, H. Damon; Gillett, Nathan P.; Stott, Peter A.; Zickfeld, Kirsten (11 July 2009). "The proportionality of global warming to cumulative carbon emissions". Nature. 459 (7248): 829–832. doi:10.1038/nature08047. ISSN 1476-4687. PMID 19516338.
- Babiker, Mustafa H.; Eckaus, Richard S. (2002-09-01). "Rethinking the Kyoto Emissions Targets". Climatic Change. 54 (4): 399–414. doi:10.1023/A:1016139500611. ISSN 0165-0009.
- Rogelj, J., Schaeffer, M., Friedlingstein, P., Gillett, N. P., Van Vuuren, D. P., Riahi, K., ... & Knutti, R. (2016). Differences between carbon budget estimates unravelled. Nature Climate Change, 6(3), 245.
- Séférian, Roland; Smith, Christopher J.; Kriegler, Elmar; Forster, Piers M.; Rogelj, Joeri (July 2019). "Estimating and tracking the remaining carbon budget for stringent climate targets". Nature. 571 (7765): 335–342. doi:10.1038/s41586-019-1368-z. ISSN 1476-4687.
- Jamieson, Naomi Oreskes,Michael Oppenheimer,Dale. "Scientists Have Been Underestimating the Pace of Climate Change". Scientific American Blog Network. Retrieved 2019-08-21.
- Comyn-Platt, Edward (2018). "Carbon budgets for 1.5 and 2 °C targets lowered by natural wetland and permafrost feedbacks". Nature Geoscience.
- Millar, Richard J.; Fuglestvedt, Jan S.; Friedlingstein, Pierre; Rogelj, Joeri; Grubb, Michael J.; Matthews, H. Damon; Skeie, Ragnhild B.; Forster, Piers M.; Frame, David J. (September 2017). "Emission budgets and pathways consistent with limiting warming to 1.5 °C" (PDF). Nature Geoscience. 10 (10): 741–747. doi:10.1038/ngeo3031. ISSN 1752-0908.
- Matthews, H. Damon; Landry, Jean-Sébastien; Partanen, Antti-Ilari; Allen, Myles; Eby, Michael; Forster, Piers M.; Friedlingstein, Pierre; Zickfeld, Kirsten (2017-03-01). "Estimating Carbon Budgets for Ambitious Climate Targets" (PDF). Current Climate Change Reports. 3 (1): 69–77. doi:10.1007/s40641-017-0055-0. ISSN 2198-6061.
- Rogelj, J.; Reisinger, A.; McCollum, D. L.; Knutti, R.; Riahi, K.; Meinshausen, M. (10 July 2015). "Mitigation choices impact carbon budget size compatible with low temperature goals". Environmental Research Letters. 10 (7): 075003. doi:10.1088/1748-9326/10/7/075003. ISSN 1748-9326.
- Zickfeld, Kirsten; Eby, Michael; Matthews, H. Damon; Weaver, Andrew J. (2009-09-22). "Setting cumulative emissions targets to reduce the risk of dangerous climate change". Proceedings of the National Academy of Sciences. 106 (38): 16129–16134. doi:10.1073/pnas.0805800106. ISSN 0027-8424. PMC 2752604. PMID 19706489.
- Climate Change 2014 Synthesis Report https://ar5-syr.ipcc.ch/topic_futurechanges.php
- Vuuren, Detlef P. van; Edmonds, James A.; Kainuma, Mikiko; Riahi, Keywan; Weyant, John (2011-11-01). "A special issue on the RCPs". Climatic Change. 109 (1–2): 1–4. doi:10.1007/s10584-011-0157-y. ISSN 0165-0009.
- 2015 Gasser et al. "Negative emissions physically needed to keep global warming below 2 °C" Nature Communications https://www.nature.com/articles/ncomms8958
- 2018 IPCC p.19 "Global warming of 1.5°C Summary for Policymakers" https://www.ipcc.ch/sr15/chapter/summary-for-policy-makers/
- Ringius, L., Torvanger, A., & Underdal, A. (2002). Burden sharing and fairness principles in international climate policy. International Environmental Agreements, 2(1), 1-22.
- Baer, P., Athanasiou, T., Kartha, S., & Kemp-Benedict, E. (2009). Greenhouse development rights: A proposal for a fair global climate treaty. Ethics place and environment, 12(3), 267-281.
- Matthews, H. Damon (January 2016). "Quantifying historical carbon and climate debts among nations". Nature Climate Change. 6 (1): 60–64. doi:10.1038/nclimate2774. ISSN 1758-6798.
- Grübler, A., & Fujii, Y. (1991). Inter-generational and spatial equity issues of carbon accounts. Energy, 16(11-12), 1397-1416.
- Smith, K. R. (1992). Allocating responsibility for global warming: The natural debt index. Ambio. Stockholm, 20(2), 95-96.
- Botzen, W. J., Gowdy, J. M., & van den Bergh, J. C. (2008). Cumulative CO
2 emissions: shifting international responsibilities for climate debt. Climate policy, 8(6), 569-576. doi:10.3763/cpol.2008.0539.
- Pan, J. (2003). Emissions rights and their transferability: equity concerns over climate change mitigation. International Environmental Agreements, 3(1), 1-16.
- Neumayer, Eric (2000). "In defence of historical accountability for greenhouse gas emissions". Ecological Economics. 33 (2): 185–192. doi:10.1016/s0921-8009(00)00135-x.
- UNFCCC (1998). "Kyoto Protocol to the United Nations Framework Convention on Climate Change".(http://unfccc.int/resource/docs/convkp/kpeng.pdf)
- European Commission (2010) 2010/384/: Commission Decision of 9 July 2010 on the Community-wide quantity of allowances to be issued under the EU Emission Trading Scheme for 2013 (notified under document C(2010) 4658). Official Journal of the European Union L 175 36-37 (http://eur-lex.europa.eu/legal-content/EN/TXT/?uri=CELEX:32010D0384)