by Sebastian Hettrich
When it comes to climate policies and whether a policy is compliant with the Paris Agreement, the remaining carbon budget is often also mentioned.
But what is this carbon budget and what are its effects?
In 2014, the Intergovernmental Panel on Climate Change (IPCC) released the Assessment Report 5 (AR 5), an article based upon latest scientific research that links the emissions of carbon dioxide from fossil fuels to the increase in global temperature .
In order to describe a carbon budget — how much carbon we can still burn before we reach a certain maximum warming temperature we don’t want to exceed — this “goal” temperature has to be linked to a certain carbon dioxide (CO2) concentration in the air (and CO2 equivalents) that must not be reached. Considering also the uptake and storage of CO2 through the global carbon cycle allows the calculation of the amount of CO2 that can be emitted before reaching that temperature target (within certain probabilities (see our article on the IPCC and their future climate scenarios) [2, 1].
In an IPCC report from 2018, a total of 420 GT CO2 emissions were calculated until reaching the 1.5°C warming target, and 1170 GT for the 2°C target . The number for the 2°C target had been calculated in an earlier report from 2013 with 990 GT . There is also a discrepancy in the calculation of data for the annual global CO2 emissions. “Our world in data” puts the global emissions of 2019 at 36.44 GT, whereas the IPCC estimates the annual emission of CO2 and CO2 equivalents, considering also land use, therefore estimating emissions of up to 42 GT per year [4, 3]. Given a steady emission of greenhouse gases, these numbers can be used to estimate how much time we still have until a certain temperature is reached. Depending on which numbers are used, the 1.5°C target is already in reach within the next 7 to 8.5 years, while the values for the 2°C range from 15 to 29 years, depending on the scenario and annual emissions (see table 1).
Table 1: Global carbon dioxide budget attributed to the 1.5°C and 2°C warming targets. Depending on the data source and whether the models of IPCC 2013 or IPCC 2018 are used, results, particularly for the 2°C target, show a large variance.
|Remaining carbon budget (report) in GT||420 (2018)||990 (2013)||1170 (2018)|
|Remaining carbon budget by 1st Jan 2021 in GT||313 (based on ),294 (based on )||672 (based on ),654 (based on ||1063 (based on  and ),1044 (based on )|
|Remaining time until budget is used up (considering steady emissions) by 1st Jan 2021||8.5 years (based on ),7 years (based on )||18.4 years (based on ),15.6 years (based on )||29.2 years (based on ),24.9 years (based on )|
These targets however are also calculated with a probability. As shown in an earlier article on the IPCC even if the carbon emissions stay below these limits there are still chances that the temperature targets can be exceeded. A nice visualisation of the remaining budget and time is represented by the carbon clock  .
So far almost no country is on track with the Paris climate goals [SDG IISD 2019] and current policies are widely insufficient to reach that goal according to Climate Action Tracker [7, 8]. If we still want to stay below a 2°C warming, climate policies globally have to become more ambitious, and they have to do that as soon as possible, since time is running out.
Sebastian has studied physics and astrophysics before pursuing his PhD degree in meteorology on atmospheric dispersion modelling of radionuclides. From 2017 to 2020 he has been working at the Max-Planck Institute for Meteorology in Hamburg as scientific project manager for the decadal climate prediction system MiKlip, which is now in operational use at the German Weather Service. Since 2019 he is working at the Leibniz University Hannover as project coordinator for MOSAIK-2, a scientific project to develop an urban climate model to be used by city planners and city administration to adapt cities to climate change. Aside from that, Sebastian has been active in several volunteer groups and organisations determined to advocate sustainability, nature and climate protection. At ClimaTalk he is working as author and editor.
 IPCC, 2014: Climate Change 2014: Synthesis Report. Contribution of Working Groups I, II and III to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Core Writing Team, R.K. Pachauri and L.A. Meyer (eds.)]. IPCC, Geneva, Switzerland, 151 pp.
 IPCC, 2013: Summary for Policymakers. In: 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.
 IPCC, 2018: 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.)]. In Press.
 Our World in Data, URL: https://ourworldindata.org/grapher/cumulative-co-emissions?tab=table&stackMode=absolute&time=2011..latest®ion=World, (last accessed 6 March 2021).
 McSweeney, R., Tandon, A., 2020, Global Carbon Project: Coronavirus causes ‘record fall’ in fossil-fuel emissions in 2020, Carbon Brief,
URL: https://www.carbonbrief.org/global-carbon-project-coronavirus-causes-record-fall-in-fossil-fuel-emissions-in-2020, (last accessed 6 March 2021).
 Carbon Clock, URL: https://www.mcc-berlin.net/fileadmin/data/clock/carbon_clock.htm, (last accessed 6 March 2021).
 SDG IISD, 2019, URL: https://sdg.iisd.org/news/no-country-on-path-compatible-with-paris-climate-targets-2020-climate-index-warns/, (last accessed 6 March 2021).
 Climate Action Tracker, URL: https://climateactiontracker.org/, (last accessed 6 March 2021.)