Transient climate response to cumulative carbon emissions

The transient climate response to cumulative carbon emissions (TCRE) is the ratio of the globally averaged surface temperature change per unit carbon dioxide (CO2) emitted.[1][2][3][4][5] As emitted CO2 exhibits atmospheric lifetimes on millennial timescales, this response is conceived as the amount that global temperature changes per the amount of total carbon dioxide in the atmosphere.[6][1][3] With respect to cumulative CO2 emissions over time, global temperature is reasonably estimated to change linearly regardless of the path taken to reach peak CO2 emissions.[6][1][7][8][3] This means that for specific amount of cumulative CO2 emissions, a known global temperature change (within a range of uncertainty) can be expected, which indicates that holding global temperature change to below specific thresholds is a problem of limiting cumulative CO2 emissions, leading to the idea of a carbon budget.[3][4]

Calculation

Formulas

The TCRE is calculated based on a formula for the ratio of temperature change to cumulative carbon emissions (measured as CO2), which is the net carbon remaining in the atmosphere after accounting for relevant sources and sinks.[1] As a measure of atmospheric carbon change, the TCRE parameterizes climate sensitivity and carbon sensitivity to formulate a value that is the temperature change (°C) per trillion tonnes of carbon emitted (Tt C).[1][6] This is represented via the following formula from Matthews et al., 2009:

${\displaystyle TCRE=\bigtriangleup T/E_{T}=(\bigtriangleup T/\bigtriangleup C_{A})\times (\bigtriangleup C_{A}/E_{T})}$[1]

where,

• ΔT = average global temperature change (°C)
• ET = cumulative carbon dioxide emissions (Tt C)
• ΔCA = change in atmospheric carbon (Tt C)

and, 1Tt C = 3.7 Tt CO2

TCRE can also be defined not in terms of temperature response to emitted carbon, but in terms of temperature response to the change in radiative forcing as in Myhre et al., 2015:[9]

${\displaystyle TCRE=\bigtriangleup T/RF}$[9]

where,

• RF = radiative forcing (W/m2) taken at the top of the atmosphere (TOA)

Here TCRE is used to assess the assumed linear effect radiative forcing has on temperature change in an historical analysis.[9]

Modeling

TCRE is modeled using climate models that simulate carbon emissions by increasing CO2 emissions by 1% per year from pre-industrial levels until the concentration of CO2 in the atmosphere is doubled (2 x CO2) or quadrupled (4 x CO2).[10][1][3][4] Since these experiments all start from the same initial atmospheric concentration of CO2 (around 285 ppm[5]), the doubling and quadrupling occur at 70 and 140 years respectively. Different modelling parameterizations of TCRE include: holding CO2 emissions constant after quadrupling;[5] modelling net negative emissions after doubling or quadrupling;[7] stopping emissions after doubling and continuing the model for up to 10,000 years;[11] or running extended RCP scenarios and assessing temperature change per cumulative emissions at high CO2 concentrations.[8]

Temperature Response

Global response

Global temperature change is approximately linearly proportional to cumulative carbon emissions.[4][3] This means that for a given amount of carbon emissions, a related amount of global warming can reasonably be expected.[1][12] Model data synthesized by the IPCC Fifth Assessment Report from available studies suggests a likely TCRE of 0.8° to 2.5 °C per Tt C (or 1000 Pg C).[4] In a TCRE focus review, Matthews et al. (2018) estimate TCRE as 0.8° to 2.4 °C per Tt C and suggest an observationally-constrained best estimate of 1.35 °C per Tt C.[3]

Regional response

Though the global average temperature response to cumulative emissions is approximately linear, this response is not uniform throughout the globe.[3][2][13] Calculations by Leduc et al., (2016) of the geographical pattern of temperature response (the regional TCRE, or RTCRE) show values of low temperature change over equatorial and tropical ocean regions and high values of temperature change exceeding 4 °C/Tt C in the Arctic.[2] Likewise, they show a pronounced temperature response difference between the land and ocean, which is largely the result of ocean heat cycling.[2][5][14]

Regional precipitation response

Unlike the positive regional temperature response, regional precipitation change to cumulative emissions are positive or negative, depending on location.[13] Partanen et al., (2017) show a strong positive precipitation response in the Arctic with negative responses (meaning reduced precipitation) in parts of Southern Africa, Australia, North and South America.[13]

Carbon Budget

The observed and calculated linear TCRE and RTCRE leads to the notion of a carbon budget.[1][4][12][15] A carbon budget is the cumulative amount of CO2, emitted anthropologically as a globe, that leads to a set limit of global warming.[1][4][12][15] The IPCC estimates the CO2-only carbon budget (with a 50% chance) for staying below 2 °C at 1210 PgC (or 1.21 Tt C).[4] Accounting for the 515 PgC of CO2 emitted between 1870 and 2011, this leaves a CO2-only carbon budget of 695 PgC, for a 50% chance of staying below a global average temperature change of 2 °C.[4]

References

1. Matthews, H.D.; Gillett, N.P.; Stott, P.A; Zickfeld, K (June 11, 2009). "The proportionality of global warming to cumulative carbon emissions". Nature. 459 (7248): 829–832. doi:10.1038/nature08047. PMID 19516338.
2. ^ a b c d Leduc, M.; Matthews, H.D.; de Elia, R. (January 4, 2016). "Regional estimates of the transient climate response to cumulative CO2 emissions". Nature Climate Change. 6 (5): 474–478. doi:10.1038/NCLIMATE2913.
3. Matthews, H.D.; Zickfeld, K.; Knutti, R.; Allen, M.R. (January 12, 2018). "Focus on cumulative emissions, global carbon budgets and the implications for climate mitigation targets". Environmental Research Letters. 13: 010201. doi:10.1088/1748-9326/aa98c9.
4. Collins, M.; Knutti, R.; Arblaster, J.; Dufresne, J.-L.; Fichefet, T.; Friedlingstein, P.; Gao, X.; Gutowski, W.J.; Johns, T.; Krinner, G.; Shongwe, M.; Tebaldi, C.; Weaver, A.J.; Wehner, M. (2013). Stocker, T.F.; Qin, D.; Plattner, G.-K.; Tignor, M.; Allen, S.K.; Boschung, J.; Nauels, A.; Xia, Y.; Bex, V. (eds.). "Long-term climate change: Projections, commitments and irreversibility". 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. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.
5. ^ a b c d Gillett, Nathan P.; Arora, Vivek K.; Matthews, Damon; Allen, Myles R. (2013-09-09). "Constraining the Ratio of Global Warming to Cumulative CO2Emissions Using CMIP5 Simulations*". Journal of Climate. 26 (18): 6844–6858. doi:10.1175/jcli-d-12-00476.1.
6. ^ a b c Allen, M.R.; Frame, D.J.; Huntingford, C.; Lowe, J.A.; Meinshausen, M.; Meinshausen, N. (April 30, 2009). "Warming caused by cumulative emissions towards the trillionth tonne". Nature. 458 (7242): 1163–1166. doi:10.1038/nature08019. PMID 19407800.
7. ^ a b Zickfeld, K.; MacDougall, A.H.; Matthews, H.D. (May 12, 2016). "On the proportionality between global temperature change and cumulative CO2 emissions during periods of net negative CO2 emissions". Environmental Research Letters. 11 (5): 055006. doi:10.1088/1748-9326/11/5/055006.
8. ^ a b Tokarska, K.B.; Gillett, N.P.; Weaver, A.J.; Arora, V.K.; Eby, M. (May 23, 2016). "The climate response to five trillion tonnes of carbon". Nature Climate Change. 6 (9): 851–855. doi:10.1038/NCLIMATE3036.
9. ^ a b c Myhre, Gunnar; Boucher, Olivier; Bréon, François-Marie; Forster, Piers; Shindell, Drew (March 2015). "Declining uncertainty in transient climate response as CO2 forcing dominates future climate change" (PDF). Nature Geoscience. 8 (3): 181–185. doi:10.1038/ngeo2371. ISSN 1752-0908.
10. ^ Williams, Richard G.; Goodwin, Philip; Roussenov, Vassil M.; Bopp, Laurent (2016). "A framework to understand the transient climate response to emissions". Environmental Research Letters. 11 (1): 015003. doi:10.1088/1748-9326/11/1/015003. ISSN 1748-9326.
11. ^ Frölicher, Thomas L.; Paynter, David J. (2015). "Extending the relationship between global warming and cumulative carbon emissions to multi-millennial timescales". Environmental Research Letters. 10 (7): 075002. doi:10.1088/1748-9326/10/7/075002. ISSN 1748-9326.
12. ^ a b c Frame, David J.; Macey, Adrian H.; Allen, Myles R. (2014-09-21). "Cumulative emissions and climate policy". Nature Geoscience. 7 (10): 692–693. doi:10.1038/ngeo2254.
13. ^ a b c Partanen, Antti-Ilari; Leduc, Martin; Matthews, H. Damon (2017). "Seasonal climate change patterns due to cumulative CO 2 emissions". Environmental Research Letters. 12 (7): 075002. doi:10.1088/1748-9326/aa6eb0. ISSN 1748-9326.
14. ^ Bryan, K.; Komro, F.G.; Manabe, S.; Spelman, M.J. (January 1, 1982). "Transient climate response to increasing atmospheric carbon dioxide". Science. 215 (4528): 56–58. doi:10.1126/science.215.4528.56. PMID 17790468.
15. ^ a b Millar, Richard; Allen, Myles; Rogelj, Joeri; Friedlingstein, Pierre (2016-01-01). "The cumulative carbon budget and its implications". Oxford Review of Economic Policy. 32 (2): 323–342. doi:10.1093/oxrep/grw009. ISSN 0266-903X.