This is the accepted version of the letter:
King, Lewis Carl; van den Bergh, Jeroen C. J. M.; Kallis, Giorgos. «Transparency
crucial to Paris climate scenarios». Science, Vol. 375 issue 6583 (Feb. 2022), p.
827-828. DOI 10.1126/science.abn7998


This version is available at https://ddd.uab.cat/record/279214

under the terms of the             license
Paris climate scenarios lack transparency

In their Policy Forum “Can updated climate pledges limit warming well below 2°C?” (5 November 2021, p. 693), Y. Ou and
coauthors explain that the updated pledges submitted as part of the Paris Agreement process, although an improvement, will
only result in a ~33% chance of staying within 2°C of warming. We agree with Ou et al. that more ambition is required, but
we think even the low chance of success they estimate is overly optimistic. The mitigation pathways based on Paris Agree-
ment pledges lack transparency and immediacy, making them difficult to evaluate and decreasing the chances of success.
           The updated pledges, if executed as written, will result in negligible global emission reductions of 1% by 2030
according to the authors. Substantive reductions are postponed until after 2030 on the assumption that countries will fulfill
their softer commitment to “long-term strategies.” In line with earlier studies using integrated assessment models (IAMs) (1),
these strategies heavily depend on negative emission technologies. For example, US fossil fuels and industry emissions are
expected to fall from 2.7 GtCO2 to –1.1 GtCO2 between 2030 and 2050 according to the “Updated pledges-Continued ambi-
tion” scenario of Ou et al. However, implementing negative emission technologies at such a large scale remains highly spec-
ulative and would present serious challenges to other sustainability dimensions (2). Analyses of this nature need to be trans-
parent about the extent that negative emission technologies are used. Only then can we evaluate the inherent risk of this goal
not being achieved.
           Other uncertainties merit structural attention as well. An energy transition to low-carbon alternatives could result in
a lower average energy return, leading to additional energy costs and indirect emissions (3). Moreover, modeled transitions
rely on decoupling gross domestic product and energy at a faster rate than historical trends, which underestimates rebound
effects caused by energy-efficiency policies (4). And differing levels of pledge ambition could lead to carbon leakage where
emissions are transferred to countries with weaker policies (5). The extent to which these neglected uncertainties are captured
in current and future IAM studies of the Paris Agreement pathways needs to be transparent and, ideally, quantified.



                                                            Lewis King1*, Jeroen van den Bergh1,2,3, Giorgos Kallis1,2
   1
       Universitat Autonoma de Barcelona, Institute of Environmental Science and Technology, Bellaterra, Catalonia, 08193, Spain.
             2
                 ICREA, Barcelona, Catalonia, 08010, Spain. 3Vrije Universiteit Amsterdam, 1081 HV, Amsterdam, The Netherlands.
                                                                           *Corresponding author. Email: lewiscarl.king@uab.cat




REFERENCES AND NOTES
1. A. A. Fawcett et al., Science 350, 1168 (2015).
2. K. Anderson, G. Peters, Science 354, 182 (2016).
3. L. C. King, J. C. J. M. van den Bergh, Nat. Energ. 3, 334 (2018).
4. P. E. Brockway, S. Sorrell, G. Semieniuk, M. K. Heun, V. Court, Renew. Sustain. Energ. Rev. 141, 110781 (2021).
5. L. C. King, J. C. J. M. van den Bergh, Clim. Change 165, 52 (2021).