Historical and anticipated future total CO2 emissions to 2050 show more agreement with Representative Concentration Pathway 8.5 (RCP8.5) than other Coupled Model Intercomparison Project 5 (CMIP5)-era RCPs (1). Hausfather and Peters (2) attempt to argue against this by emphasizing 1) RCP4.5 or RCP6.0 better match fossil fuel emissions (FF) relative to International Energy Agency scenarios; and 2) our future emissions from land-use change (ELUC) diverge from RCPs and the new Shared Socioeconomic Pathways (SSPs).
What matters for the purpose of providing input to climate models, however, is total atmospheric CO2 content, not how much of that CO2 came from FF vs. ELUC. Assumed errors in the socioeconomic assumptions behind CMIP5-era RCPs are similarly irrelevant. The CMIP5 recommendation for CO2 in each RCP is a single global constant given as an annual midyear mean mass mixing ratio (3) reflecting the sum of all pathways of CO2 into the atmosphere. In our analysis, ELUC comprises between 5 and 15% of the total cumulative CO2 emissions from 2005 to 2050. There is no justification for ignoring this emissions component in assessing RCP suitability. Also, we cannot use one RCP for FF and another RCP for ELUC. Only total atmospheric CO2 content is a useful benchmark and only one RCP can be the best match, that is, RCP8.5.
Hausfather and Peters (2) also argue that historical trends in ELUC are uncertain while criticizing our future ELUC for not trending downward as shown in RCPs and SSPs after 2020. Historical ELUC estimates vary (4). This is linked to the diversity in ELUC estimation techniques. However, Hausfather and Peters (2) show an uptick in historical ELUC (their figure 2). Focusing on decadal summaries from the Global Carbon Project also reveals a clear, albeit small, increase over the past three decades (4). There is no evidence that we have reached peak ELUC or peak deforestation. Consider a 2015 assertion that deforestation had peaked in 2012 (5) only for that “peak” value to now have been exceeded in 2016, 2017, 2018, and 2019 for tree cover loss and primary forest lost globally (6). A future downward trend in ELUC is at odds with observational evidence and emerging geopolitical drivers (7, 8).
We agree with Hausfather and Peters (2) that reducing uncertainty in ELUC is desirable. Better integration across climate and energy modeling communities is also necessary to create better scenarios for future climate assessments. Furthermore, addressing missing biotic feedbacks in global climate models—a first-order uncertainty that in extremis is on par with today’s remaining 1.5 °C carbon budget—is even more crucial. However, the stylized facts and socioeconomic assumptions used in creating the CMIP5 RCPs almost two decades ago do not inform which RCP merits use as a climate planning scenario today. Given what is known about biotic feedbacks, our current path, and the success of past forecasts to anticipate human behavior, RCP8.5 is the preferred choice for assessing the climate humans currently live in and is the best tool for assessing the risks to come through midcentury.
Footnotes
The authors declare no competing interest.
References
- 1.Schwalm C. R., Glendon S., Duffy P. B., RCP8.5 tracks cumulative CO2 emissions. Proc. Natl. Acad. Sci. U.S.A. 117, 19656–19657 (2020). [DOI] [PMC free article] [PubMed] [Google Scholar]
- 2.Hausfather Z., Peters G. P., RCP8.5 is a problematic scenario for near-term emissions. Proc. Natl. Acad. Sci. U.S.A. 117, 27791–27792 (2020). [DOI] [PMC free article] [PubMed] [Google Scholar]
- 3.Meinshausen M. et al., The RCP greenhouse gas concentrations and their extensions from 1765 to 2300. Clim. Change 109, 213–241 (2011). [Google Scholar]
- 4.Friedlingstein P. et al., Global carbon budget 2019. Earth Syst. Sci. Data 11, 1783–1838 (2019). [Google Scholar]
- 5.Union of Concerned Scientists , Deforestation has decreased (22 September 2015). https://blog.ucsusa.org/doug-boucher/deforestation-has-decreased-888. Accessed 1 September 2020.
- 6.Global Forest Watch , Global forest deforestation rates and statistics by country. https://gfw.global/3bfYy6Q. Accessed 1 September 2020.
- 7.Escobar H., Brazilian president attacks deforestation data. Science 365, 419 (2019). [DOI] [PubMed] [Google Scholar]
- 8.Fuchs R. et al., Why the US–China trade war spells disaster for the Amazon. Nature 567, 451–454 (2019). [DOI] [PubMed] [Google Scholar]