by Benjamin J. Henley and Andrew D. King
A blog post by Hare et al (2017) from Climate Analytics criticised our paper published in Geophysical Research Letters, “Trajectories toward the 1.5°C Paris target: Modulation by the Interdecadal Pacific Oscillation”. This is a response to that critique.
Our paper uses observations and model projections to determine the proximity of global temperatures to the 1.5°C level. Its main focus is to investigate the influence of the Interdecadal Pacific Oscillation (IPO) on rates of global warming and temperature trajectories towards 1.5°C. With a slowdown in the rate of warming since around 2000, due substantially to the IPO, and the likelihood of an impending shift to the warm phase of the IPO, our paper’s results highlight the critical role of decadal variability in modulating the rate of warming towards the 1.5°C level.
The critique by Hare et al (2017), hereafter HEA17, explains our paper’s contribution towards underscoring the need for urgent near-term mitigation of climate change and the influence of natural variability on the trajectory towards 1.5°C of warming.
However, the HEA17 blog post, with due respect to its authors, falsely claims that our conclusion that the 1.5°C Paris temperature goal could be breached soon is “problematic” and “exaggerated”. HEA17 discuss two elements in their critique: the semantics around the definition of the 1.5°C Paris temperature goal, and the emissions scenario used in our analysis. We respond to these two points below.
Definition of the 1.5°C threshold
Our paper uses a conditional approach to investigate the rate of warming levels under IPO positive and negative conditions at warming rates consistent with RCP8.5 in the near term. It explores a number of different definitions of the 1.5°C warming level of interest to the global community. For example, we compare the timing of the 1-year and 5-year global mean temperature exceeding 1.5°C above pre-industrial levels. As a standalone study of global temperature, our study is within its right to explore such questions and is not bound by the subjective suggestions of external parties. Other studies have used 20-30 year windows, or longer. The choice of a smoothing window is a necessary but subjective choice and on such decisions, there can be no definitively correct choice. Instead, the key point is that the conclusions and interpretations of a scientific study must be mindful of the parameter settings of the study. The reviewers and editors were satisfied with our study’s methods and conclusions, and our public communication has been clear in its explanation of the study’s methodology.
We do not state, in our paper, nor in any associated communication, that a single year, or even several decades, above a warming level of 1.5°C constitutes a failure of the Paris agreement. On the contrary, we discuss the optimistic view that should a temporary breaching of the 1.5°C level occur, strong reductions in atmospheric greenhouse gas concentrations can return Earth to temperatures of 1.5°C of warming or below. Our analysis of the first year to breach the 1.5°C mark is motivated by the interest from the public and media we expect to see when this event occurs, as opposed to any broader implications with regards to policy.
HEA17 are critical of the use of RCP8.5 in our analysis, stating that the difference between warming projections in RCP8.5 and RCP2.6 in 2030 is around 0.2°C. However, in making this point, HEA17 neglect the actual emissions in the period from 2006 to the present, a period in which emissions have tracked significantly higher than RCP2.6, at, or slightly below RCP8.5, albeit with a recent stalling in emissions growth. A key point is that temperature trajectories over the next decade are more sensitive to the total accumulation of greenhouse gases, model uncertainty, and natural variability such as the IPO, than the actual emissions path over the next few years. Achieving a 2030 global annual temperature around the level projected by RCP2.6 would require a stronger future emissions reduction pathway from 2017 than the original RCP2.6 scenario, that commenced in 2006.
In any case, we chose RCP8.5 as it is the pathway that tracks most closely with observations. The CMIP5 model experiments predate the intended nationally determined contributions (INDCs) and pledges by governments at the Paris meeting. In choosing an appropriate concentration pathway for our analysis over the next 10-20 years, we placed a higher weight on observational evidence of emissions than the pledged INDCs. The use of RCP2.6 in this analysis, for which HEA17 are advocating, would be misrepresentative of observed accumulated emissions to 2017.
We acknowledge the concern that our paper might give some people the impression that we have no chance of meeting the Paris Agreement. This is a subtle point, since, as pointed out by HEA17, some interpretations of the Paris Agreement incorporate the possibility of an overshoot of 1.5°C, and others do not. Regardless, our study is not influenced by the politics of individuals or governments.
The critique and advocated approach taken by HEA17 is problematic. HEA17 advocate for an extremely optimistic scenario, especially given the fact that observed emissions since the start of the CMIP5 projections in 2006 have tracked far closer to the “business-as-usual” RCP8.5 scenario than the substantial action on climate change modelled using RCP2.6.
Our study clearly describes its methods and definitions, and asks readers to be mindful of those parameter settings in interpreting our findings.
We are in strong agreement with Climate Analytics and its esteemed researchers that governments can, and should, pursue every effort possible to reduce global emissions to minimise climate change and its impacts, and stabilise global temperatures at 1.5°C of warming or below.
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