By Professor Myles Allen, University of Oxford and Coordinating Lead Author, Chapter 1, Framing and Context, IPCC Special Report on 1.5 °C, 2018
As one of the relevant Coordinating Lead Authors on the IPCC Special Report on 1.5°C (SR15), I was concerned to see the following sentence appear in the draft Decision text (https://unfccc.int/sites/default/files/resource/Overarching_decision_1-CP-26.pdf), and further concerned to be told that Parties were informed that it was supported by SR15:
“17. Also recognizes that limiting global warming to 1.5 °C by 2100 requires rapid, deep and sustained reductions in global greenhouse gas emissions, including reducing global carbon dioxide emissions by 45 per cent by 2030 relative to the 2010 level and to net zero around mid-century;”
As it stands, the statement is not true, and not supported by SR15, because limiting global warming to 1.5 °C by 2100 does not require reductions in carbon dioxide emissions before 2030 if (crucially) it is allowed that global temperatures can be rapidly reduced by large-scale active carbon dioxide removal in the final decades of this century (the SR15 “P4” scenario). Any re-interpretation of 1.5 °C as referring solely to temperatures in 2100 has implications for near-term mitigation strategies and discussions of loss and damage.
What the SR15 actually said was: “In model pathways with no or limited overshoot of 1.5 °C, global net anthropogenic CO2 emissions decline by about 45% from 2010 levels by 2030 (40–60% interquartile range), reaching net zero around 2050 (2045–2055 interquartile range).”
Hence the sentence in the draft decision could be made consistent with SR15 simply by replacing “limiting global warming to 1.5 °C by 2100” with the phrase “limiting global warming to 1.5 °C with no or limited overshoot”, or in plainer language, “limiting global warming close to 1.5 °C” (the year 2100 is redundant: whether or not temperatures overshoot 1.5 °C will be determined long before then). SR15 interpreted “limited overshoot” as meaning less than 0.1 °C, this being comparable to natural variability in decadal-average temperatures and less than the uncertainty in human-induced warming to date (e.g. recent revisions to observational datasets increased the estimated warming between 1880 and 2012 by approximately 0.1 °C between the IPCC 5th and 6th Assessments).
This matters, as I see it, for four reasons: first, most impacts scale with the level of global warming at the time, so whatever peak warming is reached, the impacts associated with that level of warming will be experienced by at least one generation of humans and many non-human generations, with irreversible consequences regardless of the rate of cooling thereafter.
Second, it is not known how fast it will be possible to reduce global temperatures by active carbon dioxide removal, and at what cost. The IPCC 6th Assessment noted that net zero greenhouse gas (GHG) emissions aggregated using the conventional GWP100 metric results in a cooling trend in global temperatures, as ongoing emissions of short-lived GHGs such as methane are balanced by active carbon dioxide removal. In some scenarios this reduction is approximately 0.1 °C per decade, or about 20 billion tonnes of CO2 removal per year. But simply because an outcome is implied by the definition of net zero GHG emissions under a particular aggregation rule does not make it technically or economically feasible: 20 billion tonnes per year represents half the current emission rate.
Third, allowing scenarios that limit global warming to 1.5 °C by 2100 through arbitrarily high levels of carbon dioxide removal in the second half of this century effectively removes any imperative for early emissions reductions, with immediate implications for decisions regarding investment in fossil fuel infrastructure.
Finally, discussions of loss and damage may be affected by the assessment of whether collective efforts have been sufficient to meet the long-term temperature goal of the Paris Agreement. If “pursuing efforts to limit the temperature increase to 1.5 °C” is re-interpreted to mean “returning temperatures to 1.5 °C in 2100”, then it will not be possible to assess whether it has been achieved or not until after 2100.
It is important that Parties are clear on the full implications of the apparently innocuous clarification represented by the phrase “by 2100”.
