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With Climate Week NYC underway and the 2023 United Nations Climate Change Conference (COP28) approaching, it’s a good time to ask if the Paris Agreement, ratified at COP21 in 2015, is understood in its entirety. At the Paris conference, the objective to prevent dangerous anthropogenic interference with the climate system was specified as limiting global warming to well below 2°C, relative to pre-industrial levels, and pursuing further efforts to limit it to 1.5°C.[1] This is the temperature stabilization target set out in Article 2 of the Paris Agreement. Article 4 adds to Article 2; while both aim to limit global warming, Article 4 is focused on the net-zero target, stating that global peaking of greenhouse gases (GHGs) should be as soon as possible to achieve a balance between anthropogenic emissions by sources and removals by sinks of GHGs in the second half of this century.[2] The net-zero date largely coincides with the peak, after which the temperature stops increasing.
In this article, the authors argue that current net-zero narratives in policy discourse are not sufficiently attentive to Article 2, and show why—for limiting global warming—the net-zero pathway is as important as the net-zero date. Different possible emissions trajectories between now and the net-zero date, and the level of negative emissions thereafter, lead to very different cumulative CO2 emissions and resulting warming impact.[3]
Figure 1 illustrates this. The three different decarbonization trajectories achieve net zero by 2050, and yet the temperature rise associated with the “Late action” trajectory is double that of the “Early action” trajectory. To understand why, it’s essential to pay attention to the slope of each curve depicting the rate of emissions decline, and the area under it. Cumulative CO2 emissions associated with “Late action” are double that of the “Early action” pathway, leading to double the warming impact.[4]
Next the authors use the scenarios database from the Intergovernmental Panel on Climate Change’s (IPCC) Sixth Assessment Report (AR6) cycle Working Group III (WG3) to visualize the implications of different global pathways from integrated assessment models (IAMs) used to derive science-based targets (SBTs).
Cumulative Emissions under Science-Based Pathways
SBTs are data-driven results from integrated assessment and energy system models. They project future pathways to reduce emissions aligned with what the latest climate science shows is necessary to limit warming to well below 2°C and toward 1.5°C above pre-industrial levels. For this analysis, only CO2 emissions are considered; while other GHGs contribute to global temperature warming, CO2 is the single largest driver of the currently changing climate.[5]
Figure 2 is a global CO2 emissions and mean temperature chart based on CO2 emissions pathways averaged per IPCC scenario Categories C1 to C4, of eight. What’s important, once again, is to look at both the decarbonization pathway as well as the cumulative emissions under the curves. Together these dictate when the net-zero year is reached and by how much the world will overshoot the 1.5–2°C target along the way. In scenario Category C4, for example, the temperature stops rising after 2080, based on a greater cumulative area under the emissions curve, and warming is limited to 2°C with 2080 also being the net-zero year. In contrast, under scenario Category C1, the temperature stops rising as early as 2040 and warming is limited to 1.5°C, with a net-zero year of 2050.
The United States’ Role and Responsibility
The world is already 1.1°C hotter on average than before fossil fuel combustion took off in the 1800s.[6] Homing in on the United States provides further context given the country’s outsized contribution to global warming. The US is responsible for 23 percent of the cumulative domestic fossil CO2 emissions from 1870–2020, which equates to 0.25°C.[7] Ranking second to “All others” in Figure 3 (left bar), the US undeniably has a fundamental global role and responsibility to be a leader in achieving net zero. Moreover, US responsibility for warming levels correlates to its capacity to pay for the energy transition by investing further in zero-carbon energy and eliminating non-technical barriers. Article 4 of the Paris Agreement also states that net-zero targets should be achieved based on equity, with developed countries taking the lead in climate mitigation.[8]
NDCs and the ‘Ambition-Reality Gap’
Signatories to the Paris Agreement, including the US, adopted net-zero targets in policy documents referred to as Nationally Determined Contributions (NDCs) recorded in a public registry, with the US NDCs available here. In April, the Biden administration announced an updated NDC, reducing emissions 50–52 percent below 2005 levels by 2030. This positions the US as a leader in 2030 climate ambition when it comes to economy-wide emission reductions below historical levels,[9] but the historically large contribution of the US to global emissions means that the country urgently needs to increase its rate of emissions decline—moving away from the current straight-line path[10] toward a downward “Early action” sloping curve.[11]
Figure 4 visualizes both the current NDC trajectory and the SBT derived pathway necessary for the US to move from its current emission levels to its NDC targets for 2030 and net zero by 2050.
A recent report published by the Center on Global Energy Policy, “Tallying Updated NDCs to Gauge Emissions Reductions in 2030 and Progress toward Net Zero,” highlighted this gap between the ambition of NDCs and the trajectory necessary to meet net zero emissions by 2050.[12] The recent Net Zero Stocktake 2023 report that assessed the status and trends of net-zero target-setting across countries, sub-national governments, and companies also called on those “entities that have set a target now need to urgently make a robust strategy, aligned to the requirements of science.”[13]
The evidence clearly calls for urgent action by US policymakers to ramp up the pace of emissions reductions toward “Early action” to meet the objectives set forth in its NDC. The severity and frequency of climate-induced disasters—floods, droughts, or wildfires—demonstrate the pressing need for an immediate course correction.[14]
Every Fraction of a Degree Matters
The ramifications of each fraction of a degree of warming cannot be overstated, particularly for the world’s most vulnerable populations, which are suffering disproportionately high impacts of climate change.[15] The IPCC recently delivered the sobering message that the world’s collective ability to adhere to a 1.5°C pathway hangs in the balance.[16] “There is a rapidly closing window of opportunity to secure a liveable and sustainable future for all,” the IPCC outlined.
There is a limit to the remaining global carbon budget. In its recent report “Mitigation of Climate Change,” the IPCC said there is an urgent need to quicken the pace of decarbonization, requiring emissions to peak by 2025 and be cut 43 percent by 2030.[17] With COP28 fast approaching, the global community knows that the first Global Stock Take (GST) of the Paris Agreement about to take place will show the world is off track. But to echo the words of Sultan Ahmed Al Jaber, the president of COP28, in his July letter, “the GST can also be the turning point we need on climate action over this critical decade.” Urgent action is required by the world—with the US as a global leader—to reduce CO2 emissions, realign the trajectory of the energy transition, and retain some fighting chance of limiting global warming as the window of opportunity narrows.
[11] J. McGuire et al., “The role of carbon budgets in translating the Paris Agreement into national climate policy”; J. Glynn, M. Gargiulo, A. Chiodi, P. Deane, F. Rogan, and B. Ó Gallachóir, “Zero carbon energy system pathways for Ireland consistent with the Paris Agreement,” Climate Policy, vol. 19, no. 1, 30–42, 2019, https://www.tandfonline.com/doi/full/10.1080/14693062.2018.1464893.
[16] Intergovernmental Panel on Climate Change, “Summary for Policymakers,” in Climate Change 2023: Synthesis Report. Contribution of Working Groups I, II and III to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change,” H. Lee and J. Romero, eds. 1–34, doi: https://www.ipcc.ch/report/ar6/syr/downloads/report/IPCC_AR6_SYR_SPM.pdf.
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