17
projections include five new model estimates with full GHG coverage and 10 modelling groups from the 2017 Emissions Gap Report (UNEP, 2017). Appendix A.1, which is available online, provides a full overview of the studies considered for the current policy and NDC scenarios.
3.2.3 Least-cost mitigation scenarios consistent with the Paris Agreement’s temperature limits and updates following the IPCC Special Report on Global Warming of 1.5°C
Compared with previous Emissions Gap Reports, two major changes have been made to the 2018 assessment of global GHG emission levels in 2030 that are in line with least-cost pathways to limit warming to below 2°C or 1.5°C. As section 3.3 shows, these updates have implications for the estimated emissions gap in 2030, particularly for the 1.5°C target.
The 2018 assessment builds on the new scenario database compiled in the context of the IPCC Special Report on Global Warming of 1.5°C (Huppmann et al., 2018a, 2018b). Whereas 16 scenarios were available in 2017, 85 scenarios for 1.5°C and 2°C pathways are available this year.1
This expansion has enhanced the
diversity of available scenarios that are consistent with the stringent temperature goal of the Paris Agreement. In particular, recent scenarios often set a lower maximum potential for carbon dioxide removal (CDR), which results in deeper emissions reductions over the next decades to stay within the same overall carbon budget (see section 3.5.1)2
.
rationale behind this is that a nearly linear relationship exists between the total amount of anthropogenic CO2
emissions released into the atmosphere and the
increase in global mean temperature (Allen et al., 2009; Matthews et al., 2009; Meinshausen et al., 2009; Zickfeld et al., 2009; Collins et al., 2013. See also UNEP, 2014), meaning that warming under a specific future pathway can be estimated from its cumulative CO2
emissions.
calculated. The linear relationship also implies that CO2 emissions need to become net zero at the global scale
Alternatively, if a certain temperature limit is set, a corresponding consistent carbon (CO2
if warming is to be kept below any level (Matthews et al., 2012; Knutti and Rogelj, 2015). Achieving global net zero emissions means that CDR will compensate for any remaining human-related CO2
emissions.
The use of the cumulative CO2 emission proxy for grouping pathways has the advantage of linking more
) budget can be
The second major update concerns the method for grouping pathways. Previous Emissions Gap Reports used a simple climate model to estimate the temperature outcome of pathways, which were then used to group and describe various pathway classes. This year’s assessment still groups pathways as a function of their estimated temperature outcome, but uses the pathways’ cumulative CO2
emissions as a simpler proxy. The
directly to the discussions on emissions pathways under the United Nations Framework Convention on Climate Change (UNFCCC). It also facilitates future updates and improvements in understanding how the climate responds to anthropogenic GHG emissions.
onwards. These three scenario groups provide a more nuanced overview of pathways that keep warming below 2°C to 1.5°C and also help to identify the consequences of strengthened action at various degrees of ambition: from limiting warming to 2°C, over potential interpretations of ‘well below 2°C’, to pursuits to limit warming to 1.5°C and the corresponding emission reductions (see table 3.1). Each scenario considers least-cost climate change mitigation pathways that start reductions from 2020 and follow the climate model and set-up used in the IPCC 5th 3.1).
Based on these updates, pathways are grouped in three temperature scenario groups, according to their maximum cumulative CO2
emissions from 2018 Assessment Report (see box
• Below 2°C scenario: This scenario limits maximum cumulative CO2
until the time net zero CO2
emissions from 2018 emissions are reached
(or until 2100 if net -zero is not reached before) to between 900 and 1,300 GtCO2
consistent with limiting end-of-century warming to below about 2.0°C with about 66 percent or greater probability, while limiting peak global warming during the 21st
2018-2100 emissions to at most 1,200 GtCO2 century to below 2.1°C with about 66 percent or greater probability.
• Below 1.8°C scenario: This scenario limits maximum cumulative CO2
until the time net zero CO2
emissions from 2018 emissions are reached
2018-2100 emissions to at most 900 GtCO2
(or until 2100 if net-zero is not reached before) to between 600 and 900 GtCO2
, and cumulative . It is
consistent with limiting peak and end-of-century warming to below about 1.8°C with about 66 percent or greater probability.
• Below 1.5°C in 2100 scenario: This scenario limits maximum cumulative CO2
reached (all model realizations in this scenario reach net zero before 2100) to below 600 GtCO2
2018 until the time net zero CO2
the second half of the century are included. It is consistent with limiting global warming to below 1.5°C in 2100 with about 66 percent probability, while limiting peak global warming during the 21st century to between 1.6°C and 1.7°C with about 66 percent or greater probability. This scenario group is consistent with scenarios in the IPCC Special Report that limit warming to 1.5°C with no or limited overshoot.
and cumulative 2018-2100 emissions to at most 380 GtCO2
, when net negative CO2 emissions in
emissions from emissions are
, , and cumulative . It is
1 These include several model-intercomparison studies from the ADVANCE project (Luderer et al., 2018; Vrontisi et al., 2018), CD-LINKS (McCollum et al., 2018), EMF-33 (Bauer et al., 2018) and Shared Socioeconomic Pathways (SSPs) (Riahi et al., 2017; Rogelj et al., 2018), as well as a number of individual model studies focusing on various aspects of climate stabilization in the 1.5°C to 2°C range (Bertram et al., 2018; Grubler et al., 2018; Holz et al., 2018; Kriegler et al., 2018; Strefler et al., 2018; van Vuuren et al., 2018).
2 For more information, see the Summary for Policymakers of the IPCC Special Report on Global Warming of 1.5°C (IPCC, 2018).
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