Addressing global warming
In the absence of policies global warming is expected, to reach 4.1 °C – 4.8 °C above pre-industrial by the end of the century. The emissions that drive this warming are often called Baseline scenarios (‘Baselines’ in the above figure) and are taken from the IPCC AR5 Working Group III. Current policies presently in place around the world are projected to reduce baseline emissions and result in about 3.3°C 1 warming above pre-industrial levels. The unconditional pledges and targets that governments have made, including NDCs 2 as of December 2018, would limit warming to about 3.0°C 3 above pre-industrial levels, or in probabilistic terms, likely (66% or greater chance) limit warming below 3.2°C. This result is similar to our estimate last year, reflecting the fact that little has changed in terms of government commitments and targets in the past 12 months.
We also ran an “optimistic” scenario that factors in additional as well as planned, but not yet implemented, policies and a continuation of recent developments. Under the optimistic assumption that governments will continue to meet these expectations, the median warming estimate is 3.0°C, or in probabilistic terms, likely below 3.2°C, which is equal to the globally aggregated effect of NDCs.
There remains a substantial gap between what governments have promised to do and the total level of actions they have undertaken to date. Furthermore, both the current policy and pledge trajectories lie well above emissions pathways consistent with the Paris Agreement long-term temperature goal.
Evaluating progress towards the Paris Agreement
Limiting warming to 1.5°C above pre-industrial levels by 2100 means that the emissions of greenhouse gases need to be reduced rapidly in the coming years and decades, and brought to zero around mid-century.
The CAT evaluates progress towards this global goal by quantifying the aggregate effects of current policies and the pledges and targets put forward by countries, and compares these with the emissions levels consistent over time with the 1.5°C limit. The CAT also presents results in relation to a pathway consistent with limiting warming to 2°C with likely (≥66%) probability, for comparative purposes.
Prompted by the publication of the IPCC Special Report on 1.5 °C (IPCC SR1.5) (Masson-Delmotte et al., 2018), which contained a large new set of mitigation pathways—in particular for 1.5°C—the CAT has updated its “benchmark” pathways for 1.5°C and 2°C. The IPCC SR1.5 assesses pathways that limit global warming to 1.5°C with no or limited overshoot, which the CAT uses to define “Paris Agreement compatible”—or “1.5°C compatible“—pathways.
Based on sustainability and economic constraints on Carbon Dioxide Removal (CDR), the IPCC SR1.5 identified limits to BECCS (below 5 GtCO2e/yr globally in 2050) and AFOLU (below 3.6 GtCO2/yr sequestration globally in 2050). For CAT’s “1.5°C compatible“ and “2°C compatible” benchmark pathways, the new sets of mitigation pathways have been filtered to exclude those that do not meet these CDR limits.
For consistency with IPCC SR1.5, and to show the most robust results across models, the new CAT benchmark pathways follow the approach to derive median and inter-quartile ranges (50% ranges) extracted from the IPCC SR1.5 database for total global greenhouse gas emissions (for those pathways that meet the CDR criteria above).
The central “1.5°C compatible“ benchmark is defined as the median of pathways that limit global warming to 1.5°C, or below, throughout the 21st century with no or limited (<0.1°C) overshoot (for the pathways meet the CDR criteria above). In these pathways global average temperature increases above pre-industrial are limited to below 1.6°C over the 21st century and below 1.5°C by 2100 (typically 1.3°C). “1.5°C compatible“ emissions levels in 2030 are consistent with IPCC SR1.5 SPM (25–30 GtCO2e/year based on SAR GWP) but due to CDR constraints are 1 GtCO2e/year lower for median and 2 GtCO2e/year lower for top end of range.
The “2°C” compatible benchmark pathways are drawn from the “lower-2°C” and “high-overshoot 1.5°C” pathways in the new set of IAM pathways assessed in IPCC SR1.5. This includes all pathways in the IPCC SR1.5 scenario database that have global average temperature increase held below 2°C above pre-industrial with at least a likely (>66%) probability over the 21st century, excluding pathways that qualify as “1.5°C compatible” as explained above. As above, these pathways are filtered to exclude those that exceed the BECCS and AFOLU sustainability limits identified in the IPCC SR1.5. The final IPCC SR1.5 pathway category of “higher-2°C” is not included at all, as these pathways hold warming below 2°C throughout the 21st century with a probability less than 66% (50–66%).
Previous CAT pathways were based on immediate-action 2010 scenarios, which start strong global mitigation action in 2010 and show an emissions gap in 2020 compared to Cancun targets and projected actual emissions. The new pathways are based on so-called “delayed-action” 2020 scenarios, which have higher emissions levels in 2020 and do not show a 2020 emissions gap. The higher 2020 levels lead to somewhat higher 2025 levels, somewhat lower 2030 levels and substantially lower 2050 levels to compensate for the higher emissions in the period 2010–2025, compared to the previous CAT benchmark pathways for both 1.5°C and 2°C.
Last updated: 11 December 2018
1 | 3.3°C is the median of the low and high ends of current policy projections (3.1 to 3.5°C). The uncertainty range on the figure (2.5-4.4°C) originates from carbon-cycle and climate modelling around both the low and high current policy projections. Here we give the 68% range; that is the 16th and 84th percentiles of the probability distribution.
2 | In the term NDC we also include, throughout this briefing, the “Intended” Nationally Determined Contributions of governments who have not yet ratified the Paris Agreement. We project the global effects beyond the NDC timeframes assuming policies of similar strength are implemented through to the end of the century.
3 | The uncertainty range for pledges on the CAT thermometer (2.6-4.0°C) originates from carbon-cycle and climate modelling around both the low and high current policy projections. Here we give the 68% range; that is the 16th and 84th percentiles of the probability distribution. If the more ambitious end of pledge ranges were reached, warming could be limited to a median of 2.6°C. This includes the upper end of country INDCs where a reduction range is specified and mitigation commitments that are conditional on finance.