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 3.4°C 1 warming above pre-industrial levels. The unconditional pledges or promises that governments have made, including NDCs2 as of November 2017, would limit warming to about 3.16°C 3 above pre-industrial levels, or in probabilistic terms, likely limit warming below 3.5°C.
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
We base our assessment on the emissions scenarios currently available in the scientific literature, mindful of the assumptions and limitations underlying these studies. The scenarios limit warming below 1.5°C by 2100 with ≥50% probability, and below 2°C in the 21st century with about an 80% probability. We use these scenarios as indicative of the Paris Agreement’s long-term temperature goal, but note these scenarios should not be seen as a definitive interpretation of the Paris Agreement.
A new and more diverse set of scenarios that limit warming to 1.5°C or below are now being developed by the scientific community and will become available in the open literature. The CAT will update its Paris Agreement benchmark scenarios in 2018 to reflect the most recent available scientific literature (more details at CAT Rating System Update).
Limiting warming 1.5°C above pre-industrial 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 (promises) and NDCs put forward by countries, and compares these with the emissions levels consistent over time with the 1.5°C. The CAT also presents results in relation to a pathway consistent with limiting warming to 2°C with likely (≥66%) probability, for comparative purposes.
1 | 3.4°C is the median of the low and high ends of current policy projections (3.1 to 3.7°C). The uncertainty range on the figure (2.5-4.7°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.