Article 2 of the United Nations Framework Convention on Climate Change (UNFCCC) requires its 197 parties to achieve “stabilisation of greenhouse gas concentrations in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate system”. As scientists are making clear, this cannot occur without phasing out anthropogenic emissions of greenhouse gases.

In the 2015 Paris Agreement, UNFCCC Parties committed to limiting global temperature rise “to well below 2ºC above pre-industrial levels and to pursue efforts to limit the temperature increase to 1.5ºC above pre-industrial levels”.

Staying well below the 2ºC limit and keeping pathways to 1.5ºC open means ensuring that global emissions peak by 2020 and then decline to net zero around mid century.

The Paris Agreement acknowledges this scientific imperative by requiring Parties to achieve a balance of emissions (i.e. net zero emissions) in the second half of this century and asks them to submit mid-century, long-term low greenhouse gas emissions development strategies to show how to achieve this phase out (Article 4.1 of the Paris Agreement and Decision 1/CP21, paragraph 36).

Recent scientific assessments provide an overview of what emissions reductions pathways would ensure maximum temperature rise of 2ºC and 1.5ºC.Their key conclusions are as follows:

1) To return warming to below 1.5ºC by 2100 with a more than 50% chance:

  • Global emissions of CO2 only must reach zero by around 2050 (range 2045-2050)
  • Global emissions of all greenhouse gases must reach zero by 2060-2080

The shorter time frame for CO2 emissions is to ensure a full phase out of energy-related fossil fuel emissions. The phase out for gases such as methane and agricultural related CO2 emissions may take longer.

The phase out dates above for having a more than 50% chance of returning to below 1.5ºC by 2100 correspond to having a “high probability”, i.e., about 85%, to hold warming below 2ºC during the 21st century.

2) To limit warming below 2ºC by 2100 with a “likely probability” (of more than a 66% chance):

  • Global emissions of CO2 must reach zero at a slightly later timeframe of around 2055-2070
  • Global emissions of all greenhouse gases must reach zero by a slightly later timeframe of 2080-2100

The graphic below helpfully sets out the global emissions reductions pathways implied by these carbon budgets:Source of graphic: Oil Change International (2016)

The carbon budgets upon which these phase out dates are based are set out below. These are compiled from data and scenarios from the IPCC Fifth Assessment Report (2014), the UNEP Emissions Gap Report (2014), and the IIASA/Rogelj (2015).

Year of globally zero emissions (approximate) and emissions budgets:Source of table: Climate Analytics (2016)

Although the IPCC has modelled 1.5ºC trajectories, the majority of scientists’ focus has been on modelling pathways to avoid 2ºC maximum temperature rise. The Paris Agreement aims to redress this gap in scientific literature and analysis by creating the mandate for the IPCC to assess more pathways on how to keep to a maximum 1.5ºC temperature rise.

In Paris, Parties agreed to “Invite[s] the Intergovernmental Panel on Climate Change to provide a special report in 2018 on the impacts of global warming of 1.5 ºC above pre-industrial levels and related global greenhouse gas emission pathways” (Decision 1/CP21, paragraph 21).

The IPCC has now commenced work on this important report. More details of its work schedule on the 1.5ºC Report can be found here.

It should be noted that the IPCC’s Fifth Assessment Report in 2014 and assessments made by organisations such as Climate Analytics and Climate Action Tracker in the run up to Paris already contain many scientific assessments on the implications and pathways for 1.5ºC warming. Much of this literature was assessed by the UNFCCC Structured Expert Dialogue (SED) process, which was set up after the 2009 Copenhagen COP15 to look at whether the 2oC goal should be strengthened to 1.5oC.

The scientific assessment work of the SED was one reason why so many countries came to Paris insisting the final agreement needed to contain a long-term emissions reduction goal in line with the pathways for keeping temperature rise well below 2ºC and preferably the safer limit of 1.5ºC. Their argument was that a long-term goal of carbon neutrality or net zero by mid-century should be included in the Paris Agreement to operationalise the agreed temperature limit, and that this would send a signal to the real world economy that the era of fossil fuels was over. For a list of which countries supported the long-term goal of 1.5 and phase out emissions to zero, please see Track 0 tracking resources on the Paris Negotiations.

The scientific imperative embodied in the Paris Agreement is also recognised as an evolving field that requires flexibility, measurement, reporting and increasing ambitions and financing of Parties as climate change unfolds. An integral complement to the political processes of countries submitting long-term mid-century phase out strategies is the Paris Agreement’s provision for a ‘global stocktake’ (Article 14 of the Paris Agreement). The first such opportunity for a stocktake is the Facilitative Dialogue, which is to be convened in 2018 in accordance with Decision 1/CP.21, paragraph 20. This commits Parties to the Paris Agreement “to convene a facilitative dialogue among Parties in 2018 to take stock of the collective efforts of Parties in relation to progress towards the long-term goal referred to in Article 4, paragraph 1, of the Agreement and to inform the preparation of nationally determined contributions pursuant to Article 4, paragraph 8, of the Agreement”.

The Facilitative Dialogue in 2018 and the global stocktake in 2023 will enable the UNFCCC to assess aggregate progress (total of all countries) to date against the trajectory required to stay within the 2ºC/1.5ºC global temperature limits, in order to ‘inform’ Parties’ Nationally Determined Contributions (NDCs) in an effort to enhance their ambition and stay on track to achieve net zero by the second half of the century.

Raising ambition of existing and future NDCs is key to the success of the Agreement over time, as current commitments still result in a gap between what’s on the table and what’s necessary until the Agreement comes into force in 2020, as demonstrated in the graphic below.

UNFCCC, UNEP and civil society reviews have found current commitments are consistent with a 2.7ºC pathway, demonstrating the importance of the reviews and global stocktake for keeping the Agreement on track for net zero, in line with the scientific imperative.

Source of graphic: UNFCCC Updated Synthesis Report on the Aggregate Effects of INDCs (2016)


Please refer to the links and reports in our Resources section for further information.

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