Guest post: Why 1.5C and ‘well below’ 2C are part and parcel of one temperature goal

In 2015, the Paris Agreement set out an overall objective for global climate ambition – holding global temperature rise to “well below” 2C above pre-industrial levels while “pursuing efforts” to limit warming to 1.5C. 

From a physical science perspective, carbon budgets for 1.5C and 2C have established a (seemingly) simple fact – that global average temperature rises linearly with the cumulative emissions of carbon dioxide (CO2). 

However, then things can get a little more complicated. There are other greenhouse gases (GHGs) and pollutants to consider, as well as uncertainty around exactly how sensitive the Earth’s climate is to the accumulation of CO2 in the atmosphere.

A carbon budget is also static – a fixed total amount of CO2 that can still be emitted for a specific likelihood to keep below a warming limit. But global emissions will not be stopped overnight, so creating a pathway for how emissions are reduced is needed. This adds another dimension of uncertainty as well as a subjective element – such as how these remaining emissions are distributed over time.

In its recent contribution to the sixth assessment report (AR6), Working Group III (WG3) of the Intergovernmental Panel on Climate Change (IPCC) categorises emissions pathways according to their probability of not exceeding a certain warming limit – either at peak warming or at the end of the century. 

In all cases, the categorisation is in relation to just one warming level, which implies a division between pathways as being linked to either 1.5C or 2C.

In a new paper, published in Communications Earth & Environment, we show that this division in either 1.5C or 2C pathways does not really reflect the Paris Agreement context, and suggest criteria for pathways that could be considered compatible with the Agreement’s objectives.

Scenario categorisation

The AR6 WG3 report draws on a database of more than 3,000 different future emissions pathways. They are primarily generated by integrated assessment models (IAMs) that examine energy technologies, energy use choices, land-use changes and societal trends that cause – or prevent – GHG emissions.

The report then broadly divides these scenarios in eight different “climate categories”, labelled C1 through to C8, based on 21st century warming. The lowest three of these merit a closer look here:

• C1 pathways (“limit warming to 1.5C (>50%) with no or limited overshoot”). These pathways have a greater than 67% and greater than 50% likelihood of holding peak warming and end-of-century warming, respectively, to less than 1.5C.
• C2 pathways (“return warming to 1.5C (>50%) after a high overshoot”): These pathways have a less than 33% likelihood of keeping peak warming below 1.5C, but a greater than 50% likelihood that warming in 2100 will be less than 1.5C.
• C3 pathways (“limit warming to 2C (>67%)”): These pathways have a greater than 67% chance of keeping peak warming below 2C, while excluding all pathways that would also meet more stringent criteria such as established in C1.

The figure below illustrates the joint probabilities of IPCC WG3 mitigation pathways with regards to exceeding the 1.5C (y-axis) or 2C (x-axis) warming level. These are divided into C1 (green), C2 (blue) and C3 (pink) pathways, with crosses and dots indicating pathways that do and do not achieve net-zero GHG emissions, respectively.

In general, pathways positioned towards the bottom left have a better chance of staying below 1.5C or 2C, while those towards the top right have a worse chance.

A few observations can be made. For example, emissions pathways that merely limit warming to 2C with >67% chance have up to a 90% chance of exceeding 1.5C. Such pathways clearly do not keep 1.5C within reach. On the other hand, pathways that match the C1 criterion have a close to or even higher than 90% likelihood of not exceeding 2C. For some “no overshoot” pathways, this likelihood even increases to >95%.

Put another way, only a selection of the most stringent C1 “no or limited overshoot” 1.5C pathways actually ensure – or at least have a good chance – of not ever exceeding 2C. So, if avoiding a warming of 2C was the objective, then only the emissions reductions under a selection of C1 pathways would actually deliver.

Well below 2C – constraining a potential overshoot above 1.5C

In our paper, we consider the global climate policy implications of these observations. Article 2.1 of the Paris Agreement establishes the objective of “holding the increase in the global average temperature to well below 2C above pre-industrial levels and pursuing efforts to limit the temperature increase to 1.5C above pre-industrial levels”.

There is ambiguity on how this language can be interpreted, but there is an understanding that this is one temperature goal and not an either/or choice of meeting the 1.5C or 2C limit. They are part and parcel of the same goal.

The Paris Agreement language of holding warming “well below 2C” is a clear strengthening of earlier decisions by the UN Framework Convention on Climate Change (UNFCCC) from 2010 that set a temperature goal to hold warming “below 2C”. 

A common interpretation of the previous “below 2C” goal has been in terms of a likely (or greater than 66%) chance. The more stringent “well below 2C” objective takes the intent of avoiding warming of 2C even further. The next strongest IPCC qualification category above a “likely” (>66%) probability level is a “very likely” (>=90%) outcome. We would therefore argue that a plausible interpretation of the hold “well below 2C” objective would be a “very likely” chance of never exceeding that warming level.

As we have shown above, there are indeed emission pathways that are aligned with both criteria set out in the temperature goal. That is, those that pursue efforts to limit warming to 1.5C, and in case of a temporal temperature overshoot, always keep warming to “well below 2C”. These pathways are a subset of the broader C1 category of pathways.

When reflecting on the criteria set out in the Paris Agreement, however, we also need to look beyond just a temperature-based perspective and look towards the mitigation objectives expressed in Article 4. One of these – achieving a balance between anthropogenic emissions by sources and removals by sinks – is understood as the objective to achieve net-zero GHGs, thereby establishing an additional criterion against which to benchmark pathways.

Pathways that achieve net-zero GHGs are summarised in the IPCC WG3 report pathway as subcategory “C1a”. With net-zero GHGs leading to slowly declining temperatures, thereby ensuring any overshoot above 1.5C would be temporary, this provides an internally consistent interpretation of the Paris Agreement.

Taken together, we establish three criteria for Paris Agreement compatibility: pursuing efforts to limit warming to 1.5C, well below 2C, and net-zero GHGs. For pathways meeting these criteria, the world reaches net-zero CO2 and greenhouse gas emissions around 2050 and 2065, respectively.

In our study, we also show how pathway design criteria that are not rooted in the Paris Agreement – such as the 2100 temperature outcomes – can have significant implications. 

Here, we define “additional assumptions” as any deployment of carbon dioxide removal (CDR) that is not necessary to achieve and maintain net-zero CO2 and GHG emissions. Many pathways that achieve net-zero GHG emissions, for example, then continue to become net-negative towards the end of the century. 

As a result, in pathways that meet our criteria, we find that 6-24% of the total CDR deployed is the result of such additional assumptions included in the scenario design. In individual cases, this can amount to several hundred gigatonnes of CDR.

Given the controversy around the feasibility and potential negative side-effects of large-scale CDR deployment, understanding what level of CDR is actually required to achieve the Paris Agreement objectives, and what is not, is of critical importance. 

What pathways can be considered Paris Agreement compatible?

The IPCC WG3 report – reflecting its mandate of being policy relevant, but not prescriptive – has been very careful to not provide an explicit interpretation in that regard. However, the approach of the IPCC will of course be the basis on which policy perspectives on Paris Agreement-compatible pathways are formed.

We therefore argue that it is important to ensure transparency on how pathway characteristics and design assumptions relate to the climate objectives of the Paris Agreement, to help bridge the gap between science and policy.

Our proposed Paris-aligned categorisation can provide for a good starting point for further reflections. Where disagreements exist, a clear line of reasoning which engages with the arguments – and associated pathway mapping – we lay out, can help to steer policy discussions in a more transparent manner. 

One key element of this would be to acknowledge that the conventionally perceived separation of presenting pathways as either being linked to 1.5C or 2C is neither rooted in the Paris Agreement itself, nor well motivated scientifically.