Temperatures

Limiting warming to 1.5°C above pre-industrial levels means that the greenhouse gas emissions need to be reduced rapidly in the coming years, about halved by 2030, and brought to zero soon after 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 using the MAGICC climate model (see Methodology section).

The CAT determines a 1.5°C compatible benchmark using pathways from the IPCC AR6 pathways that limit warming to 1.5°C with no or low overshoot, and that are compatible with reaching net zero greenhouse gas emissions in the second half of this century, consistent with the Paris Agreement’s Articles 2.1 and 4.1. Reaching net-zero global greenhouse gas emissions in the second half of the century is important to align with Article 4.1 of the Paris Agreement and reduce climate change impacts. Pathways that achieve this in the IPCC AR6 fall in the IPCC's "C1a" category of pathways.

The CAT uses additional criteria to avoid pathways with unsustainable levels of carbon capture and storage (CCS) and unsustainable levels of carbon dioxide removal (CDR) from afforestation/reforestation (AR) and bioenergy use with CCS (BECCS).

BECCS and fossil CCS are technologies that are still at an early stage of development, and the feasibility of large-scale deployment is very unclear. In addition, BECCS (bioenergy with CCS) and carbon sequestration via afforestation/reforestation CDR can have significant negative impacts if deployed at large scales, such as competition with food production, large-scale water and fertiliser demands and biodiversity loss (Heck et al, 2018).

Pathways with lower reliance on CCS and CDR generally reduce emissions faster than those which deploy large amounts of these technologies (Grant et al, 2021a, 2021b), and so by selecting for pathways with limited CCS/CDR deployment consistent with sustainability limits identified by the IPCC, we see faster emissions reductions out to 2030. The currently used sustainability criteria are:

1) Limiting carbon sequestration via BECCS to 5GtCO₂/y in 2050 (Fuss et al, 2018)

2) Limiting carbon sequestration via Afforestation and Reforestation to 3.6GtCO₂/y in 2050 (Fuss et al, 2018)

3) Limiting deployment of fossil CCS to 3.8GtCO₂/y in 2050 (IPCC, 2022).

To give a better sample of scenarios relevant to the period to pre-2050 the CAT also considers pathways in the IPCC AR6 C1b category that do not reach net zero GHGs at a global level by 2100 but that show similar or faster emissions cuts over the next few decades to the C1a set. Many of the C1b pathways display faster decarbonisation in the 2020-2050 period because they explore how we could limit warming to 1.5°C without achieving net-negative CO₂ emissions (Riahi et al, 2021). Without this possibility, emissions need to fall even faster in the 2020s.

These pathways do not reach net-zero GHGs because CO₂ emissions fall to zero, but do not become significantly negative. That is, they reach net-zero CO₂ emissions, but no net-zero GHG emissions. To achieve net-zero GHG, net negative CO₂ emissions would be needed to compensate for residual CH₄ and N₂O emissions that may be impossible to eliminate fully (e.g., from agriculture).

Many of the C1b pathways (16 out of 24 selected C1b pathways) do not achieve net zero GHGs because of choices made in their design to prevent CO₂ emissions becoming net negative (Riahi et al 2021). These pathways provide valuable information on the near-term emissions reductions required to minimise carbon budget overshoot and avoid reliance on net-negative CO₂ emissions, but do not provide information on the long-term energy system configurations required to reach net-zero GHGs.

Most of the remaining 8 Clb pathways limit the level of CO₂ removal to very low levels (Luderer et al 2021, Strefler et al 2021), which prevents net-zero GHGs being met.

The CAT only includes C1b pathways if they pass the sustainability filters for CCS and CDR, and if they reduce emissions from 2020-2050 faster than the corresponding C1a pathways. This ensures that, although these pathways do not hit net-zero in the long-term, they are consistent with achieving net-zero GHGs in the second half of the century, as they reach lower emissions levels by 2050 than corresponding C1a pathways. Indeed, the faster decarbonisation of the global energy system pre-2050 means that these pathways would be well placed to achieve net-zero GHGs in the second half of the century with reduced reliance on carbon dioxide removal.

This represents a precautionary approach, where these additional pathways help ensure that our final 1.5°C compatible pathway is not overly reliant on achieving negative emissions (the feasibility of achieving which is challenging, as argued above). Post-2050, these C1b pathways are not included in the CAT’s 1.5°C compatible benchmark, as they do not achieve net-zero greenhouse gas emissions as required by the Paris Agreement.

Last update: 14 November 2024