IPCC on How to Tackle Climate Change, Part 4: What would it take to limit warming to 1.5 or 2C?

Limiting global warming to 1.5C or 2C would mean “rapid and deep” emissions reductions in “all sectors” of the global economy, says the latest report from the United Nations’ Intergovernmental Panel on Climate Change (IPCC). Instead, emissions have continued to rise – albeit at a slowing rate – and it will be “impossible” to stay below 1.5C with “no or limited overshoot” without stronger climate action this decade, says the new document, which forms part of the IPCC’s sixth assessment report (AR6).  In this in-depth Q&A series, Carbon Brief unpacks the key findings of the latest report. (Read previous section here)

By Carbon Brief

4. What would it take to limit warming to 1.5 or 2C?

The WG3 report assesses a wide range of scenarios associated with 1.5C (C1 and C2) and 2C (C3 and C4) outcomes. The lowest of these scenarios (C1), where warming is limited to 1.5C with no or limited overshoot, has become notably more difficult to achieve since the IPCC special report on 1.5C was published in 2018, due to four years of flat or modestly increasing global emissions. These scenarios are now associated with higher peak warming in most models, leading to a lower likelihood of limiting warming to 1.5C, the WG3 report says.

The report features five different illustrative mitigation pathways that explore different ways the world might limit warming to 1.5C or below 2C – in addition to the Mod-Act and Cur-Pol pathways discussed above.

Three of the illustrative pathways – “shifting development pathways” (SP), low demand (LD) and high renewables (Ren) – produce climate outcomes that limit warming to 1.5C with “no or limited overshoot” (C1).

A pathway featuring extensive use of net-negative emissions (Neg) results in warming below 1.5C in 2100, but with “high overshoot” during the 21st century (C2), while a pathway characterised by a gradual strengthening of current policies (GS) likely (>66% chance) limits warming to below 2C by 2100 (C3). The Mod-Act pathway – in which countries meet their 2030 commitments, but with little additional climate action – likely results in warming of between 2.5C and 3C by 2100 (C6).

Although the report outlines various ways to limit warming to 1.5C by 2100 (C1-C2), all of these pathways show deep emissions reductions over the next few decades, with both global CO2 and all GHG emissions peaking “between 2020 and at the latest before 2025” and CO2 emissions declining between 25% (Neg) and >50% (LD) by 2030 relative to 2020 levels.

Overall, in scenarios staying below 1.5C with no or limited overshoot, global GHG emissions fall to 43% below 2019 levels by 2030 and 84% by 2050. In these scenarios, global CO2 falls 48% by 2030 and to net-zero “in the early 2050s”, says the report. Methane emissions fall 34% by 2030.

The more intermediate GS pathway, which likely limits warming to 2C but not 1.5C, has global emissions declining by around 14% by 2030 relative to 2020 levels. More broadly, it says keeping warming below 2C means cutting CO2 emissions 50% “in the 2040s” and to net-zero “around the early 2070s”.

Each of the five mitigation-focused illustrative pathways has a somewhat different approach to cutting emissions, energy use and energy production. GS has a relatively slow phase-out of fossil fuels compared to the others, with high future energy demand primarily met by renewables and a fair amount of CDR deployed late in the century – including lots of direct air capture.

Neg relies heavily on biomass and bioenergy with carbon capture and storage (BECCS) for CDR, and also has a relatively slow phase out of fossil fuels and high future energy demand. Ren has a rapid phaseout of fossil fuels, more moderate future energy demand that is primarily met by renewables, and more limited deployment of CDR.

Finally, LD and SP both have large reductions in future energy demand and rapid fossil fuel phaseouts, with LD including an even faster fossil fuel use exit with limited negative emissions other than land use change (LUC), while SP has more reliance on late-century CDR.

The figure below shows the amount of positive emissions (orange, red, and brown) and CDR (purple, blue, and green) over time in the illustrative pathways.

Each illustrative pathway and climate category has a range of possible climate outcomes, depending on two areas of uncertainty. First, the uncertainty in emissions across all scenarios in a climate category. Second, climate system uncertainties that depend on how sensitive the climate is to CO2 and other GHG emissions and how carbon cycle feedbacks might change as the Earth warms.

In addition to these new WG3 scenarios, the IPCC WG1 provided “assessed warming” estimates for five different future climate scenarios based on Shared Socioeconomic Pathways (SSPs). The SSPs reflect five specific emissions scenarios with differing end-of-century outcomes and are used by complex earth system models to simulate various aspects of the changing climate.

The WG3 scenarios, by contrast, explore a much wider variety of possible emissions futures, but are evaluated using simpler climate model emulators that only provide global average temperature changes.

The figure by Carbon Brief below provides a comparison between the possible amount of warming that may occur in the year 2100 under each of the different IP, climate category and SSP scenarios. The colour of the bar indicates the type of scenario (IP, climate category, or SSP), the dot gives a central warming estimate, the bars (when available) show the range of warming across different emissions scenarios in the category and the whiskers show the range of warming accounting for climate system uncertainties.

The climate categories and illustrative pathways provide a detailed look at a wide range of scenarios (SP to C4) that are “more likely than not” to limit warming to below 2C by 2100, similar to SSP1-1.9 and SSP1-2.6.

The report notes that while net-zero CO2 emissions are expected to lead to relatively flat temperatures, net-zero GHG emissions result in declining global temperatures.

In practice, pathways reaching net-zero GHGs tend to balance out residual non-CO2 GHG emissions with net-negative CO2 emissions. Net-zero CO2 emissions are generally reached by models around 10 to 40 years before net-zero GHG emissions, for those scenarios reaching net-zero GHGs in the 21st century.

While most scenarios that limit warming to well-below 2C (C1-C3) employ significant amounts of net-negative emissions, not all do. Scenarios with more immediate emissions reductions tend to have no or low overshoot and limit the amount of late-century CDR needed.

However, even in the most ambitious emission reduction scenarios (C1), most pathways still have a limited amount of overshoot, with the median model in that category having a 2 in 3 risk of overshooting 1.5C at some point in the 21st century.

Pathways that limit warming to 1.5C with no or limited overshoot (C1) reach net-zero CO2 emissions “between 2050-2055” on average, with a range of between 2035 and 2070 across all the scenarios assessed.

C2 scenarios, on average, reach net-zero between 2055 and 2060 with a range from 2045 to 2070, while C3 scenarios reach net-zero “between 2070-2075” with a range of 2055 to 2100 or later. The median net-zero year and range across all the scenarios are shown for each climate category and illustrative pathway limiting warming to below 2C in the Carbon Brief chart below.

Year in which net-zero CO2 emissions are reached across climate categories (C1-C4) and illustrative pathways (LD, Neg, Ren and GS) that limit warming to below 2C by 2100. Climate categories show the median and range of net-zero years across all scenarios in that category. Chart by Carbon Brief using Highcharts.

The fact that some of these models have a later net-zero emissions date does not imply that emissions reductions can be delayed, or that we “have more time to rapidly reduce current emissions levels compared to SR1.5”.

Rather, the scenarios with the latest peak year are often the ones with largest near-term emissions reductions, as they reserve more of the remaining carbon budget for later years.

There are different mitigation strategies that can be employed to limit warming to well-below 2C, as shown by the illustrative pathways. Some have a heavy reliance on renewables (Ren), a strong emphasis on energy demand reductions (LD), wide-spread deployment of CDR (Neg), or focus mitigation in the context of sustainable development (SP).

In some scenarios, a small part of the energy system is still based on fossil fuels in 2100 (Neg), while, in others, fossil fuels are almost or completely phased out (Ren).

However, all mitigation pathways show “greatly reduced” fossil fuel use, with the three that limit warming to 1.5C with no or limited overshoot (Ren, LD, SP) unabated coal use “completely” phased out by 2050. While the GS and Neg scenarios rely heavily on CDR, its use is purposely restricted in the other illustrative pathways focused on deep mitigation (Ren, LD, and SP). These mitigation scenarios tend to feature a widespread electrification of the global energy system, as direct combustion of fuels in buildings and vehicles are increasingly replaced with electric alternatives.

The figure below shows the energy use over time by source in the illustrative pathways (top part), as well as the amount of positive GHG emissions and CDR at the net-zero year, compared to 2019 (bottom part).

One area where there is a lot of new focus in WG3 compared to previous reports such as the 2018 SR15 is low-demand scenarios. These involve a significant decoupling of material and energy demands from economic growth, resulting in very low per capita demand for energy compared to present due to increased energy efficiency and less energy intensive lifestyles.

Illustrative pathways taking this approach – LD and SP – have the lowest peak and 2100 warming of all pathways considered, though it is unclear how possible these pathways would be to achieve in practice as they require energy efficiency improvement rates considerably above those previously experienced.

Similarly, the high level of CDR used in scenarios like Neg has large challenges; as the report notes, “CDR ramp-up rates and absolute deployment levels are tightly limited by techno-economic, social, political, institutional and sustainability constraints”.

Next section: How can shifting peoples’ demand for products and services cut emissions?

Authors:

Aruna Chandrasekhar holds an MSc in environmental change and management from the University of Oxford. She previously worked as a freelance journalist for a variety of publications, including the New York Times, Guardian, Scroll and Caravan.

Daisy Dunne holds a BSc in biology from the University of Bristol and a science journalism MA from City, University of London. She was The Independent’s climate correspondent from November 2020 to 2021. Prior to this, she was Carbon Brief’s science writer from 2017 to 2020.

Dr Simon Evans is our deputy editor and policy editor. Simon covers climate and energy policy. He holds a PhD in biochemistry from Bristol University and previously studied chemistry at Oxford University. He worked for environment journal The ENDS Report for six years, covering topics including climate science and air pollution.

Josh Gabbatiss covers climate and energy policy. Josh holds a BSc in zoology from the University of Bristol. He previously worked as science correspondent at The Independent and prior to that was a freelance journalist for a variety of scientific publications including New Scientist and BBC Earth.

Dr Zeke Hausfather is a climate science contributor for Carbon Brief. Zeke has masters degrees in environmental science from Yale University and Vrije Universiteit Amsterdam, and a PhD in climate science from the University of California, Berkeley. He has spent the past 10 years working as a data scientist and entrepreneur in the cleantech sector.