Multiple pathways towards sustainable development goals and climate targets

The GDP scenarios of all three SDPs feature a near-complete convergence of income levels between Global North and Global South (see suppl. table S2 for regional mapping) over the course of the century (figure 2(a)). While in SDP-EI, and to a lesser extent in SDP-MC, per-capita income in the Global North continues to grow, SDP-RC features a post-growth economic outlook with approximately constant per-capita income in high-income countries. Global GDP/capita spans a range of around 45 000 (SDP-RC) to nearly 100 000 $_{2010 PPP} (SDP-EI) in 2100, around 3–6 times the current (2020) value, with most of the growth occurring in the Global South. Within-country inequality as measured by the Gini coefficient reduces rapidly in all SDPs (fastest in SDP-RC, slightly slower in SDP-EI), leading to globally averaged values below 30 by 2050, comparable to the values in countries with the lowest income inequality today (Min et al 2024).

All SDP scenarios also assume a rapid reduction in the population at risk of hunger (SDG 2), with zero hunger being reached or nearly reached by 2050 (figure 2(b)). Reflecting the increased food demand in low-income countries, the global food demand projections for SDP-EI slightly exceed the trends-continued scenario (SSP2-Ref) until mid-century. They also feature only moderate reductions in the shares of animal-sourced food and in food waste in the Global North (Weindl et al in review). By contrast, SDP-MC and especially SDP-RC assume an ambitious demand-side transformation with a strong shift towards more plant-based nutrition (SDP-RC: over 90% of food demand by 2050) as well as a substantial reduction of food waste. In particular the Global North shifts away from the current consumption patterns with high shares of food with adverse health and/or environmental impacts and towards the diets recommended by the EAT-Lancet commission (Willett et al 2019), leading to a convergence in dietary patterns between Global North and Global South by 2050.

The SDPs also include rapid progress for several further access indicators from the SDGs (figure 2(c)), such as increased access to safe drinking water (SDG 6), a rapid reduction in the population relying on solid fuels for cooking, and near-universal electricity access (both SDG 7). The projections for energy service demands (figure 2(d)) reflect the underlying SDP narratives: SDP-EI and SDP-MC show only moderate breaks with historical trends, leading to a globally averaged passenger transport demand of 9,300–14 600 pkm/cap/yr and a floor space of 46–57 m²/cap in 2050 (range across models and both SDP-EI and SDP-MC). By contrast, the SDP-RC scenario with its ambitious sufficiency orientation features a markedly lower passenger transport demand of 5,100–9,800 pkm/cap/yr and a floor space of 41–50 m²/cap in 2050 (model range; see also suppl. figure S2 for model-specific results). Final energy (FE) consumption (also including industrial energy use; see also figure 5 below) is reduced moderately compared to today's level in SDP-EI (global average of 44–48 GJ/cap/yr in 2050), driven mostly by more efficient provisioning of energy services via electrification. SDP-MC (34–39 GJ/cap/yr) and SDP-RC (around 32–33 GJ/cap/yr) feature deeper reductions, driven by a combination of efficiency increases and demand-side shifts (SDP-MC), or a shift towards more sufficiency-oriented lifestyles especially in the Global North (SDP-RC).

We show a number of SD outcome indicators in figure 3, covering the dimensions of poverty & inequality (SDGs 1 & 10), health (SDG 3), and food and land use (SDGs 2, 15). Our multi-model analysis robustly demonstrates that a continuation of current trends (SSP2-Ref scenario) is far off-track from meeting the SDGs. For example, we project only a modest reduction of extreme poverty until 2030 (660 million people below the extreme poverty line of 1.90 $_PPP2011/day), and even by 2050 the goal of eradicating extreme poverty remains unmet (220 million). For a number of environmental indicators, such as the biodiversity intactness index (BII) or nitrogen pollution, we even project a worsening of the situation. In short, a trends-continued scenario will fail to deliver on both the social and environmental goals of the Agenda 2030.

Climate change mitigation without targeted SD policies (SSP2-1.5C scenario) can have synergistic effects for progress towards certain SDGs, e.g. preventing further worsening in BII. However, they also have adverse side effects, such as substantial increases in the prices for agricultural commodities, increased energy prices with negative consequences on access to clean cooking fuels and therefore indoor air pollution health effects, and small increases in poverty and inequality. This reinforces the findings of earlier single-model studies (e.g. Bertram et al 2018, Soergel et al 2021) that both a massive step-up in SDG implementation and a holistic strategy for integrating SD and climate change mitigation are needed.

The SDP scenarios substantially improve SD outcomes over the current-trends and climate-policy-only scenarios. For example, extreme poverty is reduced to around 280 million in 2030 and close to being fully eradicated by 2050. Furthermore, the trade-off of increasing agricultural prices due to climate policy is ameliorated or completely avoided in the SDP scenarios, as food-system emission reductions are achieved not only through price-driven measures but also facilitated through dietary change and efficiency improvements. Importantly, the enhanced levels of human development in the SDP scenarios do not come at the expense of further environmental degradation: we project moderate (SDP-EI & SDP-MC; SDP-RC from REMIND-MAgPIE) to substantial (SDP-RC from IMAGE) improvements in BII compared to today. Furthermore, the nitrogen surplus is halved by 2050 compared to the reference scenario (81–103 Tg N/yr across SDPs vs 176–200 Tg N/yr in SSP2-Ref). However, even in the scenarios with the strongest reductions (SDP-RC and SDP-MC from REMIND-MAgPIE) the boundary for nitrogen pollution remains transgressed.

For a more aggregate perspective, figure 4 shows an SDG score that normalises indicator values to display progress towards the targets between 2020 and 2030 (2050) for all five analysed scenarios (integrating also indicators from figure 2 and the deep dives on buildings and materials and climate in figures 5 and 6 below). While the SDPs generally substantially accelerate progress towards the SDGs compared to the SSP2-Ref and SSP2-1.5C scenarios, many targets remain out of reach for 2030 due to the increasingly narrow time window. However, as already noted in earlier single-model studies (Soergel et al 2021, Moallemi et al 2022, Orbons et al 2024), a push for SDG implementation until 2030 and a continuation of SD policies afterwards can ensure that the targets are largely met at a later point in time. Comparing the three different SDP scenarios, we find that the overall level of SDG achievement in 2030 and 2050 is similar, with differences between the three alternative SDPs mostly being smaller than their common advantage over the SSP2-based comparison scenarios. Where differences exist, they reflect the differences in underlying SD strategies: For indicators with relatively close links to consumption patterns, such as reducing non-CO₂-emissions, SDP-RC makes the fastest progress. On the other hand, indicators related to the efficiency and/or circularity of production systems, such as secondary steel share, improve fastest in SDP-EI.

Already today the buildings and construction sector accounts for over a third of global greenhouse gas emissions, with a growing share from embodied emissions of construction materials. The goal to provide decent housing (SDG 11), together with increasing living standards in the Global South, can be expected to further accelerate this trend, resulting in an urgent need to reduce material demands, decarbonise their production, and shift to regenerative materials (United Nations Environment Programme 2023a). As a quantification of different strategies to enable decent housing but reduce emissions, here we project the demand for the energy- and emission-intensive materials iron & steel and cement in the buildings sector, accounting for scenario-specific trends in per-capita floor space (figure 2) and material efficiency and/or substitution strategies (see modelling protocol). While material demands in SDP-EI remain comparable to current levels until 2050 and SDP-MC achieves only moderate reductions, SDP-RC reduces iron & steel demand to 0.02–04 t/cap/yr and cement demand to 0.07–0.09 t/cap/yr in 2050 (figure 5, top row), enabled by both more intensive use of the existing building stock and shifting to wood-based construction.

In the SDP-MC and SDP-RC scenarios, also the total production of iron and steel and cement (all sectors) decreases considerably compared to the SSP2-based scenarios by 2050, alongside increases in the use of secondary (recycled) steel in all three SDPs (figure 5, bottom row). Reflecting a broader shift towards sustainable cities, direct CO₂ emissions from residential and commercial buildings are reduced rapidly in all SDPs, reaching values of around 0.1 t CO₂/cap/yr across models and scenarios by 2050.

In figure 6 we compare the mitigation strategies and the associated transformations of the energy and land-use systems between the SDPs, to our SSP2-1.5C scenario, and additionally to the IPCC AR6 ensemble of 1.5 °C scenarios (Byers et al 2022, Riahi et al 2022). All SDPs, and also SSP2-1.5C, limit the increase of global mean temperature (GMT) to slightly above 1.5 °C, with peak temperatures in the range of 1.56 °C–1.64 °C (median warming from AR6-calibrated climate assessment; Kikstra et al 2022). After the peak temperature is reached, warming is gradually reduced to well below 1.5 °C at the end of the century (EoC), making the scenarios compatible with the long-term goal of the Paris Agreement to limit warming to below 1.5 °C (suppl. figures 3 and 4). SDP-RC has the lowest peak and EoC warming across the SDPs, indicating that combining the necessary decarbonisation of the supply side with deep demand-side shifts facilitates limiting warming both in the near and long term. In particular the shift towards healthier and more sustainable nutrition in SDP-RC (and to a lesser extent, SDP-MC) drives a rapid and deep reduction of non-CO₂ emissions to levels well below typical 1.5 °C scenarios, helping to limit the overshoot over 1.5 °C.

In SDP-RC and SDP-MC the need for carbon dioxide removal (CDR) is also considerably reduced compared to standard 1.5 °C scenarios: CDR in 2050 is reduced to around a quarter to half its value in SSP2-1.5C (range across models and both scenarios), demonstrating the potential of demand-side shifts to reduce the long-term reliance on CDR. SDP-EI, on the other hand, compensates for initially slower emission reductions with a higher degree of CDR, though still less than in SSP2-1.5C. However, the overall reliance on CDR remains a key uncertainty dimension in our model-scenario ensemble: REMIND-MAgPIE SDPs show consistently low CDR values (1.2–4.9 Gt CO₂/yr in 2050), at the lower end of the IPCC AR6 range of C1 (i.e. 1.5 °C with no/low overshoot) scenarios and below the typical range from the State of CDR report (25%–75% range of 7.6–11 Gt CO₂/yr; (Gidden et al 2024)). By contrast, the IMAGE SDPs show somewhat higher CDR deployment (5.0–8.7 Gt CO₂/yr) closer to the range of typical C1 scenarios.

The carbon price range required to meet the 1.5 °C target is broadly comparable between the three SDPs and across both models until mid-century (165–232 $₂₀₁₀/tCO₂ in 2050 for REMIND-MAgPIE; 167–183 $₂₀₁₀/tCO₂ for IMAGE), but lower than for a standard 1.5 °C scenario (SSP2-1.5C: 224–325 $/tCO₂). While the demand-side shifts assumed in the SDP-RC and SDP-MC scenarios generally lower the mitigation challenge, the reduced availability of mitigation technologies (e.g. bioenergy, CCS) in SDP-RC increases the challenge on the supply side, resulting in broadly comparable carbon prices. However, in the long term (suppl. figure S3) the sufficiency-oriented SDP-RC scenario requires lower carbon prices (201–253 $/tCO₂ in 2100) than SDP-MC (260–404 $/tCO₂), and substantially lower prices than SDP-EI (410–569 $/tCO₂) or SSP2-1.5C (606–665 $/tCO₂). All SDPs further feature a rapid electrification of FE demand, with electrification rates (incl. indirect electrification via hydrogen) reaching values between 44%–57% (SDP-RC) and 55%–61% (SDP-MC) by 2050, comparable to (REMIND-MAgPIE) or higher (IMAGE) than in a standard 1.5 °C scenario.

On the land side, SDP-EI relies on improving production efficiency, and also features the highest afforestation and reforestation across the SDPs (281–319 Mha by 2050), largely with fast-growing plantations to increase land carbon sequestration (for details see Weindl et al ). By contrast, SDP-RC relies on a strong dietary shift towards a healthy and environmentally friendly nutrition. This reduces the land area required for the agricultural system (cropland & pasture) by 325–705 million ha by 2050, freeing up land for natural regrowth of native tree species and thereby enabling higher gains in biodiversity intactness in SDP-RC than in SDP-EI (figure 3).

The SDP scenarios also include stylized schemes for international climate finance, assuming an international redistribution of part of the carbon pricing revenues (Soergel et al 2021; see modelling protocol for details). The internationally oriented SDP-MC scenario reaches around 330 billion $₂₀₁₀/yr of international climate finance by 2030 and nearly 550 billion $₂₀₁₀/yr by 2050, more than compensating for the near-term costs of climate policy in developing regions of the Global South (Africa, South Asia). By contrast, SDP-RC has the lowest level of international financing, with 96 billion $₂₀₁₀/yr in 2030 only at the level of the 2009 Copenhagen pledge, reflecting its regionally oriented narrative.

Our SDP scenarios show that in principle large progress towards the SDGs and the Paris climate target is possible. However, they also show that such progress would require a rapid and deep departure from historical development patterns, and as such the SDP scenarios also highlight the magnitude of the required transformations. For the underlying socio-economic scenarios we take primarily a normative approach, with the SDP scenarios reflecting different socio-economic futures broadly aligned with the goals of eradicating poverty and reducing inequality. On the other hand, we can assess the feasibility of the associated energy and food/land system transformations by comparing our scenarios to estimates of biogeophysical limits as well as constraints set by the current techno-economic and socio-cultural context (Brutschin et al 2021, Riahi et al 2022). Low feasibility concerns indicate a transformation broadly in line with historical examples, while high levels point towards a change without historical precedence (for details on method, indicator choice and threshold see SM section 3.6).

As elements of biogeophysical and technological feasibility, we consider the reliance on primary energy sourced from biomass (biogeophysical) and the scale-up of carbon capture and storage (technological). All three SDPs show substantial reductions in feasibility risks along both dimensions compared to a standard 1.5 °C mitigation scenario (SSP2-1.5C), and also compared to the range (10%–90%) of AR6 C1 scenarios (figures 7(a) and (b)). The reductions in feasibility risks are stronger in SDP-MC and especially in SDP-RC thanks to its ambitious demand-side shifts, and more moderate in SDP-EI.

Concerning economic feasibility we assess the cost of climate policy in terms of a reduction of the GDP growth rate (compared to a reference scenario without climate policy) as a high-level indicator (figure 7(c)). In SDP-MC and SDP-RC the growth rate losses and associated feasibility concerns are substantially reduced compared to SSP2-1.5C, both for the Global South and the Global North (mainly 2030–2040 decade). A more regionally disaggregated perspective reveals short-term policy gains for Africa (all SDPs) and South Asia (SDP-MC) thanks to international climate finance, but also near-term policy costs for both Middle East and the former Soviet Union countries in SDP-EI. We also assess changes in electricity prices because of the crucial role of electrification for the energy transition (figure 7(d)). Given the need for compensating mechanisms in the case of high price increases, this indicator additionally also touches upon aspects of institutional feasibility. We find that there is an initial scarcity of renewable electricity due to high initial investment requirements, which is reflected in substantial increases in electricity prices in all three SDPs until 2030, comparable to (Global North) or slightly higher (Global South) than in SSP2-1.5C. However, the associated feasibility risks only affect a transition period, as electricity prices reduce to around their 2020 levels after 2030.

As all SDPs rely on changes in energy, material and food demand to a certain extent (SDP-RC most strongly, SDP-EI the least), the socio-cultural feasibility dimension is of particular importance. Here we focus on the share of calories from livestock products and the use of FE per capita as high-level proxies, and quantify feasibility challenges in terms of rates of change over decadal periods (figures 7(e) and (f)). The rapid reductions in livestock share assumed in SDP-MC and especially SDP-RC lead to substantially increased feasibility concerns in the Global North, and in SDP-RC also in the Global South from 2030 onwards. Similarly, the ambitious FE demand reductions assumed in the Global North lead to increased feasibility concerns, especially for SDP-MC and SDP-RC in the next two decades. Feasibility concerns remain mostly moderate for the Global South, as the SDP scenarios aim for sufficient energy for decent living standards (Kikstra et al 2021), which can even lead to increasing FE values. As an exception, there are initial reductions of FE per capita around or above 10% in the initial 2020–2030 decade of SDP-RC and SDP-MC. However, these are mostly related to the phase-out of traditional biomass with its poor relation between FE use and provided energy service (Baltruszewicz et al 2021). This points to changes in FE being an imperfect proxy of feasibility, and future refinements of the framework should work towards separating changes in energy service demands and efficiency.

Relatively low absolute values of FE/capita in some Global South regions in SDP-RC could nonetheless indicate a remaining challenge with providing sufficient energy for positive human development outcomes. However, the SDP scenarios also assume ambitious reductions in income inequality (strongest in SDP-RC, figure 2(a)), which also imply strong reductions in energy inequality. Together with energy efficiency improvements, this largely ensures sufficient provision for a development towards decent living standards despite low average energy use (Kikstra et al in review).