Storyline and main scenario drivers of RCP8.5

The RCP8.5 is based on the A2r scenario (Riahi et al. 2007), which provides an updated and revised quantification of the original IPCC A2 SRES scenario storyline (Nakicenovic et al. 2000). With a few exceptions, including an updated base year calibration (to 2005) and a revised representation of short-term energy trends, especially in developing countries, the RCP8.5 builds thus upon the socio-economic and demographic background, resource assumptions and technological base of the A2r scenario.Footnote 7

The scenario’s storyline describes a heterogeneous world with continuously increasing global population, resulting in a global population of 12 billion by 2100. Per capita income growth is slow and both internationally as well as regionally there is only little convergence between high and low income countries. Global GDP reaches around 250 trillion US2005$ in 2100. The slow economic development also implies little progress in terms of efficiency. Combined with the high population growth, this leads to high energy demands. Still, international trade in energy and technology is limited and overall rates of technological progress is modest. The inherent emphasis on greater self-sufficiency of individual countries and regions assumed in the scenario implies a reliance on domestically available resources. Resource availability is not necessarily a constraint but easily accessible conventional oil and gas become relatively scarce in comparison to more difficult to harvest unconventional fuels like tar sands or oil shale. Given the overall slow rate of technological improvements in low-carbon technologies, the future energy system moves toward coal-intensive technology choices with high GHG emissions. Environmental concerns in the A2 world are locally strong, especially in high and medium income regions. Food security is also a major concern, especially in low-income regions and agricultural productivity increases to feed a steadily increasing population.Footnote 8

Compared to the broader integrated assessment literature, the RCP8.5 represents thus a scenario with high global population and intermediate development in terms of total GDP (Fig. 4). Per capita income, however, stays at comparatively low levels of about 20,000 US$2005 in the long term (2100), which is considerably below the median of the scenario literature. Another important characteristic of the RCP8.5 scenario is its relatively slow improvement in primary energy intensity of 0.5% per year over the course of the century. This trend reflects the storyline assumption of slow technological change. Energy intensity improvement rates are thus well below historical average (about 1% per year between 1940 and 2000). Compared to the scenario literature RCP8.5 depicts thus a relatively conservative business as usual case with low income, high population and high energy demand due to only modest improvements in energy intensity (Fig. 4).

Fig. 4 Global development of main scenario drivers in RCP 8.5 (red lines) compared to the range of scenarios from the literature (grey areas: IPCC AR4 scenario database; Fisher et al. 2007; Nakicenovic et al. 2006). Right hand vertical lines give the AR4 database range in 2100, including the 5th, 25th, 50th, 75th, and 95th percentile of the AR4 scenario distribution Full size image

Development of the energy system

Energy system of RCP8.5

As discussed earlier, the RCP 8.5 is a baseline scenario with no explicit climate policy, representing the highest RCP scenario in terms of GHG emissions. In this section we will first briefly describe the main energy system changes of the RCP 8.5 baseline. In addition to baseline trends, we will congruently analyze also the required GHG emissions reductions in order to limit radiative forcing to levels comparable to the other RCPs highlighted in this SI. We primarily focus in this section on the transition of the energy system and move later to results for land-use (Section 3.3) and GHG and pollutant emissions (Section 3.4).

A growing population and economy combined with assumptions about slow improvements of energy efficiency lead in RCP8.5 to a large scale increase of primary energy demand by almost a factor of three over the course of the century (Fig. 5). This demand is primarily met by fossil fuels in RCP 8.5. There are two main reasons for this trend. First, the scenario assumes consistent with its storyline a relatively slow pace for innovation in advanced non-fossil technology, leading for these technologies to modest cost and performance improvements (e.g., learning rates for renewables are below 10% per doubling of capacity; see also Riahi et al. 2007 for further detail). Fossil fuel technologies remain thus economically more attractive in RCP8.5. Secondly, availability of large amounts of unconventional fossil resources extends the use of fossil fuels beyond presently extractable reserves (BP 2010). The cumulative extraction of unconventional fossil resources lies, however, within the upper bounds of theoretically extractable occurrences from the literature (Rogner 1997; BGR 2009; WEC 2007).Footnote 9

Fig. 5 Development of global primary energy supply in RCP8.5 (left-hand panel) and global primary energy supply in 2100 in the associated mitigation cases stabilizing radiative forcing at levels of 6, 4.5, and 2.6 W/m2 (right-hand bars). Note that primary energy is accounted using the direct equivalent method Full size image

Coal use in particular increases almost 10 fold by 2100 and there is a continued reliance on oil in the transportation sector. This fossil fuel continuance does not necessarily mean a complete lack of technological progress. In contrast to most other technologies, there are significant improvements in existing fossil alternatives as well as the penetration of a number of new advanced fossil technologies, thus increasing their efficiency and performance in the longer-term. In the electricity sector, this results in a shift towards clean coal technologies from current sub-critical coal capacities. In addition, with conventional oil becoming increasingly scarce, a shift toward more expensive unconventional oil sources takes place by 2050 and the subsequent increases in fossil fuel prices also leads an increased penetration of “synthetic” fuels like coal-based liquids. The increase in fossil fuel prices (about a doubling of both natural gas and oil prices by mid-century) triggers also some growth for nuclear electricity and hydro power, especially in the longer-term. Overall, however, fossil fuels continue to dominate the primary energy portfolio over the entire time horizon of the RCP8.5 scenario (Fig. 5).

In terms of final energy, significant transformations occur in the manner in which energy is used in RCP8.5 (Fig. 6). Particularly electricity continues its historical growth and becomes the dominant mode of energy use mostly in the residential and partly also in the industrial sector. In the long term (beyond 2050) electricity is provided in RCP8.5 to a large extent from non-fossil sources (nuclear and biomass).

Fig. 6 Development of global final energy in RCP8.5 (left-hand panel), and global final energy in 2100 in the associated mitigation cases stabilizing radiative forcing at levels of 6, 4.5, and 2.6 W/m2 Full size image

Impact of mitigation measures

The high energy demand and fossil intensity associated with RCP8.5 implies that achieving climate stabilization will require a massive reduction of emissions and drastic energy system transformations compared to the baseline. In fact, previous studies indicated that achieving low climate stabilization levels from the A2r scenario—the predecessor of RCP8.5—may technically not be feasible (Rao et al. 2008). The earlier studies employed though a qualitative criterion for target attainability that limited energy intensity improvement of a given stabilization targets to stay within relatively narrow margins of the baseline scenario storyline (see Riahi et al. 2007 and Rao et al. 2008). In our assessment, however, we allow pronounced reductions in energy demand beyond this criterion and observe that 2.6 W/m2 target under a fossil intensive RCP8.5 scenario would become feasible, if more rapid energy intensity improvements were possible to achieve.

In addition to responses in energy demand, our analysis considers a number of options for reducing energy-related CO 2 emissions on the supply-side of the energy system (see Riahi et al. 2007 for details). These include switching from fossil fuels to renewable or nuclear power; fuel switching to low-carbon fossil fuels (e.g., from coal to natural gas); and carbon capture and storage (both fossil and biomass based). Also included in this analysis is the full basket of non-CO 2 gases and related mitigation options (see Rao and Riahi 2006 for details), both energy related (e.g. extraction and transport of coal, natural gas, and oil) and non-energy related (livestock, municipal solid waste, manure management, rice cultivation, wastewater, and crop residue burning).Footnote 10

The primary energy mix of the climate mitigation scenarios (reaching 6, 4.5, and 2.6 W/m2 radiative forcing by the end of the century) are illustrated in the right bars of Fig. 5. In the short and medium term, transition options like fossil based CCS (in particular natural gas with CCS) become particularly important while in the longer-term, dominant technological options include energy conservation and efficiency improvements, nuclear, and biomass with carbon capture (BECCS). This trend is robust across all analyzed stabilization targets, but is obviously most pronounced in the low 2.6 W/m2 forcing scenario. While electricity from other renewables, like solar PV, increase their contribution in the longer-term, the majority of the carbon free electricity comes from centralized nuclear and biomass power plants. This technology choice reflects the underlying storyline of the RCP8.5 and related technology assumptions, which favor traditional centralized supply-options (including fossil CCS, nuclear and biomass). The results highlight that in principle lower stabilization goals might be possible to reach from high baselines as the RCP8.5, and that mitigation solutions would not necessarily require a shift from large-scale centralized energy production to dispersed intermittent sources (for a discussion of alternative mitigation paradigms with higher shares of intermittent renewables see Riahi et al. 2007).

In terms of final energy, the pace of electrification is accelerated further in the climate mitigation scenarios, where non-fossil electricity becomes a major driver of the decarbonization, leading to electricity shares in final energy of up to about 60% by 2100 (compared to about 30% in RCP8.5). Oil use peaks around middle of the century and declines in the longer term. In RCP8.5 the resulting gap for the supply of liquid fuels is filled by other liquefaction processes like coal- and biomass-based liquids. In the climate mitigation scenarios, hydrogen becomes an additional important long-term final energy carrier in the transport sector. Important wide ranging consequences of the transformation away from oil-products to electricity and hydrogen are at the one hand improvements of regional energy security in terms of decreased oil dependency (oil imports). At the other hand the transformation enables also major environmental improvements through decreasing pollutant emissions, particularly in urban areas (see Section 3.5).

Figure 7 compares the required pace of energy intensity and carbon intensity improvements in the RCP8.5 and the mitigation scenarios that have been derived with historical trends and selected scenarios from the literature (SRES B1 and B2). Reducing GHG emissions requires both demand-side changes (improvements in energy intensity) as well as supply-side structural changes (improvements in carbon intensity of the economy). The required pace of the transition is particularly challenging in the case of the low target of 2.6 W/m2. In terms of carbon intensity the 2.6 W/m2 scenario shows for example a six-fold increase in the rate of decarbonization compared to the RCP8.5 baseline. This corresponds also to a major trend-break and a five-fold acceleration of the decarbonization pace compared to the long run historical improvement rate for the world (1940 to 2000). With respect to energy intensity the 2.6 W/m2 is less ambitious. It depicts improvement rates roughly in line with historical trends between 1940 and 2000 of about 1% per year. This rate is also comparable to assumptions for intermediate baseline scenarios in the literature such as the B2 SRES (Fig. 7). While this improvement rate is quite modest considering the stringent climate target, it means nevertheless a drastic departure from the RCP8.5 baseline, where energy intensity improves at only half this rate (0.5% per year). Our results thus also indicate the importance of path dependency and conditionality of the transformation strategy depending on the choice of the baseline and its underlying assumptions. Clearly, any of the climate targets would have been achieved by a different mix of measures (and costs) if we had used for example the sustainable SRES B1scenario with its relatively high rates of improvements as the counterfactual of our analysis (see Fig. 7).

Fig. 7 Long-term energy intensity and carbon intensity improvement rates between 2000 and 2100 for RCP8.5, related mitigation scenarios developed with the MESSAGE model, and the B2/B1 scenarios from SRES (Nakicenovic et al. 2000). The “cross” indicates the relative position of historical intensity improvements compared to future developments of the scenarios Full size image

Land-use and land-cover change

Some 1.6 billion ha of land are currently used for crop production, with nearly 1 billion ha under cultivation in the developing countries. During the last 30 years the world’s crop area expanded by some 5 million ha annually, with Latin America alone accounting for 35% of this increase. The potential for arable land expansion exists predominately in South America and Africa where just seven countries account for 70% of this potential. There is relatively little scope for arable land expansion in Asia, which is home to some 60% of the world’s population. These constraints are also reflected by the land-use change dynamics of the RCP 8.5 scenario. Projected global use of cultivated land in the RCP8.5 scenario increases by about 185 million ha during 2000 to 2050 and another 120 million hectares during 2050 to 2100. While aggregate arable land use in developed countries slightly decreases, all of the net increases occur in developing countries. Africa and South America together account for 85% of the increase. This strong expansion in agricultural resource use is driven by the socio-economic context assumed in the underlying emission scenario with a population increase to over 10 billion people in 2050 rising to 12 billion people by 2100. Even then yield improvements and intensification are assumed to account for most of the needed production increases: while global agricultural output in the scenario increases by 85% until 2050 and 135% until 2080, cultivated land expands respectively by 12% and 16% above year 2000 levels (Fig. 8).

Fig. 8 Global land use by category in RCP8.5 Full size image

An important characteristic of RCP8.5 are transformative changes the biomass use for energy purposes from presently traditional (non-commercial) use in the developing world to commercial use in dedicated bio-energy conversion facilities (for power and heat) in the future. Globally the contribution of bioenergy is increasing in RCP8.5 from about 40 EJ in 2000 to more than 150 EJ by 2100. The vast majority of this biomass is harvested in forests, resulting in increased land-requirements for secondary managed forests. While total area of forests is declining in RCP8.5 (Fig. 8), the share of managed forests and harvested areas for biomass are thus increasing considerably. The latter grows from about 17 million ha to more than 26 million ha by 2100. Uncertainties in the interpretation of the underlying land developments are nevertheless very large. Hurtt et al. (2011) for example estimate about a factor of six higher land requirements for the same amount of wood harvest for the year 2000. Differences between the estimates increase over time. The results indicate the need for further harmonization of underlying data and definitions of carbon harvest in forest models.

GHG emissions

GHG emissions in RCP8.5

GHG emissions of the RCP8.5 continue to rise as a result of the high fossil-intensity of the energy sector as well as increasing population and associated high demand for food. The development of main GHG emissions of RCP8.5 and the corresponding mitigation scenarios is shown in Fig. 9. The RCP8.5 emissions are high, not only compared to the overall emissions scenario literature, but also compared to the set of baseline scenarios. In RCP8.5 CO 2 -eq. emissions more than double by 2050 and increase by three fold to about 120 GtCO 2 -eq. by 2100 (compared to 2000). Roughly about three quarter of this increase is due to rising CO 2 emissions from the energy sector. The rest of the increase is mainly due to increasing use of fertilizers and intensification of agricultural production, giving rise to the main source of N 2 O emissions. In addition, increases in life-stock population, rice production, and enteric fermentation processes drive emissions of methane (CH 4 ).

Fig. 9 Development of global GHG emissions (CO2-eq., CO 2 , CH 4 , and N 2 O) in RCP8.5 and MESSAGE mitigation scenarios of this study (brown lines). For a comparison the trends of the official RCPs described elsewhere in this SI are shown as well (red = RCP6, blue = RCP4.5, green = RCP3-PD) Full size image

The high GHG emissions in RCP8.5 imply the need of large-scale emissions reductions to limit radiative forcing to levels comparable to the other RCPs. For the mitigation potentials from livestock and agricultural sectors we rely on estimates from Rao and Riahi (2006), which assumes no major technological breakthroughs in these sectors. Globally the mitigation potential is thus limited to about 50% and 30% of the RCP8.5 baseline emissions for CH 4 and N 2 O respectively. This explains also the comparatively limited role of CH 4 and N 2 O emissions mitigation in our mitigation scenarios compared to the official RCP2.6,Footnote 11 RCP4.5, and RCP6 (see Fig. 9 and papers on the other RCPs in this SI).

GHG Emissions in the mitigation scenarios

The comparatively limited potential for non-CO 2 mitigation options in RCP8.5 implies also that the bulk of the emissions reductions in the longer term will need to come from CO 2 in the energy sector (Fig. 9). Cumulative CO 2 emissions in RCP8.5 amount to about 7300 GtCO 2 over the course of the entire century. In order to limit forcing to 6 W/m2 about 40% of these emissions would need to be avoided. The more stringent targets require further emissions mitigation in the order of 60% and 87% of the RCP8.5 emissions to stay below the 4.5 and 2.6 W/m2 target. The cumulative mitigation requirements have large implications for the emissions pathways, which in all mitigation scenarios are characterized by a peak and decline of CO 2 emissions. As indicated in Fig. 9, the peak of emissions in the scenario leading to 6 W/m2 occurs around middle of the century. If, however, emissions growth over the next decades is considerably slower than in our scenarios (as illustrated by the official RCP6), the same target could be achieved with a later peaking date around 2080. Staying below 2.6 W/m2 requires much more rapid emissions reductions, leading to comparatively limited flexibility for the peak of emissions. Both the official RCP2.6 and our 2.6 W/m2 scenario indicate the need of emissions to peak before around 2020. This finding is also consistent with other assessments in the literature (e.g., van Vuuren and Riahi 2011). There are nevertheless important differences between the CO 2 emissions pathways, particularly with respect to the required negative emissions for limiting forcing to below 2.6 W/m2. As illustrated by Fig. 9, there is a considerably larger need for negative emissions in our scenario than in the official RCP2.6. The main reason for this difference is the higher non-CO 2 emissions in our scenario, which are compensated by more pronounced negative CO 2 emissions compared to the official RCP2.6 in the long term (Fig. 9).

Emission of air pollutants

Air pollutants in RCP8.5

While RCP8.5 depicts baseline developments in absence of climate mitigation policies, air quality legislation plays an important role for the scenarios’ projection of pollutant emissions. This reflects the fact that in contrast to climate policies, air quality measures have already been introduced in many parts of the world. Specifically, RCP8.5 assumes the successful implementation of present and planned environmental legislation over the next two decades to 2030. Beyond 2030 we further assume that increasing affluence may lead to tightening of pollutant legislation in the long term (see also Section 2.3.1).

RCP8.5 explicitly considers varying levels of legislation, economic growth and technological progress across regions, resulting in regionally different developments for emission intensities as illustrated in Fig. 10. Air quality standards are presently the highest in the OECD region. Emission intensities in the OECD are thus already comparatively low, and planned legislation is expected to reduce emissions intensities even further by 2030. For economies in transition and regions with medium development,Footnote 12 current legislations imply most significant declines across all regions by 2030. This trend reflects tightening of policies particularly in the power sector (e.g., through application of flue gas desulfurization or DENOx) and for vehicles (e.g., catalytic converters). Today’s low income regions are generally characterized by modest air quality controls. These regions show also the least pronounced declines in emissions coefficients to 2030, reflecting the lack of concrete plans for future legislation over the short term.

Fig. 10 Illustrative examples for the development of emissions intensities for different pollutant emissions and sectors. Current and planned environmental legislation drive improvements in emissions coefficients to 2030. Thereafter technology shifts and EKC assumptions explain further improvements. Colored ranges depict sub-regional differences between regions at similar economic development stages (slow development, medium development, and OECD) Full size image

In RCP8.5 many regions exhibit a catch-up in economic levels beyond 2030 to income levels greater than 5000$/capita (Fig. 10). After this point the regions follow the EKC assumptions of declining emissions coefficients explained in Section 2.3.1. In addition, an important trend in RCP8.5 is the pervasive shift in the energy system towards cleaner fuels and advanced fossil technologies, which together with the EKC assumptions explain the long-term decline in pollutant emissions intensities (Fig. 10). For example in the case of SO 2 emissions in the power sector, tightening of legislation results in emissions reductions from end-of-the-pipe technologies, but at the same time a growing share of inherently cleaner coal technologies (e.g., through gasification processes) fosters additional emissions reductions through technology shifts.

Assumptions about environmental legislations in combination with ongoing structural and technological change imply thus in RCP8.5 that pollutant emissions decline significantly as seen in the example of SO 2 emissions in Figs. 11 and 12. Growing regional environmental concerns combined with the lack of a global climate change regime thus also imply a clear decoupling of CO 2 emissions from pollutants. For example, the power sector remains a major contributor to CO 2 emissions by the end of the century; although SO 2 emissions from this sector are almost negligible due to increasing use of advanced coal technologies. Also in the transport and residential sector, CO 2 emissions continue to rise globally while in most developing regions, there is either a slowing down of growth of pollutants from this sector or even a decline where air quality legislations are stringent enough to offset growing demand. This is important as the RCP8.5 while representing the highest levels of GHG emissions among the RCP set, is not necessarily a ‘high pollution’ case as well.Footnote 13

Fig. 11 Distribution of SO 2 Emissions in RCP8.5 for the years 2000, 2020, 2050, and 2100 Full size image

Fig. 12 Global SO 2 Emissions by sector in the RCP8.5 baseline and the mitigation scenarios for 6, 4.5, and 2.6 W/m2 Full size image

While globally aggregated trends for pollutants show continues improvements and declines in emissions, there are pronounced regional and spatial differences with local implications for human health, environment, and climate change. The maps of Fig. 11 illustrate some of the main spatial dynamics for the evolution of SO 2 emissions in RCP8.5. The spatial dynamics are similar for other pollutant emissions and to large extent also for the mitigation scenarios. Initially, the majority of the reductions happen in OECD countries whereas developing regions, in particular Asia, continues to grow in terms of SO 2 emissions, mainly due to growing energy demands (see map for 2020). This clearly indicates that currently legislated environmental policies are most likely not sufficient in reducing pollution levels of emerging economies where growth in energy demands can offset the effects of control policies. This may particularly be the case in China and India. In the longer-term, however, increasing affluence and technological shifts in these regions (Fig. 10) imply in RCP8.5 that global emission levels decline significantly, leading to reduced impacts from pollutants at global scale.

Air pollutants in the mitigation scenarios

With respect to the mitigation scenarios, we observe significant co-benefits from climate mitigation for pollutant emissions. As explained earlier, the greenhouse gas emissions reductions in the mitigation scenarios lead to major improvements of the carbon-intensity and the energy-intensity compared to the RCP8.5 baseline. This switch to carbon-free and non-fossil technologies is generally associated with lower pollutant emissions. In addition, also the application of CCS requires cleaner combustion processes, and thus reduces pollutant emissions in the climate mitigation scenarios further. Perhaps most importantly, the higher rates of energy-intensity improvements in the climate mitigation scenarios leads to pronounced energy savings, and each unit of energy that is not consumed is obviously climate friendly as well as pollution free.