GCAM reference scenario
Each of the RCPs was produced by a different integrated assessment model; therefore, each has its own reference scenario (Vuuren et al. 2011b; Riahi et al. 2011; Masui et al. 2011). Thus, the reference scenario for RCP4.5 is not RCP8.5 but rather a GCAM reference scenario. The GCAM reference scenario (Clarke et al. 2007) depicts a world in which global population reaches a maximum of more than 9 billion in 2065 and then declines to 8.7 billion in 2100 while global GDP grows by an order of magnitudeFootnote 3 and global primary energy consumption triples (Fig. 1). The reference scenario includes no explicit policies to limit carbon emissions, and therefore fossil fuels continue to dominate global energy consumption, despite substantial growth in nuclear and renewable energy. Atmospheric CO2 concentrations rise throughout the century and reach 792 ppmv by 2100, with total radiative forcing approaching 7 W m−2 (Fig. 2). Emissions of CH4 and N2O also continue to rise with the continued use of fossil fuels and the expansion of agricultural lands. Forest land declines in the reference scenario to accommodate increases in land use for food and bioenergy crops. Even with the assumed agricultural productivity increases, crop land increases in the first half of the century due to increases in population and income, which drives an increase in land-intensive meat consumption. After 2050 the rate of growth in food demand slows, in part due to declining population. As a result the area of cropland and land use change (LUC) emissions decline.
RCP4.5 stabilization scenario
The RCP4.5 scenario is based on the same population and income drivers as the GCAM reference scenario but applies greenhouse gas emissions valuation policies to stabilize atmospheric radiative forcing at 4.5 W m−2 in 2100 (Fig. 2). This imperative to stabilize climate change drives anthropogenic CO2 emissions downward throughout the next century (Fig. 3) and results in an atmospheric CO2 concentration of 526 ppm in 2100,Footnote 4 compared to 792 ppm in the GCAM reference case. This stabilization is achieved in 2080 when radiative forcing reaches 4.5 W m−2 and the emissions price becomes roughly constant. CO2 emissions also become roughly constant. RCP4.5 depicts declines in overall energy use, as well as declines in fossil fuel use compared to the reference case, while substantial increases in renewable energy forms and nuclear energy both occur (Fig. 4a). The proportion of total final energy that is supplied by electricity also increases due to fuel switching in the end-use sectors. The emergence of large-scale carbon dioxide capture and storage (CCS) (Fig. 5) allows continued use of fossil fuels for electricity generation and cement manufacture, among other uses, though total use is lower than in the reference scenario. Bioenergy with CCS is used to produce electricity, providing an energy source that is carbon-negative with respect to the atmosphere. The amount of bioenergy deployed is limited by the availability of dedicated crop and crop residue feedstocks from the land system.
One important feature influencing the availability of bioenergy feedstocks in the RCP4.5 is the expansion of forests as part of the larger emissions mitigation strategy. The extent of afforestation follows Wise et al. 2009b. This idealized case assumes that all carbon from fossil fuel and land use emissions are charged an equal penalty price, and thus reductions in LUC emissions constitute an available strategy for global emissions mitigation. The GCAM therefore simulates the preservation of large stocks of terrestrial carbon in forests, with some crop and pasture lands converted to bioenergy crops (Fig. 6). Under this policy environment, dedicated bioenergy crops still provide an important source of fuel, >50 EJ/yr, to meet global energy demand in 2100.Footnote 5 This is accomplished while still providing for the world’s dietary need by shifting toward food products with a smaller carbon footprint, principally by reducing beef consumption relative to the reference scenario. While this results in an increase in global food commodity prices, the overall food expenditure (cost of food as fraction of income) declines. These dynamics, and the role of agricultural productivity assumptions, are fully explored in Thomson et al. (2010).
Carbon prices reach $85 per ton of CO2 by 2100 (Fig. 7) which transforms the global economy. Electric power generation changes from the largest source of emissions in the world to a system with net negative emissions—made possible by increased reliance on nuclear and renewable energy forms such as wind, solar and geothermal, and the application of CO2 capture and storage technology to both fossil fuel sources and bioenergy (Figs. 4a and 5). Buildings and industry largely de-carbonize by employing more efficient end-use technologies and by electrifying. Annual land-use change emissions are reduced to 0.13 GtCO2/yr (Fig. 3b). Total anthropogenic CO2 emissions for the RCP4.5 peak around 42 Gt CO2 per year (Fig. 3) around 2040 and decline to 2080 before leveling off around 15 Gt CO2 per year for the remainder of the century. Other greenhouse gases respond to the mitigation price signals in the GCAM using marginal abatement cost curves (Smith and Wigley 2006) (Fig. 8).
The spatial distribution of these emissions leads to additional insights about future non-CO2 emissions that are important considerations for global climate and atmospheric chemistry models. For example, aggregate emissions and the associated radiative forcing contribution of CH4 from all sectors are relatively constant over time at the global level (Fig. 8) in the RCP4.5 scenario, which represent a 70% reduction from reference case levels by the end of the century. Methane emissions in RCP4.5 exhibit significant geographical shifts (Fig. 9), however, due to regional differences in driving forces and mitigation over time. CH4 emissions in South America and Africa increase over the century while those from China, India the US and Western Europe decline.
CO2 constitutes the largest contribution to total radiative forcing in the RCP4.5, followed by CH4, halocarbons, tropospheric ozone, and N2O (Fig. 8). The relative proportion of the non-CO2 components of positive radiative forcing remains constant over time. Increases in activity levels as income and population grow tend to increase emissions; however, since these substances are greenhouse gases, we assume that emissions controls are implemented as the carbon price rises. The net result is that emissions of these gases are roughly constant over time. Sulfate forcing is net negative throughout the century but this influence declines over time largely due to assumed increases in pollution control with income, although there are also indirect sulfur dioxide emissions reductions due to the greenhouse gas mitigation policy (Smith et al. 2005).
Comparison to the literature
Several other scenario studies have examined variations of the 4.5 Wm−2 stabilization target. The Climate Change Science Program (Clarke et al. 2007) included a 4.7 Wm−2 stabilization scenario in its report. This scenario, referred to in the report as “Level 2”, stabilized Kyoto gas forcing rather than total radiative forcing. Thus, the two scenarios, while similar, are not exactly alike. The Energy Modeling Forum 22 study (Clarke et al. 2009) included two 4.5 Wm−2 stabilization scenarios, one with all countries participating immediately and another with delayed participation by some regions. As with the CCSP, the EMF 22 stabilization scenarios focus on Kyoto gas forcing and not total radiative forcing. Nonetheless, we compare the RCP4.5 to the CCSP and EMF22 scenarios (Fig. 9). We have highlighted the RCP4.5 and the scenario on which it was based (CCSP – RCP4.5 Marker) in Fig. 10.
Population in the RCP4.5 (Fig. 10a) is among the lowest of the scenarios considered. There are six EMF 22 scenarios and one CCSP scenario with similar population trajectories to the RCP4.5; however, it should be noted that all of these scenarios were produced by integrated assessment models from the Pacific Northwest National Laboratory.Footnote 6 However, the range in population estimates in 2100 across the 28 scenarios is small; the largest population estimate is only 20% higher than the lowest.
Global GDP (Fig. 10b) varies more significantly across the 28 scenarios. The highest GDP estimate in 2100 is more than double the lowest estimate. The RCP4.5 and the CCSP RCP4.5 marker scenario on which the RCP was based fall in the middle of these estimates. Additionally, the RCP4.5 has slightly higher GDP than the CCSP, as discussed previously.
Cumulative energy and industrial CO2 emissions, as well as the time path of emissions, vary across models (Fig. 10c). Cumulative emissions range from 2043 GtCO2 to 3573 GtCO2 between 2000 and 2100. RCP4.5 falls in the middle of this range with 3010 GtCO2 emitted by the energy and industrial systems over the century. Several reasons exist for differences in cumulative emissions. First, these are only energy and industrial CO2 emissions and do not include land use and land-use change CO2 emissions. The amount of CO2 emitted from the terrestrial sphere is also likely to vary across the 28 scenarios. Second, the models use different climate models and have different characterizations of the carbon cycle. Thus, two models with the same level of cumulative total anthropogenic CO2 emissions may reach different atmospheric CO2 concentrations (see Smith and Edmonds 2006). Next, these scenarios limit radiative forcing to 4.5 Wm−2. Different models may find different contributions of the various gases to radiative forcing due to underlying pollution abatement assumptions.
The time path of emissions also varies across models for a variety of reasons, including (1) differences in reference scenario emissions, (2) differences in the cost of abatement over time, (3) differences in the speed at which capital stock can be replaced, and (4) differences in assumptions about foresight (Fawcett et al. 2009). The RCP4.5 has a slightly different emissions time path than its predecessor, the CCSP marker scenario. These two scenarios have similar cumulative CO2 emissions (RCP4.5 has 3010 GtCO2; CCSP has 3212 GtCO2) but the RCP4.5 has higher emissions in the near term and lower emissions in the long-term than the CCSP. These two scenarios use the same model, and thus the same assumptions about foresight and capital stock turnover. Additionally, they use very similar reference scenarios. One major difference between these two scenarios is the inclusion of biomass with CCS in the RCP4.5. This technology generates net negative CO2 emissions therefore making it cost effective to delay some emissions mitigation until the second half of the century. Thus, we observe higher emissions in the near term in the RCP4.5 and lower in the long term.
Finally, we compare the carbon price needed to reach 4.5 Wm−2 in the 28 scenarios (Fig. 10d). This price varies significantly across the models, ranging from $55/tCO2 in 2100 to $2141/tCO2 in 2100. RCP4.5 and the CCSP Marker Scenario both fall in the lower part of this price range. Differences in carbon prices can be attributed to differences in reference scenario emissions, and thus the level of abatement required, along with differences in the cost of abatement technologies.
GCAM-simulation of the four pathways
In order to facilitate model intercomparisons and further explore the characteristics of the RCPs, the participating models simulated their assigned RCP as well as the other three defined radiative forcing levels. The results for all models are discussed in van Vuuren et al. (2011a) The GCAM was used to simulate a 2.6 W m−2 peak-and-decline scenario for use in the evaluation of low radiative forcing targets during the planning stages of the RCPs (Weyant et al. 2009) and is fully documented in Calvin et al. (2009). The GCAM6 was simulated as a stabilization following the same methods as the RCP4.5 but resulting in lower carbon prices and a longer time to stabilization. All three of the mitigation cases with GCAM (2.6, 4.5 and 6.0 W m−2) used the same technology, population and economic assumptions described earlier. The GCAM reference case with these assumptions and no climate mitigation policy reaches around 7.0 W m−2 radiative forcing in 2100. Thus, to reach the RCP level of 8.5 W m2 required altering some underlying assumptions. Several modifications were tested; the case selected for the GCAM8.5 follows the same population and economic drivers as the other GCAM scenarios, but assumes no technological improvement in energy technologies or agricultural productivity. In other words, the GCAM8.5 is the GCAM reference case if all technological development is frozen after 2005. This is a hypothetical experiment in exploring high levels of radiative forcing which also allows exploration of the role of technological development in scenarios. Global primary energy consumption across the four scenarios is depicted in Fig. 4, and total global GHG emissions are shown in Fig. 11.
The CO2 emissions from the four pathways simulated with GCAM are illustrated in Fig. 12 along with the four officially released RCPs. The GCAM2.6 results in even lower emissions of CO2 than the RCP2.6 (van Vuuren et al. 2011b). GCAM2.6 has 310 GtCO2 cumulative emissions, while RCP2.6 has 390 GtCO2. Total radiative forcing still declines to 2.6 W m−2 due to higher CH4 and N2O emissions in GCAM than in IMAGE for this scenario. Conversely, CO2 emissions for the GCAM8.5 are higher than the RCP8.5 (Riahi et al. 2011); GCAM8.5 emits 2077 GtCO2 over the century, while RCP8.5 emits 1816 GtCO2. However, the same radiative forcing is reached due to lower CH4 and N2O emissions in GCAM than in MESSAGE for this scenario.
Differences in emissions by gas between the official RCPs and the GCAM pathways that follow the RCP forcing levels can be attributed to any number of factors. First, GCAM employs different population and GDP assumptions than the other three models. GCAM has the smallest population and the second highest GDP of the four models (van Vuuren et al. 2011a). Second, the four models have different assumptions about technological change and resource availability. Third, the GCAM model uses a terrestrial carbon policy that has a significant impact on land use and land-use change emissions. While each of the models consider abatement opportunities in the terrestrial system, the method of attaining these opportunities differs across the four RCP models. The differences listed here are only a subset of differences between the four models. However, they illustrate an important point; namely, there are numerous ways that a given radiative forcing goal can be achieved. The RCP4.5 is only one possible pathway to stabilization of radiative forcing at 4.5 W/m2.