2.1 Representative Concentration Pathways

The IPCC currently uses four ‘possible futures’ called Representative Concentration Pathways (RCPs). These were defined for the IPCC Fifth Assessment Report in 2013–4 (van Vuuren et al., 2011). The process of defining an RCP begins with imagining a plausible description of the future: a socio-economic scenario of possible population, global domestic product (GDP), energy usage and sources, and so on. This scenario is then translated into a set of:

• a.greenhouse gas concentrations
• b.air pollutant concentrations (such as industrial sulfates)
• c.changes in land use (e.g. conversion of forest to cropland).

These are then used by climate models to predict future climate change for each RCP.

The RCPs are called ‘representative’ concentration pathways because they represent the range of futures that have been suggested as possible by previous studies. The four scenarios have the rather technical names RCP2.6, RCP4.5, RCP6.0 and RCP8.5.

Figure 1 shows the predicted changes in population and primary energy consumption for the scenarios underlying the four RCPs. Primary energy is the energy embodied in natural resources such as coal, natural gas, crude oil, solar energy, wind energy and biomass before they are transformed for use.

Figure 1 (a) Population and (b) primary energy consumption in the four scenarios underlying the RCPs (van Vuuren et al., 2011).

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Figure 1(a) is a graph displaying four lines. It plots population (mlns) on the y or vertical axis, against year from 2000 to 2100. All four lines start from about 6000 and represent RCP2.6, RCP4.5, RCP6 and RCP8.5. RCP2.6, RCP4.5 and RCP6 follow a similar curve where they steadily increase up to 9000 and then very slightly and gradually decline to just under 9000 by 2100. RCP8.5 has a much steeper curve, reching 12000 in 2090 and then very slightly dropping down again by 2100. Figure 1(b) is a graph displaying four lines plotting primary energy consumption (EJ) for the four RCPs shown in figure 1(a). The y or vertical axis plots energy consumption (EJ) against year from 2000 t 2100. RCP2.6, RCP4.5 and RCP6 all start just under 500 EJ and follow a similar curve where they steadily increase up to 750 EJ by 2100, with RCP4.5 being just above and nearly reaching 1000. RCP6 follows the same path but has a much steeper incline, which drops again at 2060 and then slowly increases by 2080. RCP 8.5 starts at 500 EJ and then follows a steep incline up to 1750 EJ by 2100.

Figure 1 (a) Population and (b) primary energy consumption in the four scenarios underlying the RCPs (van Vuuren et al., 2011).

Actually, this is indeed the case. The four RCPs described in more detail are:

• RCP8.5: an energy-intensive scenario of very high greenhouse gas concentrations, in which emissions continue to increase through the 21st century
• RCP6.0 and RCP4.5: two stabilisation scenarios, in which emissions stop increasing by the end of the 21st century
• RCP2.6: a mitigation scenario, in which emissions peak in the middle of the twenty-first century and then decline.

The different greenhouse gas concentrations in the RCPs are produced different human emissions of the gases. Figure 2a and 2b show an example for one of the greenhouse gases: it shows the changing CO₂ emissions into the atmosphere and corresponding CO₂ concentration in the atmosphere.

Remember: there is a difference between emissions and concentrations. Think of the atmosphere as like a bath, and carbon dioxide as like the water. The rate of emissions of CO₂ into the atmosphere is like the rate of water flowing in, while the concentration of CO₂ in the atmosphere is like the amount of water in the bath. Burning fossil fuels releases CO₂ into the atmosphere (emissions), which increases the amount in the atmosphere (concentrations).

Figure 2 (a) Emissions and (b) concentrations of CO2 in the four RCPs (van Vuuren et al., 2011).

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Figure 2(a) is a graph displaying four lines plotting carbon dioxide emissions/GtC for the four RCPs. The y or vertical axis plots emissions (in GtC) against year from 2000 to 2100. RCP2.6 shows a short increase in emissions until about 2020 then a steady decrease to zero. RCP 4.5 shows an increase until about 2045, a decrease until about 2080 then a steady value of about 5 GtC. RCP6 shows an increase until about 2060 then a decrease to about 13 GtC. RCP8.5 shows an increase to almost 30 GtC by 2100. Figure 2(b) is a graph displaying four lines plotting primary energy consumption (EJ) for the four RCPs. RCP2.6 starts at 370 EJ and shows a sharp increase to about 920 EJ by 2100. RCP 4.5 starts at 370 EJ and shows a much more gradual increase to 650 EJ by 2100. RCP6 starts at 370 EJ and shows an increase to about 520 EJ at 2062 and then more or less stays at a similar level until 2100. RCP8.5 starts at 370 EJ and shows an increase until 2025 and then plateus gradually decreasing until 2100, ending on 440 EJ.

Figure 2 (a) Emissions and (b) concentrations of CO2 in the four RCPs (van Vuuren et al., 2011).

All four RCPs show increases in greenhouse gas concentrations in the future (Figure 2b). This gives a net positive forcing: in other words, all of them lead to a warming of the climate. Of the four, RCP2.6 has the smallest total forcing (the least warming effect on climate), followed by RCP4.5, RCP6.0 and RCP8.5, which has the highest.

The RCPs are the standard scenarios of predicting possible future climate change. You will see how scientists predict climate change for different possible geoengineering actions later in the course. One important question you will consider is: how could we use geoengineering to achieve negative emissions of CO₂?

Scientists try to predict climate change for each RCP using climate models. The climate models take the concentrations of greenhouse gases and pollutants, and the land use changes, and simulate the Earth system response: predicting changes in temperatures, rainfall, sea ice, and so on.