Stabilisation of carbon dioxide concentrations

Carbon dioxide, as we have seen, is the most important of the greenhouse gases that is increasing through human activities. As we saw in Chapter 3, emissions of carbon dioxide into the atmosphere from anthropogenic sources result from fossil fuel burning (about 80%) and from land-use changes (about 20%) - mainly deforestation. Reduction in emissions from land-use changes was considered earlier in the chapter. Reductions in emissions from fossil fuel burning will be the subject of the next chapter.

Under all the SRES scenarios, the concentration of carbon dioxide rises continuously throughout the twenty-first century and apart from scenarios B1 and A1T none comes anywhere near to stabilisation of concentration by 2100. Since the year 2000, the growth of global carbon dioxide emissions (Figure 10.1) at close to 3% per year has been faster than assumed in most of the SRES scenarios where the average growth was around 1%. During the 1980s and 1990s global carbon dioxide emissions grew at an average of just over 1% per year, the average during the 1990s being kept down because of substantial falls in emissions in the former Soviet Union and eastern Europe. Projections for the next few years indicate that global emissions are likely to continue rising at about the current rate.

Here we consider what sort of emissions scenario would lead to stabilisation of the carbon dioxide concentration. Suppose for instance that it were possible to keep global emissions for the future at the same level as now, would that be enough? Stabilising concentrations is, however, very different from stabilising emissions. With constant emissions from now, the concentration in the atmosphere would continue to rise, would reach at least 500 ppm by the year 2100 and continue to increase thereafter, although more slowly, for many centuries. Further, because of the long lifetime of carbon dioxide in the atmosphere, even if very severe action is taken to curb emissions, stabilisation of its concentration and hence stabilisation of climate will take many decades.

A large number of studies focusing on climate stabilisation and emissions profiles leading to stabilisation of greenhouse gases have been brought together in the IPCC AR4 Report, where they have been grouped together under categories leading to different stabilisation levels. For each of the chosen levels, the range of emissions profiles shown by the studies to bring about stabilisation of carbon dioxide concentration is shown in Figure 10.2 and Table 10.3. Studies that have included greenhouse gases other than carbon dioxide have been put together with carbon dioxide-only studies through the use of carbon dioxide equivalent CO2e (for definition see Chapter 6, page 147).

Note that stabilisation at any level shown in the figure, even at an extremely high level, requires that anthropogenic carbon dioxide emissions eventually fall to a small fraction of current emissions. This highlights the fact that to maintain a constant future carbon dioxide concentration, emissions must eventually be no greater than the level of persistent natural sinks. The main known such sink is due to the dissolution of calcium carbonate from the oceans into ocean sediments which, for high levels of carbon dioxide concentration, is probably less than 0.1 GtC per year.23 This means, for instance, that for the lowest category in Figure 10.2, anthropogenic emissions of greenhouse gases need to fall close to zero by 2100.

In the work presented in Figure 10.2, many different pathways to stabilisation could have been chosen. The particular emission profiles illustrated in Figure 10.2 begin by following the current average rate of increase of emissions and then provide a smooth transition to the time of stabilisation. To a






1940 1960 1980 2000

2020 Year

2040 2060 2080 2100

280 300 400 500 600 700 800 900 1000 Greenhouse gas concentration stabilisation level (ppm CO2)

1940 1960 1980 2000

2020 Year

2040 2060 2080 2100

280 300 400 500 600 700 800 900 1000 Greenhouse gas concentration stabilisation level (ppm CO2)

I: 445-490 ppm CO2e II: 490-535 ppm CO2e

III: 535-590 ppm CO2e IV: 590-710 ppm CO2e

V: 710-855 ppm CO2e VI: 855-1130 ppm CO2e

Post-SRES range

Figure 10.2 (a) Global carbon dioxide emissions for 1940-2000 and emissions ranges for categories of stabilisation scenarios from 2000 to 2100; colours show stabilisation scenarios grouped according to different levels (categories I to VI in inset and Table 10.3). The range shown covers the 10th to 90th percentiles of the full scenario distribution (numbers of scenarios included in Table 10.3). The thin dashed lines denote the lower end of the range for that category in cases where there is overlap between the categories. To convert Gt CO2 to Gt C, divide by 3.66. The thick dashed black lines indicate the range of emissions scenarios published since 2000. (b) Relationship between stabilisation level and the likely equilibrium global average temperature increase above pre-industrial level; the dark blue line assumes a best estimate of climate sensitivity of 3 °C and the colours indicate the effect of a range in climate sensitivity from 2 to 4.5 °C. For calculating the equilibrium temperature, the simple relationship Teq = T2xC02 x ln([C02]/280)/ln(2) is employed with mean, lower and upper values of T2xC02 of 3, 2 and 4.5 °C. The relationship between radiative forcing (R in W m-2) and concentration (C in ppm) is R = 5.3ln (C/C0) where C0 is the pre-industrial C02 concentration of 280 ppm.

first approximation, the stabilised concentration level depends more on the accumulated amount of carbon emitted up to the time of stabilisation than on the exact concentration path followed en route to stabilisation. This means that alternative pathways that assume higher emissions in earlier years would require steeper reductions in later years. For instance, if the atmospheric concentration of carbon dioxide is to remain below about 550 ppm, the future global annual emissions averaged over the twenty-first century cannot exceed the level of global annual emissions in the year 2000. For lower levels of stabilisation, to bring down the twenty-first-century accumulated emissions to a much lower level will require urgent and large carbon dioxide emissions reductions. Note also that for the lowest stabilisation level, Category 1, in Figure 10.2a some of the scenarios later in the century require negative emissions implying a need for substantial removal of CO2 from the atmosphere.

Table 10.3 Characteristics of stabilisation scenarios and resulting long-term equilibrium global average3


Anthropogenic addition to radiative forcing at stabilisation (Witt2)

C02 equivalent concentration at stabilisation'' (ppm)

Peaking year for C02 emissions3-c (year)

Change in global C02 emissions in 2050 (per cent of 2000 emissions) (percent)3-c

Global average temperature increase above pre-industrial at equilibrium, using 'best estimate' climate sensitivity^ e (°C)

Number of assessed scenarios





-85 to -50







-60 to -30







-30 to +5







+10 to +60







+25 to +85







+90 to +140



a The emission reductions to meet a particular stabilisation level reported in the mitigation studies assessed here might be underestimated due to missing carbon cycle feedbacks b Atmospheric C02 concentrations were 379 ppm in 2005. The best estimate of total C02 equivalent concentration in 2005 for all long-lived greenhouse gases is about 455 ppm, while the corresponding value including the net effect of all anthropogenic forcing agents e.g. aerosols is 375 ppm C02e. c Ranges correspond to the 15th to 85th percentile of the scenario distribution. C02 emissions are shown so multi-gas scenarios can be compared with C02-only scenarios.

d The best estimate of climate sensitivity is 3 °C.

e Note that global average temperature at equilibrium is different from expected global average temperature at the time of stabilisation of greenhouse gas concentrations due to the inertia of the climate system. For the majority of scenarios assessed, stabilisation of greenhouse gas concentrations occurs between 2100 and 2150. For Categories I or II, equilibrium temperature may be reached earlier.

Figure 10.3 Global CO2 emission profiles that would stabilise CO2 at 450 ppm (pink) and 550 ppm (light blue). The shaded areas show the range of uncertainty arising because of the climate-carbon-cycle feedback (see text). Also shown are emissions profiles for the IEA scenarios ACT Map (red) and BLUE Map (blue) for fossil fuel emissions (see Chapter 11, page 332) to both of which has been added a constant 7.3 GtCO2 (2GtC) per year to allow for emissions from deforestation and land use change. A further profile (green) shows the effect on the BLUE Map profile if emissions from deforestation and land use change are halted completely by 2050.

8 30

8 30











Many of the scenarios included in compiling the results shown in Figure 10.2 and Table 10.3 do not include the effect of climate feedbacks on the carbon cycle (see box in Chapter 3 on page 48-9). Two of the feedbacks are important in the context of the consideration of stabilisation scenarios; namely, increased respiration from the soil as the temperature rises and a decrease in net uptake of carbon by plants in some regions as the climate warms (e.g. in forests this can be perceived as dieback). The area over which such decrease occurs becomes larger for greater warming. As we saw in Chapter 3, the effect of these feedbacks could lead to the biosphere becoming a substantial source of carbon dioxide during the twenty-first century. The size of that source will depend on the amount of climate change. In Figure 10.3 are shown profi les for stabilisation of carbon dioxide alone at 450 and 550 ppm both without these climate change feedbacks included (the upper line) and with the range of feedbacks represented by the models studied in the IPCC 2007 Report, the largest value of the combined feedbacks being that derived by the Hadley Centre in the UK. Compared with no feedback, the effect of this largest value is to reduce the accumulated emissions allowable in the twenty-first century, for instance, for the 450 ppm and 550 ppm stabilisation scenarios, by about 600 Gt CO2 and 900 Gt CO i respectively.24 Also under this largest feedback case, emissions scenarios not allowing for feedback that are aiming at 450 ppm stabilisation, when the feedbacks are included, would in fact achieve around 500 ppm.

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