Chapter 3 A global emissions budget for 2 degrees or less
Chapter 2 set out the severe potential impacts, for the world and for Australia, of a global temperature rise of greater than 2 degrees. This chapter looks at what it will take to have a reasonable chance of limiting warming to no more than 2 degrees.
Limiting global emissions to keep warming to no more than 2 degrees above pre-industrial levels is challenging, but remains technologically and economically feasible. Immediate and strong action is required by all countries, especially the major emitting economies.
Global emissions budgets help quantify the challenge of limiting global warming. They specify the total amount of emissions projected to result in a given rise in global temperature. They are expressed in terms of probabilities to reflect uncertainties about the exact temperature effect of a given amount of emissions; a tighter global budget provides a higher probability of keeping global warming to 2 degrees or less but reduces the amount of emissions allowed.
The Authority recommends (Chapter 8) that Australia’s emissions goals include a long term national budget. The global emissions budget provides important guidance for recommending this national budget, and the appropriate set of targets and trajectories over time for national emissions reductions consistent with the national budget.
The Authority considers that the global emissions budget adopted as a reference point for consideration of Australia’s national emissions budget in this Review should provide a likely chance (defined here as a 67 per cent probability) of limiting warming to 2 degrees or less. This requires that global emissions of greenhouse gases covered by the Kyoto Protocol not exceed a budget of approximately 1 700 gigatonnes of carbon dioxide equivalent (Gt CO2-e) between 2000 and 2050. About 35 per cent of this budget has already been used between 2000 and 2012.
As outlined in Chapter 2, keeping global temperature rises to no more than 2 degrees is strongly in Australia’s interests. Australia’s emissions targets cannot be seen in isolation from the global task of limiting emissions. A global emissions budget sets out the total amount of global emissions consistent with the aim of limiting warming to 2 degrees or less, but does not dictate a particular emissions pathway, so long as the budget is not breached. This chapter examines global emissions budgets, including:
- whether limiting global warming to no more than 2 degrees above pre-industrial levels remains feasible, and the scope and timing of action required to maintain a global emissions pathway consistent with that limit;
- the global emissions budgets that set out a maximum level of global emissions to provide a given probability of limiting temperature increases to no more than 2 degrees; and
- the characteristics of global emissions budgets to be used as a reference for the Targets and Progress Review, including the level of probability and the greenhouse gases included.
3.1 Feasibility of limiting global warming to 2 degrees or less
Lessening the impacts of climate change will require strong international action to reduce emissions. The Authority has accepted Australia’s interest in limiting global warming to 2 degrees or less as a given for the Targets and Progress Review. This is consistent with the below 2 degrees global goal agreed by the international community. It has also been adopted by other organisations as a starting point in their consideration of national emissions reduction goals, including the United Kingdom Committee on Climate Change and the German Advisory Council on Global Change.
Two critical questions for policy-makers are whether a 2 degree temperature limit remains feasible, and the scope and timing of action required to maintain an emissions pathway consistent with that limit. Global emissions are currently tracking towards the upper bound of projections, on a pathway consistent with a 4 degree increase in global average temperature by 2100 (World Bank 2012, p. xiii). The longer emissions diverge from a 2 degree pathway, the faster the available global emissions budget will be used up, requiring greater efforts to reduce emissions in future and eventually ruling out the possibility of limiting warming to 2 degrees or less.
The feasibility of the 2 degree temperature limit has been considered extensively in the research literature, providing consensus that a range of 2 degree emissions scenarios remain technically and economically feasible (see, for example, Rogelj et al. 2011; UNEP 2012). Feasible 2 degree pathways generally share several important characteristics:
- Early emissions reductions. A near-universal finding is that early action is critical to limit future costs and maintain feasibility of limiting temperature increases, with many studies pointing to the importance of global emissions peaking by 2020 (see, for example, Rogelj et al. 2012). Delaying emissions reductions increases the rate of decarbonisation required in the future, increases costs of meeting emissions targets, reduces flexibility in choosing how to reduce emissions, and increases reliance on the development and commercialisation of currently speculative technologies to achieve net negative emissions (see, for example, Rogelj et al. 2013). Figure 3.1 illustrates alternative emissions trajectories that result in the same amount of cumulative emissions, but with different peaking years and maximum rates of emissions reductions. Analysis by the United Nations Environment Programme (UNEP) finds that there is an ‘emissions gap’ of approximately 813 Gt CO2-e between projected emissions levels in 2020 and the global emissions in 2020 consistent with a ‘likely’ (greater than 66 per cent) chance of 2 degrees, but that it is still possible to close this gap (UNEP 2012, pp. 1–7). The UNEP emissions gap analysis is discussed further in Chapter 4.
- Steep decarbonisation rates. Even with early peaking of global emissions, scenarios to remain within 2 degrees generally require high, sustained rates of emission reductions for much of the rest of this century. The maximum rate of global emission reductions that can be maintained is a key constraint for feasible pathways, with one recent study (den Elzen et al. 2010) estimating a maximum rate of about 3–4 per cent per year without the use of bioenergy with carbon capture and storage, or 4–5 per cent if this technology becomes viable.
Figure 3.1: Illustrative alternative global emissions trajectories for a given global emissions budget
Source: Climate Change Authority
- Demand-side reductions in energy use. Under the IEA ‘450 Scenario’, over half of the required emissions savings from energy are achieved by energy efficiency improvements (IEA 2012b, p. 241). Another study found that strong action on energy efficiency can allow some flexibility in the choice and timing of other emissions reduction measures (Rogelj et al. 2013).
- Negative emissions. Many 2 degree scenarios assume the use of negative emissions technology (for example, bioenergy with carbon capture and storage), in the second half of this century. Of a large number of emissions scenarios analysed by UNEP, 40 per cent of those considered to provide a likely chance of a not more than 2 degree temperature increase require net negative emissions before 2100 (UNEP 2012, p. 26). If net negative emissions prove to be infeasible, a radical shift in mitigation options may come too late to limit warming to 2 degrees or less.
- Technology investment and diversification. A number of studies raise the importance of investment in technology. The more ambitious the scenario, the earlier large investments in technology development are required. This highlights the importance of pursuing multiple technology options simultaneously to reduce the risk of particular technologies proving unviable.
Several stakeholder submissions to the Issues Paper for this Review requested that the Authority use a 1.5 degree rather than a 2 degree temperature limit.
Pathways that provide a 50 per cent or greater chance of limiting warming to 1.5 degrees have received limited consideration in the scenario literature, although there is some evidence that a 1.5 degree limit remains viable. The limited analyses available suggest that for the first half of this century 1.5 degree pathways share many of the same characteristics as 2 degree pathways (Rogelj 2013). This opens up the possibility that a 2 degree pathway could provide scope, with increased effort in future, to shift to a more ambitious 1.5 degree pathway.
Scenarios for 1.5 degrees are, however, likely to rely even more strongly on large-scale implementation of negative emissions technology in the second half of this century. The increased reliance on negative emissions and carbon capture and storage creates larger risks for 1.5 degree scenarios should such technologies prove to be infeasible.
The Authority’s assessment is that the global 2 degree limit remains feasible, but that immediate and strong international action is required, especially by all major emitting economies, as discussed further in Chapter 4. Failure to take global action at the scale required will progressively close off emissions scenarios, increase costs and eventually foreclose a reasonable possibility of limiting warming to 2 degrees or less. The Authority considers that global emissions and the ongoing requirements to remain within the 2 degree limit should be monitored in future.
C.1 Limiting global emissions to keep warming to no more than 2 degrees is still feasible, but only with immediate and strong international action, and especially by the major emitting economies.
3.2 Global emissions budgets
The magnitude of global temperature increases is not determined by emissions in any one year, but by the concentration of greenhouse gases in the atmosphere. This is the net outcome of total emissions and removals of greenhouse gases from the atmosphere over an extended time period.
Global emissions budgets estimate the total amount of greenhouse gas emissions that will result in a given temperature increase, within a probability range. The emissions budget approach links cumulative emissions of greenhouse gases directly to temperature, without focusing on the intermediate steps shown in Figure 3.2 and discussed in Box 3.1. The relationship is expressed as a probability, to reflect the variability of the climate response to a given amount of greenhouse gas emissions.
While global emissions budgets identify the overall limit on global emissions, they do not identify a particular timing of peak emissions or the rate at which emissions are reduced, so long as the overall budget is not breached. There will be a large number of trajectories that could lead to the budgeted level of cumulative emissions (and the related expected temperature increase) over time, as illustrated by Figure 3.1. Because the emissions budget is ultimately fixed, however, delays in reducing emissions must be compensated with more rapid emissions reductions in future years.
Figure 3.2: Relationship between greenhouse gas emissions and global temperature
Source: Adapted from Raupach, Harman & Canadell 2011
Box 3.1: Global emissions budgets, atmospheric concentration and radiative forcing
Global emissions budgets, also referred to as carbon budgets, have gained prominence as a method of analysing and communicating the scale of emissions reductions required to remain within a global temperature limit. Emissions budgets provide a useful way of linking emissions targets and trajectories to the underlying science of climate change.
Emissions limits that keep global temperature increases to 2 degrees or less can be expressed in a number of ways. Two other measures are the concentration of greenhouse gases in the atmosphere, and the radiative forcing of greenhouse gases and other substances. As set out in Figure 3.2, these measures reflect different intermediate steps in the chain between emissions and global temperature rises. Atmospheric concentration has been a commonly used measure to communicate the limit consistent with a certain level of temperature rise (for example, an atmospheric concentration of 450 parts per million (ppm) is consistent with about a 50 per cent chance of limiting warming to 2 degrees or less.
For example, an approximately 67 per cent probability of limiting warming to 2 degrees or lower could be expressed using the following measures:
- an equilibrium concentration of 415 ppm of CO2-e in the atmosphere;
- an equilibrium radiative forcing of about 2.1 watts per square metre; or
- a global emissions budget of 1 700 Gt CO2-e from 2000 to 2050.
The Authority is required, under the Clean Energy Act 2011 (Cth) (s 289), to have regard to estimates of the global greenhouse gas emissions budget.
The concept of a global emissions budget provides important guidance for setting Australia’s national targets. The global budget links to Australia’s ultimate aim of limiting warming to no more than 2 degrees and provides clear guidance on the scale of the global challenge. Australia’s national emissions budget, discussed in Chapter 9, can be thought of as Australia’s fair share of this future global budget.
Two issues relevant to selecting a global emissions budget as a reference point for this Review are:
- the associated probability of limiting warming to 2 degrees or less; and
- whether to specify the budget in terms of CO2 only or of multiple greenhouse gases.
3.3 Probability level for budgets
As discussed, budgets are expressed in terms of their probability of remaining within a given temperature limit. A higher or lower likelihood of a temperature increase of 2 degrees or less corresponds to different budgets. For example, a 50 per cent probability of limiting warming to 2 degrees or less gives an allowable budget of Kyoto gases of approximately 2 020 Gt CO2-e over the period 2000–2050. A 67 per cent probability reduces the allowable budget to approximately 1 700 Gt CO2-e (adapted from Meinshausen et al. 2009, p. 1 161).
Choosing a budget with a higher probability better manages risks from:
- uncertainties over the precise temperature increase from a given budget, and the possibility of greater warming; and
- the severity of impacts of a temperature increase above 2 degrees.
Tighter budgets will, however, require more action to reduce emissions.
A number of submissions to the Issues Paper for this Review indicated that budgets with higher, rather than lower, levels of probability should be the chosen reference for the Authority. Other submissions indicated a preference for budgets with a relatively high probability such as 80 per cent, but opted for a lower probability budget (such as 67 per cent) on the basis that higher probability budgets are no longer practicably attainable.
The Authority considers that the global budget used as a reference point for Australia’s national carbon budget and targets should have at least a likely probability (greater than 66 per cent, but defined here as a 67 per cent probability for the purposes of selecting a reference point) of limiting warming to 2 degrees or less. Some widely used limits (such as a maximum allowable atmospheric greenhouse gas concentration of 450 ppm) characterised as consistent with the 2 degree limit carry only an approximately 50 per cent probability of limiting warming to 2 degrees or less. In light of the severe global and national risks of the impacts projected at temperatures of up to and beyond 2 degrees, the Authority considers that a global budget with a higher probability is the more responsible reference for Australia’s national emissions budget, and represents a more appropriate risk management approach. A 67 per cent probability level is also consistent with the greater than 66 per cent probability global emissions budget referred to by the Intergovernmental Panel on Climate Change (IPCC), discussed in Box 3.2. Section 9.4 discusses setting a national emissions budget that corresponds to a global budget with a 67 per cent probability level. The implications of the choice of different global reference budgets for Australia’s national emissions budget are discussed further in Chapter 11.
Figure 3.3: Probability of staying below specific temperature increases above pre-industrial levels given carbon dioxide equivalent stabilisation levels
Note: The left scale indicates a CO2-e concentration level at equilibrium from all greenhouse forcing agents. The arrow illustrates that to limit global temperature increase to below 2 degrees with a likely (greater than 66 per cent) probability, CO2-e concentrations should be should be lower than 415 ppm.
Source: Adapted from Rogelj, Meinshausen & Knutti 2012
A budget that provides a higher probability of limiting temperature increases to 2 degrees or less also provides a lower probability of higher temperature rises, as shown in Figure 3.3. Figure 3.3 sets out probabilities (the coloured bands) of remaining below a specified temperature increase for different concentrations of greenhouse gases in the atmosphere. An atmospheric CO2-e concentration that provides an approximately 67 per cent probability of limiting warming below 2 degrees, shown by the horizontal dotted line, is also projected to provide an approximately 90 per cent probability of staying below 3 degrees and a greater than 95 per cent probability of staying below 4 degrees. It also provides a less than 10 per cent probability of staying below a 1 degree temperature increase.
3.3.1 Reviewing the global emissions budget over time
The Authority proposes that the appropriateness of the chosen global reference budget can be reviewed and adjusted, if necessary, over the longer term. This can occur as part of periodic reviews of Australia’s national emissions budget, which the Authority recommends be conducted at least every five years (see Chapter 8). This flexibility would better position Australia to respond should the international community choose a more stringent temperature goal in future or if increased scientific understanding of climate uncertainties reduces estimates of the allowable global emissions budget. Conversely, if the scale and pace of international action in future is such that a greater than 66 per cent probability of limiting warming to 2 degrees becomes infeasible, the Authority could review whether to move to a reference budget with a lower probability of achieving 2 degrees.
3.4 Carbon dioxide-only or multi-gas budgets
The Authority considered whether to adopt a CO2-only or a multi-gas budget that includes all the Kyoto gases. CO2 is long-lived in the atmosphere and is the dominant contributor to human-induced climate change. CO2-only budgets can give a good indication of the extent of likely long-term temperature rise, are simple and target the most significant greenhouse gas. A multi-gas budget is most closely aligned with Australia’s international obligations under the Kyoto Protocol, but has some scientific limitations. In part, this is because different gases behave differently in the atmosphere and remain there for different lengths of time.
The Authority received a small number of submissions on the use of multi-gas budgets, with one submitter highlighting the potential limitations of a budget approach that includes both short- and long-lived gases. Another supported a focus on CO2 as the longest lived greenhouse gas, suggesting that additional separate budgets should be provided for the other gases.
While acknowledging the limitations, the Authority considers that a multi-gas approach is preferable for the purposes of setting Australia’s national emissions budget. Multi-gas approaches are consistent with Australia’s international commitments and with the approach adopted by other nations. Non-CO2 greenhouse gases are also a significant component of Australia’s emissions – about 28 per cent in 2011 (adapted from DIICCSRTE 2013, Vol. 1 p. 29). A multi-gas approach acknowledges the importance of reducing these emissions.
Table 3.1: Estimates of global emissions budgets 2000–2050
Probability of remaining within 2 degree limit
80 per cent
75 per cent (74 for Kyoto gases)
67 per cent
50 per cent
Note: The budget figures in Meinshausen et al. are specified for 2000–2049; an extra year of estimated emissions has been added to give a budget to 2050.
Figures rounded to the nearest 10 Gt.
In 2009 the greenhouse gases covered by the Kyoto Protocol were CO2, methane, nitrous oxide, hydrofluorocarbons, perfluorocarbons and sulphur hexafluouride. A seventh gas, nitrogen trifluoride, has been added for the second commitment period of the Protocol; overall emissions of this gas are expected to be relatively small.
Source: Adapted from Meinshausen et al. 2009, p. 1 161.
3.5 Global budget estimates
The Authority is using the global emissions budget estimates developed in a 2009 study by Meinshausen et al., Greenhouse-gas emission targets for limiting global warming to 2°C (Table 3.1). These estimates have been widely cited by other scientific studies and used by national and international bodies as a reference for global emissions budgets.
The Authority has extended the 2000–2049 budget to 2050, and rounded to the nearest 10 Gt CO2-e. The Meinshausen et al. emissions budget estimates account for the temperature effects of aerosol pollution such as sulphates created by the burning of coal and oil (discussed in Chapter 2).
Approximately 35 per cent of the budget of 1 700 Gt CO2-e that would provide a 67 per cent probability of limiting temperature increases to 2 degrees or less has already been used between 2000 and 2012 (based on IEA 2012a; see Appendix C.6).
A budget to 2050 provides a robust indication of the probability that warming this century will not exceed 2 degrees (Meinshausen et al. 2009, p. 1158) and is consistent with the timeframes for long term domestic emissions reduction targets set by Australia and a number of other countries, shown in Table 4.4. It is important to note, however, that continued global effort to reduce emissions will be required after 2050.
Box 3.2: The IPCC global emissions budget
For the first time, the IPCC quantifies a cumulative emissions budget in its Fifth Assessment Report on the physical science basis of climate change, released in September 2013 (IPCC 2013). The IPCC refers to a global emissions budget of 1 000 Gt of carbon to provide a likely (greater than 66 per cent) chance to limit global warming to 2 degrees or less, and notes that about half that budget has already been emitted.
The IPCC’s estimated emissions budget is consistent with the budgets described in the Meinshausen et al study discussed above and used in this Review. The two studies, however, use some different assumptions and report in different units, resulting in different budget figures. These differences include:
- The IPCC budget is specified in carbon (C) whereas the Meinshausen budget used as a reference for the Review is specified in carbon dioxide equivalent (CO2-e). A tonne of carbon is equivalent to approximately 3.7 tonnes of carbon dioxide, with a 1 000 Gt C budget equating to a 3 700 Gt CO2 budget.
- The IPCC budget considers the period of 1861–1880 to 2100 while the Meinshausen budget only covers the period from 2000 to 2050. Both budgets, however, provide a robust indication of global warming likely remaining below a 2 degree temperature increase.
- The IPCC budget covers the effect of CO2 only and does not include the warming or cooling effects of other substances such as non-CO2 greenhouse gases and aerosol pollution. The IPCC notes that the budget would be lower if these other effects were included.
As discussed above, the Authority has chosen to use a multi-gas budget for a specified time period to 2050 as the most appropriate reference budget for this Review.
This chapter set out a global emissions budget that is consistent with a likely chance of limiting global warming to 2 degrees or less. Climate change is a global problem and immediate and strong international action will be required for the world to remain within this budget. Chapter 4 sets out the global context and assesses international trends in emissions reduction activities.
C.2 A global emissions budget that provides at least a likely (defined here as a 67 per cent probability) chance of limiting warming to no more than 2 degrees above pre-industrial levels should be used as a reference for the Review. This equates to a global budget of no more than 1 700 Gt CO2-e emissions of Kyoto gases from 2000 to 2050.