Also in this chapter:
Are the Copenhagen Accord Pledges Sufficient to Limit Global Warming to 2°C or 1.5°C?
The Copenhagen Accord declared that deep cuts in global emissions are required “so as to hold the increase in global temperature below 2 degrees Celsius”. The Accord called for an assessment that would consider strengthening the long-term goal including “temperature rises of 1.5 degrees”. Since December 2009, 140 countries1 have associated themselves with the Copenhagen Accord. Of these, 85 countries have pledged to reduce their emissions or constrain their growth up to 2020.
The question remains, however, whether these pledges are sufficient to achieve the Accord’s temperature limits, or if there will be a gap between what is needed and what is expected as a result of the pledges.
Many scientific groups have identified global emission pathways2, or emissions trajectories, that are consistent with various temperature limits, while others have estimated global emissions in 2020 based on the Copenhagen Accord pledges. Some groups have calculated both. Not surprisingly, different groups have come up with different estimates. The range of estimates is caused, for example, by the fact that some of the pledges have conditions attached, such as the provision of finance and technology or ambitious action from other countries. This leads to a range of potential outcomes rather than a single estimate.
To understand and interpret the range of results coming from different studies, the United Nations Environment Programme (UNEP), in conjunction with the European Climate Foundation and the National Institute of Ecology, Mexico, convened a six-month preliminary assessment of these studies. This assessment aims to provide policy-makers with an overview of results from various studies, as well as their areas of agreement and disagreement. Individuals from twenty-five groups have contributed to the assessment and co-authored this publication. This report is a summary of that work.
Notably, the 2020 emissions reduction pledges analysed in this report were not decided under a quantitative top-down approach to emissions management — one that starts with temperature limits for which the mitigation effort is distributed among countries by negotiation. Therefore, at this time we are only analysing the effect of the offers brought forward by countries in the form of pledges under the Copenhagen Accord3.
This assessment addresses four main questions:
The total stock of greenhouse gases in the atmosphere has a strong effect on climate forcing related to climate change. This stock is determined by the accumulated emissions of greenhouse gases in the atmosphere. It follows that cumulative emissions have a profound influence on the long-term increase of global temperature6.
An important point is that several different emission pathways can result in the same cumulative emissions over a period of time. But not all pathways are considered equally feasible; some are thought to be constrained by an upper ceiling on the rate of emission reductions due to technological, economic, social and political factors. Hence, the feasibility of reduction rates plays a central role in determining which 2020 emission levels are consistent with temperature limits. Also important are assumptions about the feasibility of “negative emissions”, i.e. the net removal of carbon dioxide (CO2) from the atmosphere through, for example, planting forests or capturing CO2 from biomass (see Box 3).
Studies show that there is a trade-off between the timing of the peak and the rate of decrease in emissions afterwards – the sooner and lower the peak, the slower the rate of decrease can be afterwards. Conversely, the longer the peak is delayed and the higher it is, the faster emissions must decline afterwards, and/or the stronger the negative emissions over the long term, in order to stay within the temperature limit (see Figure A).
Many recent modelling studies have assumed that it would be unrealistic for global emissions to immediately start decreasing (because of political and economic factors) and therefore have focused on scenarios in which global emissions continue to increase for a few years and then decrease sharply afterwards.
Emission pathways assessed in this report that provide a “likely” (greater than 66 per cent) chance of staying within the 2°C limit, have the following characteristics:
Accepting a “medium” (50-66 per cent) rather than “likely” chance of staying below the 2°C limit relaxes the constraints only slightly: emissions in 2020 could be 1 GtCO2e higher, and average rates of reduction after 2020 could be 2.5 per cent per year (range 2.2-3.0 per cent). Nevertheless, global emissions still need to peak before 2020 in the majority of cases.
In this assessment we have identified some emission pathways that keep the increase in temperature below 1.5°C by 2100, but “overshoot” this limit by a small amount for a few decades prior to 2100. However, the chance of doing so is low (range: 27-35 per cent probability). The emission levels in 2020 of these pathways are about the same as those in Point 2 above, i.e. they are consistent with a likely chance of staying below the 2°C limit throughout the twenty-first century11.
In addition, the most ambitious “stylized” pathways show that staying within the 1.5°C limit with overshoot (and with a “medium” or “likely” chance) have emission reduction rates after 2020 that are at the high end or faster than presently found in the IAM literature. Lower emission levels in 2020 would allow slower emission reduction rates after 2020.
These findings should be considered preliminary, however, as few studies have explicitly looked at the question of achieving the 1.5°C target.
The range in estimates of emission levels comes from model uncertainties including the omission of feedback phenomena in the climate system and (in some models) the impact of aerosols on climate forcing. The uncertainty of key assumptions, such as baseline emissions, also has an influence on calculations.
As a reference point, without pledges global greenhouse gas emissions may increase from 45 GtCO2e in 2005 to around 56 GtCO2e in 2020 (range: 54-60 GtCO2e) according to business-as-usual projections. These results come from thirteen studies that have been reviewed in this assessment.
Results show that the pledges, if implemented, are expected to reduce global emissions in 2020 compared to business-as-usual projections. How much lower will depend on:
For the purposes of this report, we have developed four cases that provide a range of plausible outcomes from the UNFCCC negotiations, each with different combinations of the factors mentioned above. We use the term “lenient rules” to refer to cases in which countries maximise the use of surplus emission units and “lenient LULUCF credits”, and thereby weaken mitigation targets13. We use “strict rules” for the cases in which they do not14.
Thus, under the most ambitious outcome, the pledges could result in 2020 emissions that are 7 GtCO2e lower than business-as-usual.
The estimates reflected in the four cases do not take into account all factors that could affect emissions in 2020.
Two factors could increase emissions and lessen the impact of the pledges. If industrialized countries were to use offsets to meet their targets, and the developing countries that supplied the offsets also counted them towards their pledges, then emissions would be higher than estimated in Point 5. This “double counting” of offsets could increase emissions in 2020 by up to 1.3 GtCO2e in 2020. Similarly, if domestic policies were to be ineffective in meeting the pledges, emissions could be higher in 2020.
There are also factors that could further decrease emissions in 2020. If substantial international funds were to become available as agreed to in the Copenhagen Accord, emissions could be as much as 2.5 GtCO2e lower in 2020 than in the four cases above. Similarly, if domestic policies went beyond international pledges or if pledges were strengthened, emissions could be substantially lower.
There is a large range between different groups’ estimates for 2020 emission levels, even under the same assumptions regarding conditionality of pledges and accounting rules (range: -4 to +8 GtCO2e around the median estimate, depending on the case). The range of estimates is caused, for example, by differences in the underlying data sets, the treatment of emissions from LULUCF, the estimates of emissions from international transport, and the assumptions made about business-as-usual emissions growth of developing countries.
As a reference point, we saw in Point 2 that to have a “likely” chance of staying below the 2°C temperature limit, global emissions should be around 44 GtCO2e (range: 39-44 GtCO2e). But according to business-as-usual projections global emissions in 2020 may be around 56 GtCO2e (range: 54-60 GtCO2e). This leaves a gap of about 12 GtCO2e (range: 10-21 GtCO2e).
The four pledge cases, each with different assumptions about the future outcome of the UNFCCC negotiations, result in different gaps as follows16:
These results can be seen in Figure C.
Double-counting of international emission offsets could also increase the gap up to 1.3 GtCO2e. This is a real risk since the Copenhagen Accord does not include rules regarding the use of international offsets.
As a final point here, to have a “medium” rather than a “likely” chance of staying within the 2°C limit, global emissions in 2020 can be about 1 GtCO2e higher and the gap also narrows by about 1 GtCO2e.
Since the emissions gap is the difference between emission levels for different temperature targets and expected emissions in 2020, the gap also inherits the uncertainties of these two components. The reader will note that the range around median estimates (Figure C) is not symmetric; the lower bound extends about 1-2 GtCO2e below the median, whereas the upper bound rises 7-9 GtCO2e above it (for a “likely” chance of staying below 2oC). One way to interpret this skewed range is that the gap may turn out to be higher rather than lower than the median.
This assessment focuses on the majority (20th – 80th percentile) of emission pathways. But there are obviously also results outside of this range. In the extreme case, if we combine the highest 2oC emission levels with the lowest estimate of expected emissions, the gap disappears. At the opposite extreme, if we combine the lowest 2oC emission levels with the highest estimate of expected emissions, the gap would be greater than 20 GtCO2e.
a) Reducing the gap through higher ambition pledges.
The gap can be reduced by around 2-3 GtCO2e (with a range of estimates from 2 to 5 GtCO2e) by moving from the unconditional (lower ambition) pledges to the conditional (higher ambition) pledges.
b) Reducing the gap by tightening the rules
The gap can be reduced by around 1-2 GtCO2e by ensuring that “strict” rules apply to the use of LULUCF credits and surplus emission units.
LULUCF accounting: If industrialized countries apply “strict” accounting rules to minimise the use of what we refer to as ‘lenient LULUCF credits’17 , they would strengthen the effect of their pledges and thus reduce the emissions gap by up to 0.8 GtCO2e.
We note that policy options (a) and (b) are interdependent and so their benefits cannot necessarily be added together. But we estimate that the two options combined could reduce emissions by around 4 GtCO2e in 2020 (with a range of estimates of 4-6 GtCO2e) compared with the least ambitious case (case 1).
In addition, the risk of the gap increasing in size can be avoided if the negotiations set rules regarding international offsets to prevent them from being counted towards both industrialized and developing country pledges. “Double-counting” would increase the gap by up to 1.3 GtCO2e.
If the above measures were to be taken, there might still be a gap of 5 GtCO2e compared with a 2°C limit. This gap could be closed if countries were to adopt more ambitious actions or pledges. The results from integrated assessment models (IAM) suggest that it is possible to reach emission levels where there is no gap, using mitigation measures that are economically and technologically feasible.
Analysis also shows that international climate finance in line with the Copenhagen Accord could help achieve some of these reductions in developing countries.
The results of the IAM pathways that have a “likely” (greater than 66 per cent) or even “medium” (50-66 per cent) chance of limiting temperature increase to 2°C show average annual emission reduction rates of greater than 2 per cent per year after 2020. Achieving this over the long-term would be unprecedented because, on the contrary, global emissions have almost continuously grown since the industrial revolution.
The higher the emissions in 2020, the faster the rate of decline required thereafter to meet temperature targets. Therefore, if targets are to be met, it will be essential to lay the groundwork now for such rates of reduction. This can be done, for example, by avoiding lock-in of high carbon infrastructure with long life-spans and developing and introducing advanced clean technologies.