To assess the climatic impact of the targets put forward by countries, we first construct a global emissions pathway to 2100. This global pathway is then used as input to a reduced-complexity carbon-cycle / climate model (MAGICC6) which is calibrated against data from complex general circulation models (GCMs), including climate sensitivity and carbon cycle information. The MAGICC emulations reflect the complex model response ranges for the assessed scenarios in the calibration datasets, in particular the Representative Concentration Pathways (RCPs). MAGICC6 is run multiple times in order to obtain a probability distribution of outcomes such as global mean temperature, CO2 concentration, and total greenhouse gas concentration. These distributions are used for deriving the central median estimate of e.g. the global mean warming in 2100 and corresponding temperature exceedance likelihoods over the 21st century. The detailed methodology of the climate model is outlined in Meinshausen et al. 2009 & 2011.
Based on the individual CAT country analyses, global pathways are generated for 3 different scenarios: current policy projections, short-term pledges (up to 2030), and long-term pledges (up to 2050). For each of these three scenarios, we also evaluate an upper and lower pathway, based on the uncertainties in the underlying country analyses, or the ranges of pledges.
To aggregate these scenarios to a global emissions pathway, several steps are required.
- First, we include emissions pathways for countries the CAT does not individually assess.
- Second, we determine the impact of Kyoto Protocol accounting rules where relevant.
- Third, near-term pathways are extended to 2100 based on the AR5 database scenarios.
- Fourth, emissions from international shipping and aviation are added to the national emission profiles to complete the global emission pathway.
- Fifth, individual greenhouse-gas emission pathways are derived from the resulting total global GHG emissions pathways.
- Finally, CO2 emissions from land use change are added to the national emission profiles to complete the global emission pathway. Each of these steps is described below.
Inclusion of non-CAT countries
Countries assessed by the Climate Action Tracker were responsible for 81% of global emissions in 2010 (excluding LULUCF). For countries that are not individually assessed, we currently assume that the emissions of these countries will follow a ‘business-as-usual’ (BAU) pathway. The BAU pathways used in this analysis are from the PRIMAP4 baseline, which is constructed from EDGAR, CDIAC, and CRF historic data, and projected using WEO 2009 and POLES data. The baseline construction is described in greater detail on the PRIMAP website.
Although the CAT doesn’t directly assess many countries that have submitted an NDC, these countries together are only estimated to result in ~2.5-3 GtCO2e of emissions in 2030 under our BAU scenario. Any reduction from BAU for the non-CAT countries would only be a fraction of this amount which is small compared with total emissions in 2030 in the pledge scenario that are estimated to be 54-57 GtCO2e. As of 1 October 2015, we therefore do not assume any deviation from BAU for these countries as the impact on global emissions and temperatures is expected to be small.
The Climate Action Tracker does not have sufficient resources to individually assess all NDCs. However, we continue to monitor the overall coverage and level of ambition for non-CAT countries. If a major deviation from the BAU scenario described above is found, an adjustment will be applied.
Extension of emission scenarios to 2100
The CAT uses a range of emissions pathways from the IPCC AR5 scenario database to extend the short-term CAT analysis pathways out to 2100. The methodology was revised in October 2015 in order to incorporate the latest science into this key step.
The revised methodology is based on the idea that the level of mitigation effort corresponds to the relative position of an emissions pathway in a set of pathways. For each CAT case (current policy projections, short-term pledges (up to 2030), and long-term pledges (up to 2050)), we identify this relative level of effort and keep it constant until 2100. This methodology ensures that the long-term projection is as consistent as possible with the shorter-term action or pledges by accounting for the inertia of near-term actions.
For each CAT case the country information is aggregated to the level of the 5 RCP regions (ASIA, MAF (Middle East and Africa), LAM (Latin America), OECD, and REF (reforming economies, ie. Russia and former Soviet states)).
We take into account all AR5 emission pathways from models that simulate all sectors and gases. An exception we make here is for pathways that fall in the negative emissions category 2 and have negative emissions of more than 20 GtCO2e per year. These pathways are outliers compared to other scenarios since they have extremely deep negative emissions from land use and bio energy and can therefore reach virtually any target, independent of the reductions in fossil fuel related emissions. Almost all of these pathways come from the GCAM model. The models that we use are GCAM, IMAGE, MERGE, MESSAGE, REMIND, and WITCH (see references for details). Some AR5 pathways do not include information for all regions explicitly. Those pathways are only taken into account for the regions they contain.
We exclude the scenarios where climate action only starts after 2030 (IPCC categories P3 and P4) from the AR5 database, because they are inconsistent with the storyline of continuing the implied level of mitigation effort associated with the CAT projections, beyond the last date of these projections. The 2050 target could in general resemble 2030 delayed action scenarios, however the current 2050 targets are consistent with the 2030 targets in the sense that 2020, 2030, and 2050 targets lie almost on a straight line.
To identify the relative level of effort of the CAT scenarios, we calculate the quantiles in the AR5DB pathway distribution which correspond to the CAT scenario for the last scenario years. We use a linear fit to determine the quantile in the first year after the scenario end which we then keep constant until 2100 to maintain the level of effort. In the short-term pledges case and the policy projection case, the period over which the quantiles are calculated is 2020 to 2030. In the long-term pledge case, the period is 2030 to 2050. For countries with no long-term (2050) pledge, we use the regional growth-rates from the previously calculated short-term pledge pathway to extend the country pathway to 2050.
Figure 1: Illustrative case of selecting long-term scenarios from the AR5 scenario database (light blue) that are closest to a particular near-term CAT assessment (2010-2030 or 2010-2050). In this illustration the analysis is carried out for the OECD region.
The whole analysis is carried out on the level of the 5 regions, so the last step is to sum the regional pathways to a global pathway, which is then used to calculate the global temperature after emissions from bunkers and deforestation are also included.
Additional emissions sources
A business-as-usual pathway for global deforestation emissions is provided by the median of baseline scenarios of land-use emissions from the LIMITS project (Kriegler et al., 2013). This pathway is virtually equal to the median of the wide range of baseline scenarios assessed by Working Group III in IPCC's Fifth Assessment Report (AR5), and somewhat lower than RCP8.5 (Riahi et al. 2007), the high side of the range of new emission scenarios (see iiasa.ac.at).
This global pathway is consistent with global carbon budget modelling (making sure that historical observed changes in atmospheric CO2 concentrations are reproduced) and is subsequently split into contributions by individual countries (Houghton 2009; van der Werf et al. 2009; Houghton et al., 2012).
Proposed reductions by individual countries are then applied to the business-as-usual pathways for these countries that are thus consistent with the historical global carbon budget.
The aggregate Kyoto gas pathway is transformed into a multi-gas pathway using the Equal-Quantile-Walk method developed in Meinshausen et. al (2009). This pathway is used as input to the reduced-complexity climate model MAGICC (www.magicc.org). MAGICC is calibrated to emulate higher complexity global circulation models (AOGCMs) in a probabilistic manner. The CAT temperature estimate and its uncertainty resemble the model and parameter uncertainty in the AOGCMs (see Meinshausen et al 2011).
- Gütschow, J. (2013). CP2 Surplus Calculator
- Houghton RA, House JI, Pongratz J, van der Werf GR, DeFries RS, Hansen MC, Le Quere C, and Ramankutty N (2012) Carbon emissions from land use and land-cover change, Biogeosciences, 9, 5125-5142
- Kriegler E, Tavoni M, Aboumahboub T, Luderer G, Calvin K, Demaere G, Krey V, Riahi K, Rösler H, Schaeffer M, and Van Vuuren DP (2013) What does the 2C target imply for a global climate agreement in 2020? The LIMITS study in Durpan Platform scenarios, Climate Change Ecoomics, 4, 4
- Meinshausen M, Meinshausen N, Hare W, Raper S C B, Frieler K, Knutti R, Frame D J and Allen M R (2009) Greenhouse-gas emission targets for limiting global warming to 2 °C. Nature 458 1158–62
- Meinshausen, M. et al. Multi-gas Emissions Pathways to Meet Climate Targets. Clim. Change 75, 151–194 (2006).
- Meinshausen, M., S. C. B. Raper and T. M. L. Wigley (2011) "Emulating coupled atmosphere-ocean and carbon cycle models with a simpler model, MAGICC6: Part I – Model Description and Calibration", Atmospheric Chemistry and Physics 11 1417-1456
- PRIMAP4 baseline
- UNEP (2013) The Emissions Gap Report 2013. United Nations Environment Programme (UNEP), Nairobi
More info on integrated assessment models
This is a list of models providing scenarios used by the Climate Action Tracker:
- GCAM: http://www.globalchange.umd.edu/models/gcam/
- MESSAGE: http://www.iiasa.ac.at/web/home/research/modelsData/MESSAGE/MESSAGE.en.html
- REMIND: https://www.pik-potsdam.de/research/sustainable-solutions/models/remind
- IMAGE: http://www.pbl.nl/image
- MERGE: http://web.stanford.edu/group/MERGE/
- WITCH: http://www.witchmodel.org/index.html