The Climate Action Tracker estimates a “best-guess” (median) global warming by 2100 of 2.7°C above pre-industrial, based on its assessment of pledges and policies in INDCs from 1 October 2015, with likely (>66%) chance of warming below 3°C. Climate Interactive estimates a best guess, median warming level of 3.5°C, significantly higher than the CAT estimate.
This results principally from significantly different emissions assumptions, both post-2030 and, to a lesser extent, in the period 2020-2030. The CAT assumes that a similar level of effort will be undertaken by countries post-2030 as applies in the period 2020-2030, whereas Climate Interactive assumes no direct connection between efforts before and after 2030. The climate science is consistent between the two assessments and does not explain the differences.
The Climate Action Tracker estimates what global warming would result by 2100, assuming that the mitigation effort over 2020-2030 will continue throughout the 21st century. This assumption therefore leads to continued emissions reductions in future decades, and by future generations, but at a level of effort no more, or less, ambitious than that implied by the INDCs. Also, explicit 2050 targets, from several of the larger emitters (e.g. USA, EU, and Japan) help to further constrain, and lower, post-2030 emissions trajectories.
For its estimate of 3.5°C warming by 2100, Climate Interactive assessed only the immediate effect of INDCs. Any post-2030 action is not assumed with the INDCs. Consequently, the Climate Interactive assessment presumes that emissions from regions/countries such as the EU and USA remain frozen at 2030 levels post-2030. For other regions, they assume that post-2030 emissions growth continues at a higher rate than levels consistent with the effort in the period 2020-2030.
The resulting Climate Interactive temperature of 3.5°C therefore only reflects short-term deviations in emissions trajectories resulting from INDCs in the period 2020-2030, and assumes that these deviations will not significantly affect the long-term national or global emissions profiles. If the “emissions gap” over 2020-2030 were closed, thus INDCs were indeed consistent with a 2°C pathway during the 2020s, and Climate Interactive’s approach applied, the projected warming would still exceed 2°C by a significant margin because of the assumptions regarding an effectively frozen level of emissions reduction action post-2030.
Limiting warming to below 2°C would only be achieved if subsequently, further and deeper reductions consistent with 2°C were incorporated as an explicit, additional post-2030 assumption, still not connected directly to the levels of effort represented by INDCs. Indeed, further work by Climate Interactive evaluates explicit scenarios of additional mitigation effort post 2030, leading to lower levels of warming by 2100 than the 3.5°C warming by 2100 warming estimate.
A further factor, of lesser significance in the temperature level, is that the CAT estimates significantly lower aggregate emissions resulting from pledges and INDCs in the 2020-2030 period than those estimated by Climate Interactive. CAT’s global emission levels in 2020 (52 GtCO2e) are in line with the UNEP Emissions Gap reports, but much lower than the estimate of Climate Interactive (59 GtCO2e). In 2030, CAT aggregate emissions in 2030 are 2 GtCO2e lower than Climate Interactive.
Post 2030 emission levels
The CAT assumes that a similar level of effort will be undertaken by countries post 2030 as applies in the period 2020-2030. Climate Interactive in their 3.5°C scenario assumes no direct connection between efforts before and after 2030. Consequently the emissions projected by CAT post-2030 are substantially lower than those projected by Climate Interactive.
CAT has confidence that the current INDC emissions levels are consistent with the long-term pathway post-2030 selected for the temperature calculation, because we have compared our results with other detailed assessments in the literature (e.g. LIMITS database). This difference accounts for the largest part of the temperature difference.
The CAT temperature reflects what would happen if the same level of effort were continued to the end of the century and any existing long-term, 2050 targets were met. The CAT approach aims to define a long-term CAT pathway that represents an equal relative level of effort continuing throughout the century. To quantify the level of effort, the CAT compares the 2020-2030 emissions reduction effort to the AR5 emissions scenario database. We then construct a long-term emissions pathway consistent with the same level of effort within the whole range of scenarios in the AR5 emissions scenario database (more details here).
As a result, if emissions in a certain region over 2020-2030 are revised downwards, this will result in an “automatic” transition to lower-carbon pathways and associated lower emissions in the long term. For example, the actual and draft INDC’s of China and India lower total projected emissions of the Asia region, leading to selection of a lower extended emissions pathway and contributing to the 0.4°C reduction in estimated temperature between the Lima and October 2015 temperature updates.
The level of 2020-2030 effort from INDCs across (non-OECD) Asia is generally consistent with long-term scenarios in the LIMITS database that, even with low mitigation effort, show a peaking of emissions from the Asia region around 2050, with China peaking very early (2030-2040) and India much later (2050-2070).
By contrast, Climate Interactive uses fixed assumptions post-2030, like stabilised emission levels for the US and EU, and continued growth for all other developed and developing countries/regions. This does not take into account governments’ existing post-2030 goals nor is it consistent with the policy changes embedded in developing country INDCs (if fully implemented.). The resulting temperature therefore only reflects short-term deviations in emissions trajectories resulting from INDCs and assumes that these deviations will not significantly affect the long-term national or global emissions profiles.
The consequence of Climate Interactive’s approach used for the 3.5°C scenario is that no plausible action by 2020-2030 would ever be sufficient to get on an emissions and policy pathway that limits warming to 2°C. If INDCs were consistent with a 2°C pathway during the 2020s, and Climate Interactive’s approach would be applied, warming would exceed 2°C because they assume an effectively frozen level of emissions reduction action post-2030. We note that Climate Interactive also provides scenarios with different post 2030 assumptions arriving at lower emissions and lower temperature estimates (2.4 and 2.8°C).
Aggregate emissions from pledges and INDCs 2020 - 2030.
The second key difference is the aggregate emissions resulting from pledges and INDCs in the years 2020 to 2030.
The CAT estimates total aggregate emissions in the 2020 to 2030 period to be slightly lower than that estimated by Climate Interactive, although aggregate emissions in 2030 are quite similar (a 2 GtCO2e difference). If it were possible to run the (higher) 2020-2030 regional Climate Interactive pathway through the CAT pathway-extension methodology, the resulting warming would be above CAT’s current estimates (2.7°C), but probably still well below Climate Interactive’s estimate (3.5°C), given the different post-2030 assumptions described above.
CAT’s global emission levels in 2020 (52 GtCO2e) are in line with the UNEP Emissions Gap reports, but much lower than the estimate of Climate Interactive (59 GtCO2e). Regionally, Climate Interactive emissions over 2020-2030 are comparable to those calculated by CAT for the US, the EU and India, but little higher for other developed countries, and much higher for China and other developing countries.
What we can learn from the above comparison is that both short-term and long-term policies, pledges and actions are needed to hold warming below 2°C. If global emissions are not limited in the short-term (until 2030), then the rapid reductions needed to return to a 2°C consistent trajectory will become politically and economically infeasible. Conversely, the trajectory until 2030 is only a building block and further, major reductions will be required post-2030 in order to ensure that the near-term efforts are successful.