Comparison Between Climate Action Tracker and the MIT “Energy and Climate Outlook” of the effect of INDCs on warming
The Climate Action Tracker estimates a median global warming of 2.7°C above pre-industrial by 2100, assuming full implementation of targets and policies proposed in INDCs from 1 October 2015, and of 3.6°C taking only the policies that governments have actually implemented at a national level.
The MIT Energy and Climate Outlook 2015 estimates a range of 3.1 to 5.2°C, with a central estimate of 3.7°C by 2100, significantly higher than the CAT estimate.
The higher MIT warming estimate is principally due to its assumption that government effort is effectively frozen post-2030, whereas the CAT takes into account governments’ 2050 targets and, for all others and for the post-2030 period, assumes a continuation of the 2020-2030 “level of effort” implied by the INDCs. Consequently, post-2030 emissions in the MIT approach are much higher than the CAT’s post-2030 INDC pathway, and are quite close to CAT’s “current policy projections” (currently implemented policies in countries) which leads to 3.6°C warming by 2100.
The CAT approach is aimed at ensuring that the post-2030 emissions are as consistent as possible with the shorter-term action, pledges and/or INDCs by accounting for the inertia of near-term actions. On the other hand, the MIT Outlook scenario effectively assumes that the overall level of effort worldwide will dissipate over time post-2030.
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.
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).
The MIT “Energy and Climate Outlook 2015” calculates the effects of currently implemented and/or planned policies presented in the major emitters’ INDCs through 2030, and assumes that these policies remain in place in the post-2030 period. Effectively, this approach creates a new post-2030 “business as usual” pathway, in which the policy status quo over the 2020-2030 period is maintained but no further policies are added.
During the 2020-2030 period, the policies considered are still new and being implemented, and are assumed to incentivise a push towards low-carbon transformation, technology developments and investments. In the MIT scenario, such effects would taper out post-2030. Hence, the MIT Outlook pathway represents a much smaller effort towards low-carbon transformation after 2030 than during 2020-2030. This is different from the CAT method that assumes a post-2030 continuation of a 2020-2030 “level of effort” that would keep pushing the emissions pathway further down from “business as usual” developments, compared to the new MIT post-2030 “business as usual” pathway based on a 2020-2030 policy status quo.
Consequently, post-2030 emissions in the MIT approach are higher than the CAT’s post-2030 INDC pathway.
However, the MIT approach delivers a warming quite close to the CAT “current policy projections” (which is CAT’s version of a “business-as-usual”, as it considers policies governments currently have in place) of 3.6°C warming by 2100.
The central scenario includes the assessment of a post-2020 international agreement on emissions mitigation that is likely achievable at COP21 in Paris. With the picture for likely post-2020 policies becoming clearer (as opposed to the 2014 Outlook that represented the Copenhagen–Cancun emissions commitments as being achieved via a broad cap on emissions in each region that had made a commitment), the MIT scenario explicitly models a mix of measures to achieve emissions reductions—such as vehicle emissions standards, renewable requirements, coal power generation restrictions, and carbon pricing—to reflect as closely as possible the policies that different countries appear likely to pursue.
Note that these two scenarios are, however, not perfect analogues, due to the fact that the Outlook scenario considers emissions reductions targets in the INDCs being realised in 2030 and achieves considerably lower levels of global emissions in 2030 (roughly 54 GtCO2e), while the CAT’s “current policies scenario” achieves levels of 58-61 GtCO2e in 2030. The CAT pathway also assumes that a similar level of effort to mitigate emissions will continue to the end of the century, which means that some additional policies are put in place post-2030 in this scenario.
The CAT 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. This is not the case for the MIT Outlook scenario, which effectively assumes that the overall level of effort worldwide will dissipate overtime. The MIT report notes itself: “given that substantial progress has been made in bending the curve of emissions growth, and even reversing it in the Developed region (if the proposed policies are implemented effectively), we may now have more hope that subsequent rounds of international negotiation after COP21 will lead to more policies with further reductions. Thus the extension of the forecast beyond 2025 or 2030 is not a prediction of what is most likely. Rather it is intended to indicate the need for continued effort, and a measure of the magnitude of effort needed.”