Global roll-out of comprehensive policy measures may aid in bridging emissions gap

Closing the emissions gap between Nationally Determined Contributions (NDCs) and the global emissions levels needed to achieve the Paris Agreement’s climate goals will require a comprehensive package of policy measures. National and sectoral policies can help fill the gap, but success stories in one country cannot be automatically replicated in other countries. They need to be adapted to the local context. Here, we develop a new Bridge scenario based on nationally relevant, short-term measures informed by interactions with country experts. These good practice policies are rolled out globally between now and 2030 and combined with carbon pricing thereafter. We implement this scenario with an ensemble of global integrated assessment models. We show that the Bridge scenario closes two-thirds of the emissions gap between NDC and 2 °C scenarios by 2030 and enables a pathway in line with the 2 °C goal when combined with the necessary long-term changes, i.e. more comprehensive pricing measures after 2030. The Bridge scenario leads to a scale-up of renewable energy (reaching 52%–88% of global electricity supply by 2050), electrification of end-uses, efficiency improvements in energy demand sectors, and enhanced afforestation and reforestation. Our analysis suggests that early action via good-practice policies is less costly than a delay in global climate cooperation.


Supplementary Tables: implementation of good practice policies per model
The set of good practice policies was defined in dialogue with national model teams. In some cases, this process led to higher ambition than initially defined: for example, the target for the share of electric and hydrogen vehicles in new sales was brought forward by five years for China, and China was placed in the group of countries with highest assumed afforestation rates. In other cases, it led to a delay: for example, the target level for final energy intensity of buildings in the EU was adjusted, and for the USA, the carbon price was assumed to be introduced in 2025 rather than 2020.

Introduction
In response to the global stocktake under the UNFCCC, scenarios will be developed that represent a ratcheted up mitigation ambition level by Parties to the Paris agreement (hereafter: countries). The scenario suite described here consists of the following scenarios: • baseline, • current policy, • nationally determined contribution (NDC), • good practice policies, • bridging, and • 2 ˚C mitigation scenarios.
The good practice policies and bridging scenarios, which aim to bridge the gap between the ambition levels set out by countries and the required ambition levels to meet the mitigation goals agreed to in the Paris Agreement, are new, and most important. All other scenarios are added for comparison -and could be taken from earlier modelling exercises (although we require consistent model versions).

Workflow and submission deadlines
Protocol development and scenario submission will take place in two rounds.
In Round 1, the national modelling teams got an opportunity to respond to the proposed policies as mentioned in the draft bridging scenario protocol (attached spreadsheet). PBL/PIK/COPPE gathered all comments and used them to construct the final bridging scenario protocol.
In Round 2, the final bridging scenario protocol will be distributed to both the national and global modelling teams. This will ensure that a common protocol is followed.
Please use this reporting template.

Brief description of the scenarios
In line with the global stocktake, the ratcheting up mechanism has been applied in constructing the scenario protocol. This means that the scenarios build upon one another in terms of ambition and modelling assumptions. The Baseline scenario is the least ambitious and the 2 ˚C scenario is the most ambitious.
Supplementary The Good practice policies (GPP) scenario builds upon the CurPol scenario and assumes that certain good practice policies as defined in the spreadsheet, which have shown to be effective in some countries, will be implemented globally until 2050. For the list of policies to be implemented, see the spreadsheet Bridging Scenario GPP list 20 April 2020.xlsx. That spreadsheet also contains tabs categorising countries in low / high income or other tiers. A distinction is made between low/medium income (columns K and L of the first tab) and high income countries (columns I and J) in terms of timing and stringency (applied to all model regions). See also the fifth tab, 'Country categorisation', for a classification of all countries with their ISO codes: if the majority of countries in a region is classified as high income, the region can be considered high income (and vice versa for low/medium income). For some measures, we distinguish between three country tiers (columns M and N in the first tab, and see the third tab '7. CarbonPrice' and '16. Afforestation' for the country tiers applying to these measures).
-If CurPol is more stringent than GPP in certain sectors, take that value.
The Bridging (Bridge) scenario builds upon the GPP scenario. For the list of policies to be implemented until 2030, see the spreadsheet Bridging Scenario GPP list 20 April 2020.xlsx. After 2030, the bridge scenario transitions to the 2 °C scenario (see chapter 5).
The 2˚C (2Deg2020 and 2Deg2030) scenarios assume that a radiative forcing target of 2.6 Wm -2 is reached by 2100 in a cost-effective way. National modelling teams can work with a carbon budget derived from the global carbon budget of 1000 Gt CO 2 in the period 2011-2100 (including 2011 emissions), as done in CD-LINKS (the '2020_low' budget for 2Deg 2020 and the '2030_low' budget for 2Deg 2030). Updated national carbon budget numbers for 2015-2050 are attached (NationalCbudgetsCOMMIT.xlsx), including for the teams that did not participate in CD-LINKS, and distinguishing total CO 2 and only CO 2 from energy and industry for those models that do not represent land use. Global model teams can use the NPi2020_1000 (2Deg2020) and INDC2030_1000 (2Deg2030) scenarios.
General specifications for all scenarios Naming When uploading results to the IIASA database, scenario names as mentioned in the column Scenario id of Supplementary Table 3 should be used with the extension of the version number, with _V4 for this round. That means: please submit the following scenarios, regardless of whether you have submitted in previous rounds: Time horizon

Post policy period
For the Current policy, NDC/MCS and Good Practice Policy scenarios, the ambition levels reached in the target year should remain at least constant throughout the rest of the century. This should be implemented by extrapolating the "equivalent" carbon price in 2030, using the GDP growth rate of regions. The equivalent carbon price represents the value of carbon that would yield in a region the same emissions reduction as the NDC policies. For most modelling teams this requires to run a set of (cost-optimal) sensitivity scenarios in order to derive the carbon price that would result in the same reductions as in the NDC cases. Importantly, if a region has a carbon price of zero while implementing the (I)NDC in 2030, please assume a minimum carbon price of 1 $/tCO 2 in 2030 (= 8 $/tCO 2 in 2100 with 3%/y GDP growth). If a region has a negative carbon price in 2030, offset the trajectory resulting from 1 $/tCO 2 to your own 2030 starting point. For land use, a carbon price ceiling of $200/tCO 2 should be applied.

Optional, additional scenario for global models: NDC_2050convergence
In order to explore the implications of a scenario narrative "if the 2050 MCS in all countries become similarly stringent as the NDC targets of OECD countries for 2030", this scenario foresees a global convergence to a globally harmonized carbon price in 2050, at the level of the average carbon prices in OECD countries (which are increasing from 2030 onwards with regional GDP growth rates). Exact calculation of carbon prices: Initially, regional carbon prices after 2030 are taken from the NDC scenario where they are determined by applying the regional GDP growth rate to the effective carbon price in the region in 2030. The "global" carbon price for all years > 2030 is then calculated as the GDP-weighted average carbon price of OECD regions.

Regions
Apart from model specific regions, a mapping to the 5 RCP regions should be made by global models.
Policy coverage in the model If you are unable to implement the policy or target in the model, please adopt the provided proxy values instead. If that is not possible either, please indicate in the protocol spreadsheet which policies you were not able to capture. Proxy values are provided by IMAGE in the tab 'Indicators'.
For global sectors such as international aviation and shipping, measure 18 is included for aviation (no distinction between countries). For shipping, take the baseline trends (as no current policies are included for that sector).

Detailed specifications of scenarios Updated CD-LINKS scenarios
To a large degree, the scenarios developed in the CD-LINKS project can be used. A mapping between the scenarios can be found in Supplementary Table 4. The third column indicates the novelty of the scenario. The fourth indicates the corresponding CD-LINKS protocol that can be used as reference. The table shows that, with the exception of the good practice policies / bridging and NDC-plus scenario, the protocols from CD-LINKS can be used for all scenarios. Supplementary Figure 13 gives an example of what the increasing ambition emission profiles might look like.