The short answer: Activate “Use detailed settings” in the Methane & Other Gases advanced view, and then set the “Adoption of best practices for energy, industry, and waste (CH₄, N₂O, & F-gases)” slider to 18%.

Natural gas is mostly methane. Somewhere between 2 and 9% of natural gas leaks into the atmosphere somewhere along the flow from extraction to use. 17% of all human-caused methane emissions are due to natural gas leakage, or about 10% of non-CO₂ greenhouse gases (EPA 2019; Howarth et al. 2011). Other sources of methane include coal and oil mining, refining, and transport; industrial processes; agriculture; and waste.

We group methane and other non-CO₂ gases together because the efforts to curb their release are similar or could be based on similar agreements and regulations. For most users, this coordinated action is simple and adequate — the main slider reduces emissions of all non-CO₂ gases. Other users will want the added control gained by clicking the “Use detailed settings” switch. 

If you want an estimate of what would happen if you took the isolated action of fixing the leaks in natural gas infrastructure, use the detailed settings and set the “Adoption of best practices for energy, industry, and waste (CH₄, N₂O & F-gases)” slider to 18%.

The math behind this is: natural gas leaks represent about 10% of all non-CO₂ GHG emissions, as measured in CO₂eq (EPA 2019). In 2060, non-CO₂ emissions in the Baseline scenario in En-ROADS are 20.1 Gt CO₂eq/year, so natural gas leaks would be about 2.01 Gt CO₂eq/year. If action can bring the leak rate from 3%  to 1%, emissions would fall by 2/3, or about 1.34 Gt CO₂eq/year.  That emissions reduction corresponds to 18% on the “Adoption of best practices for energy, industry, and waste” slider. This slider controls more than methane from leaking natural gas facilities, but a 18% reduction using this slider is a reasonable estimation of leaking pipelines given that reductions of the gases is calculated using CO₂ equivalents. The inclusion of more gases than just methane does not change the policy implications and insights.

To see the results of your policy, show the graph “Non-CO₂ Greenhouse Gas Emissions.” Note that even 100% action on both the “Adoption of best practices for agriculture (CH₄ & N₂O)” and the “Adoption of best practices for energy, industry, and waste (CH₄, N₂O, & F-gases)” sliders does not reduce emissions to zero—we calibrate the sliders to maximum achievable reductions since some of these emissions are thought to be unavoidable. 18% on the “Adoption of best practices for energy, industry, and waste” slider, appropriate for a “fix the leaks” effort, does lower emissions compared to the baseline, but makes less than 0.1 degree C difference in temperature change by 2100. 

** En-ROADS formerly had a separate slider for the leak rate of natural gas. There was some overlap between using that slider and the sliders under Methane & Other Gases, which could confuse users. The Methane and Other Gases sector now cover all the leaks and emissions from methane, including from natural gas.

References:

EPA. (2019). Global Non-CO₂ Greenhouse Gas Emission Projections & Mitigation Potential: 2015-2050. 

Howarth, R.W., Santoro, R. & Ingraffea, A. (2011). Methane and the greenhouse-gas footprint of natural gas from shale formations. Climatic Change 106, 679–690.