The great gas debate

Introduction from the editor

Climate Spectator has run a number of pieces over the past few months, including this week, examining the extent to which substituting coal with gas in power generation might reduce temperature rise. Below is another by Alan Pears summarising work by Tom Wigley. Having myself been a strong advocate for the substitution of coal with gas in power generation, this article led me to take pause and reflect. Tom Wigley is a top notch climate scientist. He is a contributing author to the Intergovernmental Panel on Climate Change assessment reports, and was appointed a fellow of the American Association for the Advancement of Science for his major contributions to climate and carbon-cycle modeling and to climate data analysis. Wigley's article described below was published in the peer-reviewed scientific journal, Climatic Change.  

The great gas debate

The gas industry has promoted shifting to gas as the panacea to cut greenhouse gas emissions. A recent study by climate specialist Tom Wigley has challenged this. Wigley uses a climate model to explore the year-by-year warming effects of replacing half of global coal use with gas by 2050 (phased in at 1.25 per cent additional coal replacement each year to 2050). He includes a range of options for methane leakage from gas production from zero to ten per cent. This provides some interesting insights.

Wigley's work is much more useful than the Worley Parsons industry study, which uses warming factors averaged over 100 years: this understates the significance of the short-term impacts of methane leakage and simplifies the complexities of atmospheric processes.

There are actually two independent factors at work in Wigley's study. First, there is the effect of a reduction in coal use, which cuts emissions of CO2 and methane leakage from coal mines, reducing warming. But it also reduces air pollutants such as oxides of sulphur and carbon particulates, which reduces their short-term cooling effects. Wigley's paper suggests this loss of cooling will offset most of the reduction in warming from cutting coal use until mid-century, when the long-term effect of reducing CO2 begins to swamp the air pollution effect.

Reduction in coal use could occur independent of gas use, driven by strong energy efficiency improvement, rapid adoption of renewables or even economic collapse. Indeed, the only way to achieve significant reduction in net warming by 2050 from cutting coal use seems to be through replacing it with zero emission options, because of the loss of the air pollution cooling effect.

Second is the impact of increasing gas consumption, which depends on how much methane leaks during production, the amount and type(s) of energy used for processing, liquefaction and regasification (if sold as LNG), transport, and the efficiency of gas usage compared with the coal it replaces. Wigley assumes all extra gas is used at 60 per cent efficiency to produce electricity that replaces 32 per cent efficient coal-fired electricity. He ignores LNG (liquified natural gas) production and transport (which, according to the Worley Parsons industry study data adds 22 per cent to gas CO2 emissions). So Wigley's assumptions are generous to gas.

If we consider CO2 emissions only, replacing coal with gas does reduce net warming progressively from the first year, due to its lower greenhouse intensity and higher assumed efficiency of use.

The change in methane warming impact depends on the balance between the reduction in leakage from coal mines relative to the extra leakage from gas production. A net increase in methane leakage gives net warming, particularly in the first few decades after the methane is released, as methane is a very greenhouse-active gas over this period.

In Wigley's study, the reduction in cooling from sulphur oxides and particulates as coal use declines, offsets the reduction in warming from gas replacing coal (at 1.25 per cent additional substitution per year) until 2050. As I pointed out earlier, this air pollution effect will happen whatever causes a decline in coal use, not just gas.

Overall, the loss of air pollution cooling offsets the reduction in warming through gas replacing half of coal usage to 2050, even with no methane leakage from gas. At 2.5 per cent leakage, the breakeven point is around 2055. At 10 per cent leakage (a high scenario), it is 2140. Not good. So the net climate benefit of replacing coal with gas over the next century is very sensitive to the overall efficiency of production and use of gas relative to coal, and the extent of methane leakage.

If gas is to help achieve a sustainable energy future, the industry must change. It must drive efficiency improvement in gas production and usage hard, so that gas consumption at many sites actually declines. For example, combining on-site efficiency improvement with cogeneration can reduce total site gas consumption while replacing imported electricity.

The industry should use renewable energy for production and transport where possible. It must aim for zero methane (and CO2 from gas fields) leakage, and accept independent monitoring for credibility.
Gas companies should also buy offsets to balance all emissions in their supply chains, while encouraging consumers to buy offsets to balance emissions from gas usage.

They must invest in zero emission options such as biogas, renewable synthetic gas and possibly hydrogen. Lastly, coal seam and shale gas must prove they won't damage our underground water resources.

Then the gas industry might be able to claim a transitional role in the path towards a sustainable future.

Alan Pears has worked in the energy efficiency field for over 20 years as an engineer and educator. He is Adjunct Professor at RMIT University and co-director of environmental consultancy Sustainable Solutions.

This is an edited version of an article that first appeared in ReNew Magazine. Reproduced with the permission of the author.