Which fuel source used to generate electricity emits the most carbon dioxide? the most methane?

Methane is a powerful greenhouses gas with a 100-year global warming potential 28-34 times that of CO2. Measured over a 20-year period, that ratio grows to 84-86 times.

About 60% of global methane emissions are due to human activities. The main sources of anthropogenic methane emissions are the oil and gas industries, agriculture (including fermentation, manure management, and rice cultivation), landfills, wastewater treatment, and emissions from coal mines. Fossil fuel production, distribution and use are estimated to emit 110 million tonnes of methane annually.

Methane is the primary component of natural gas, with some emitted to the atmosphere during its production, processing, storage, transmission, distribution, and use. It is estimated that around 3% of total worldwide natural gas production is lost annually to venting, leakage, and flaring, resulting in substantial economic and environmental costs.

Coal is another important source of methane emissions. Coal mining related activities (extraction, crushing, distribution, etc.) release some of the methane trapped around and within the rock. Methane is emitted from active underground and surface mines as well as from abandoned mines and undeveloped coal seams.

The geological formation of oil can also create large methane deposits that get released during drilling and extraction. The production, refinement, transportation and storage of oil are all sources of methane emissions, as is incomplete combustion of fossil fuels. No combustion process is perfectly efficient, so when fossil fuels are used to generate electricity, heat, or power vehicles these all contribute as sources of methane emissions. On a global scale, methane emissions from oil and natural gas systems account for 1,680 MtCO2e. The estimates are considered to be uncertain and are thought to be low. Based on the best currently available data, around 3.6 trillion cubic feet (Tcf) (or 102 billion cubic meters (bcm)) of natural gas escaped into the atmosphere in 2012 from global oil and gas operations. This wasted gas translates into roughly U.S. $30 billion of lost revenue at average 2012 delivered prices, and representes about 3% of global natural gas production. Emissions are expected to grow under a central growth scenario by 23% between 2012 and 2030. Regarding the global reduction potential by 2030, it is estimated that emissions could be reduced by 26% using existing technology (equal to 1,219 MtCO2e). Despite methane’s short residence time, the fact that it has a much higher warming potential than CO2 and that its atmospheric volumes are continuously replenished make effective methane management a potentially important element in countries’ climate change mitigation strategies. As of today, however, there is neither a common technological approach to monitoring and recording methane emissions, nor a standard method for reporting them.

Natural gas, which primarily consists of methane, is the cleanest burning fossil fuel. When methane is produced from non-fossil sources such as food and green waste , it can literally take carbon out of the air. Methane provides a great environmental benefit, producing more heat and light energy by mass than other hydrocarbon, or fossil fuel, including coal and gasoline refined from oil, while producing significantly less carbon dioxide and other pollutants that contribute to smog and unhealthy air. This means the more natural gas is used, in place of coal, to generate electricity or instead of gasoline to fuel cars, trucks and buses, the less greenhouse gas emissions and smog related pollutants are produced.

However, methane that is released into the atmosphere before it is burned is harmful to the environment. Because it is able to trap heat in the atmosphere, methane contributes to climate change. Although methane’s lifespan in the atmosphere is relatively short compared to those of other greenhouse gases, it is more efficient at trapping heat than are those other gases. And while there are natural processes in soil and chemical reactions in the atmosphere that help remove methane from the atmosphere, it is important for all human activities that contribute methane to the atmosphere to be conducted in ways that reduce their methane emissions. This includes the development of processes to capture methane that would otherwise be released to the atmosphere and use it as a fuel. For example, methane from waste water treatment plants or dairies can be captured and used as a fuel to reduce the amount of methane entering the atmosphere as well as reduce dependency on fossil fuels.

There is very little dispute about the emissions associated with combustion of fossil fuels and the differences between them: CO2 emissions per unit of energy produced from gas are around 40% lower than coal and around 20% lower than oil. However, there is much less consensus over the indirect emissions on the path from oil or gas production to final consumer, in particular the level of methane emissions that can occur – whether by accident or by design – along the way.

Total indirect greenhouse gas (GHG) emissions from oil and gas operations today are around 5 200 million tonnes (Mt) of carbon-dioxide equivalent (CO2-eq), 15% of total energy sector GHG emissions. Methane, a much more powerful (though shorter-lived) GHG than CO2, is the largest single component of these indirect emissions.

The World Energy Outlook includes detailed analysis of the indirect emissions associated with producing, processing and transporting the oil and natural gas consumed today.

This analysis highlighted the very broad range in the indirect emissions intensity of different sources of oil and gas. The most-emitting sources of oil and gas produce more than four times the indirect emissions than the least-emitting sources. Indirect emissions from oil are between 10% and 30% of its full lifecycle emissions intensity, while for natural gas they are between 15% and 40%.

We estimate there were 82 Mt methane emissions from oil and gas operations in 2019, split in roughly equal parts between the two.

These emissions came from a wide variety of sources along the oil and gas value chains, from conventional and unconventional production, from the collection and processing of gas, as well as from its transmission and distribution to end-use consumers. Some emissions are accidental, for example because of a faulty seal or leaking valve, while others are deliberate, often carried out for safety reasons or due to the design of the facility or equipment.

The CO2 emissions from the combustion of natural gas are certainly lower than those from coal. But are they also lower when assessing full lifecycle greenhouse-gas emissions, after taking account of methane emissions released during the supply of the respective fuels?

Most of the gas and coal produced today is used for power generation and as a source of heat for industry and buildings.

Our detailed estimates, taking into account both CO2 and methane, show a wide variation across different sources of coal and gas. Nonetheless, an estimated 98% of gas consumed today has a lower lifecycle emissions intensity than coal when used for power or heat (this comparison excludes any coal use for which gas could not be a reasonable substitute, such as coking coal used in steel production).

This analysis shows that, on average, coal-to-gas switching reduces emissions by 50% when producing electricity and by 33% when providing heat.

This comparison underpins the emissions gains seen in many countries from switching from coal to natural gas, but it sets the bar too low. The environmental case for gas does not depend on beating the emissions performance of the most carbon-intensive fuel, but in ensuring that its emissions intensity is as low as practicable.

The longer-term comparison between the fuels also depends on the extent to which emissions are mitigated by large-scale deployment of carbon capture, utilisation and storage technologies.

Despite the emergence of new data from satellites and other measurement campaigns, there remains a high degree of uncertainty in estimates of methane emissions from oil and gas operations globally.

For example, a recent paper published in Nature examined historical ice cores and found that prior to 1950, natural geological sources of methane were much smaller (around 1-2 Mt/year) than has been generally assumed (between 40-60 Mt/year). As a result, the paper concluded that the aggregate level of methane emissions from fossil fuel production and consumption in recent years has been closer to 175 Mt/year rather than 120 Mt/year (as in IEA estimates).

What is clear is that the concentration of methane in the atmosphere has risen steadily since the mid-2000s. However, the cause of this increase is the subject of an active debate in the scientific community. Possible explanations include:

The natural mechanisms that break down methane in the atmosphere are becoming weaker
There has been a rise in biogenic sources of methane (e.g. from agriculture or waste)
There has been a rise in natural sources of emissions (e.g. wetlands and other flood zones)
There has been a rise in emissions from the extraction of fossil fuels

Further research is ongoing by a number of scholars, but it is still not entirely clear which of these reasons is the dominant factor. The only way to reduce this uncertainty – and to improve understanding of abatement opportunities – is to continue to improve data transparency, and expand and deepen measurement activities. We are keeping a close eye on this and will further update and refine our estimates as more information becomes available.

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