It´s basic chemistry: coal is mainly carbon, if you burn it you get lots of CO2. Natural gas is mostly methane, and that’s a different story. With methane just ’20 percent of the burned atoms’ are carbon, the rest is hydrogen, which transforms to pure, drinkable water if you force it to bind with oxygen.

Although the energetic value of hydrogen is not as high as that of carbon, this by catch still helps to make the burning of natural gas considerably cleaner and less carbon-intensive, with fewer grams of CO2 per Joule of energy – hence gas is ‘better’ for the climate than coal.

Or so we thought.

But climate is not just about CO2. It can be about other agents that influence parts of Earth’s energy balance, either by absorbing infrared or reflecting ultraviolet radiance.

Methane warms and sulfur cools

To be more precise: climate is very much about leaking methane and emitting sulfur too. Methane to air emissions are an accidental byproduct of natural gas production [as well as of oil production – sloppy business apparently, as any methane leaked to the atmosphere is missed revenue for the energy companies – these people should think of something clever].

Sulfur aerosols, like sulfur dioxide and sulfates, can be considered a byproduct of the coal industry. Coal may be mostly carbon, it is far from pure. Apart from hydrogen and sulfur coal also contains heavy metals like mercury – and coal-fired power plants release more radioactive trace elements to the environment than nuclear plants. But let’s stick to the sulfur here.

However damaging that byproduct may be for human health and the wider environment, sulfate and sulfur dioxide aerosols do help to raise the atmosphere’s albedo, thereby reflecting more sunlight and offsetting part of the warming that coal’s CO2 causes.

Coal’s climate benefit in practice

In July a publication in PNAS stated over the period between 1998 and 2008 a doubling of Chinese coal consumption has offset* part of the climate forcing of the worldwide rise in greenhouse gases over that period [but over that same time both a cooling phase in ENSO and declining insolation were of a larger magnitude]. In this respect the Chinese coal industry could be different from the Western coal industry because many plants are old-fashioned, with poor filtering and high emissions of additional pollutants, like sulfate.

[*) In order not to confuse matters: we of course owe more climatic warming to the Chinese coal expansion – as it released more CO2 than the sulfur could compensate. Besides, the old-fashioned part of the coal industry is a major emitter of atmosphere-warming soot aerosols as well – and soot is currently the most-ignored climate forcer.]

Many small methane leaks can also change the picture

Of course it’s true that gas should be much cleaner than coal. But that is if we burned it all, neatly to water vapour and to CO2. Any leaked natural gas however, which is current practice in the gas industry, is equivalent to leaked methane. Depending on how you compare the two greenhouse gases, methane is a 20-25 times more potent climate forcer than CO2. That difference can be of crucial importance to our climate. With the tundra climate feedback for instance we don’t quite know how we’ll have all those gigatonnes of extra carbon served to the atmosphere. If it would be as CO2 it would be bad news – but as methane it would be disaster.

Quantifying the coal to gas energy transition

All right, even if we accept sulfur is good and methane is bad for the climate. Does saying coal is nothing worse than natural gas hurt your intuitive understanding of climate change? Then you’re absolutely right.

Because on a single day you may be able to compare the climatic effects of sulfur, methane and CO2 – it becomes a lot more complicated if you look at longer timescales. Sulfur aerosols are only airborne for days, perhaps weeks. Methane’s atmospheric half-life is supposed around 8.4 years. However the CO2 we emit today is there for – practically speaking – ever.

That’s why a new publication in Climatic Change by the National Center for Atmospheric Research (NCAR) is so interesting. It has further quantified the long-term effects of a transition from coal to natural gas – and concludes the climate benefits are still quite marginal then.

A 50 percent energy shift doesn’t cool in decades

In the NCAR study a computer model was used to simulate the effects of a 50 percent decrease in coal consumption and a simultaneous 50 percent increase in natural gas consumption.

Under the researched scenario a range of methane leaks were assessed, from 0-10 percent, in accordance with practice. For any leakage rate temperatures initially rose in the model study as sulfur aerosol concentrations dropped, with a transition from coal to gas.

According to the model it would take to 2050 before the hypothetical zero leaks runs went into relative cooling mode. For the other methane leakage rates there would be net warming resulting from the energy transition up to 0.06 degrees Celsius for that year.

The 5 percent methane leaks runs dropped to relative cooling shortly after 2060. And with 10 percent methane leaks coming from natural gas production, the energy transition from coal would simply be bad for the climate – for the rest of the century.

Won’t cut a multi-degree warming trend

What may be equally surprising are the small benefits if we did manage to clamp down on those sloppy leaks. The zero leaks runs go to 0.14 degrees of relative cooling by 2100. If we lost 5 percent methane on average the whole energy transition would only amount to some 0.03 degrees of relative cooling over the century.

Maybe the people at NCAR need to tune their model. Or maybe we need to realise replacing the one fossil fuel with the other is not going to do the trick.

© Rolf Schuttenhelm | www.bitsofscience.org