The 'natural gas bridge' concept has by now been so widely postulated and discussed that it has become almost cliché in energy circles. It started with the shale gas surge in the US and the economic benefits it brought with it.
The sudden abundance of cheap unconventional gas and the rise of the reliable, if controversial, hydraulic fracturing method to secure its supply for decades to come prompted the idea of gas as a 'bridge fuel' to carry us smoothly from our fossil fuel-addicted present to our low-carbon, renewable future.
The hypothesis of natural gas as a bridge fuel is a simple one. In a country like the US, which is much farther down the shale gas path than the rest of the world, the ever-increasing supply of low-cost natural gas will lead to the gradual displacement of carbon-intensive coal in the country's near-future energy mix. As natural gas burns with around half the CO2 emissions when compared with coal, climate-changing emissions will be reduced in the medium-term while intermittent renewable energy technologies are refined and made commercially ready to take over in the coming decades.
The 'bridge fuel' narrative
Or so the theory goes. It's a compelling and intuitive narrative that emphasises the power of market forces over strong state intervention. One of the key texts in this narrative is a 2011 report by a Massachusetts Institute of Technology (MIT) research team led by physics professor and then-director of the MIT Energy Initiative Ernest Moniz, which predicted an increased prominence of natural gas in the US economy and in its efforts to curb emissions.
"The newly realized abundance of low cost gas provides an enormous potential benefit to the nation, providing a cost-effective bridge to a secure and low-carbon future," the paper noted. "It is critical that the additional time created by this new resource is spent wisely, in creating lower cost technology options for the longer term, and thereby ensuring that the natural gas bridge has a safe landing place in a low-carbon future."
Shale gas is on the rise in the US, projected to account for 46% of the country's gas output by 2035
That report was partly funded by the American Clean Skies Foundation, a think tank created by the natural gas industry, and its lead author Ernest Moniz is now the US Secretary of Energy. His appointment is a reflection of President Obama's 'all of the above' energy strategy, of which Moniz is a champion, making it easier to understand why the 'bridge fuel' theme was prominent in Obama's 2014 State of the Union address. "If extracted safely, [natural gas is] the bridge fuel that can power our economy with less of the carbon pollution that causes climate change," said the president in January.
Shale gas: a bridge to nowhere?
It's interesting to note that Obama's State of the Union address was carefully worded to emphasise the economic benefits of the shale gas revolution over the environmental factors, and that the rise of natural gas was not placed into a broader context of the transition to a clean energy mix. That's because the 'natural gas bridge' theory is strongly contested in a number of areas, and in some ways the whole concept seems more like a neat way to justify cashing in on the economic benefits of the shale gas boom rather than presenting a realistic bridge to a sustainable energy mix that preserves a chance to limit global warming below 2°C.
Firstly, the gas bridge model relies to a certain extent on natural gas displacing coal in the energy market while reinforcing the development and deployment of clean energy technology in the meantime. But several recent studies, including another MIT study, dispute this assumption, arguing that as well as disrupting coal, the favourable economics brought by the gas glut will also hamper low-carbon renewable and nuclear development. This has led to some academics suggesting that, in certain scenarios, abundant gas could actually increase net emissions in the future. After all, if change is to be driven by the market, who could be blamed for choosing the most economic energy option over more expensive alternatives?
"Shale gas is a great advantage to the US in the short term, for the next few decades," said Henry Jacoby, lead author of the 2012 MIT study. "But it is so attractive that it threatens other energy sources we ultimately will need."
There is also considerable opposition concerning the timescale of a bridging mechanism between gas and low-carbon generation. As mapped out by Michael Levi, senior fellow for energy and the environment at the Council on Foreign Relations, limiting CO2 in the atmosphere to around 450 parts per million is the most achievable goal to limit warming to less than 2°C, and achieving that would mean gas use would have to be on the decline by 2030, an exceptionally short 'bridge' that shale gas developers are clearly not factoring into their decades-long business models.
And then there's the question of methane, a cause of significant disagreement in the wider conversation about the natural gas bridge, because although the natural gas supply chain involves considerably fewer CO2 emissions, it does leak methane, an even more potent (though quicker to disperse) greenhouse gas.
Worryingly, there is a wide variation of estimates on how much methane is leaked by fracking and other gas operations, from the 2% estimated by the Environmental Protection Agency, a level that would keep gas's greenhouse gas footprint beneath coal's, to the National Oceanic and Atmospheric Administration study that pegged leakage at 4% in one region in Colorado, to a controversial Cornell University that claimed the figure could be 6%. The general academic consensus tends to skew towards the lower end of the percentage scale, but the uncertainty around this vital factor is a cause for concern, and emphasises the importance of starting to scale back gas generation between 2020 and 2030, which seems increasingly unlikely.
Finding a renewable role for natural gas
So the narrative of the natural gas bridge clearly isn't as simple as 'gas replaces coal before giving way to a carbon-free future'. Nevertheless, the shale gas revolution is undeniable and market analyses virtually across the board predict a significant rise in its deployment in the coming years. Therefore it's imperative that the technical and economic strengths of natural gas are leveraged to help secure a low-carbon future; even if it's not the perfect tool for the job, it still might be the best a market-driven energy system can offer.
And there are clear advantages to work from, even if natural gas is of questionable use as a direct means of reducing emissions. For example, gas and renewables are naturally complementary if deployed in tandem. The intermittency of common methods of renewable energy supply - based on the variation of wind speeds, hours of sunshine and so on - means that it will need to be supported by back-up plants to make up the shortfall if supply is low. Coal and nuclear baseload aren't flexible enough to play this role effectively, but gas turbines' ability to quickly ramp up and down based on demand makes them an excellent partner.
In fact, a 2012 Citigroup report asserted that more gas plants will be needed to fulfil this role as renewables deployment increases, to the extent that it concluded: "Gas-fired power is not only compatible with renewables, it is in many ways essential for its large-scale adoption."
Of course, given the current economic advantages of natural gas and the possibility that governments and companies will take the easy route by simply relying on gas alone, this symbiotic relationship will have to be supported by legislation like carbon pricing to even the playing field somewhat. The introduction of capacity markets, which are beginning to roll out around the world, could also prove vital in maintaining the profitability of gas back-up plants even if they are rarely needed.
More than anything, however, it's vital that a good chunk of the economic rewards brought about by the natural gas boom be channelled towards speeding up the development and deployment of clean energy tech. As Alex Trembath, senior analyst at US think tank the Breakthrough Institute, concluded in an October article: "In the United States, natural gas is lowering emissions, boosting the national economy, enabling the deployment of intermittent renewables and, ideally, providing the economic surplus to be invested in accelerating innovation in zero-carbon energy technologies." The natural gas bridge might not buy much time for the opportune deployment of our clean energy future, but the cash it brings could certainly help.