My previous post examined the short-term impact of the North American shale gas boom on CO2 emissions, both at home and in Europe. Today’s entry will take a broader view by looking at the long-term effects that rising natural gas consumption will have on the climate. In particular, I want to examine whether the much-touted notion of a natural gas climate “bridge” is supported by the available evidence.
The makings of a bridge
To attempt a discussion of natural gas in the context of climate policy is to invite various opinions on its viability as a putative “bridge fuel”. Proponents highlight the pressing need to phase out coal from the global energy system, and the role that gas can play in precipitating this dramatic shift. Gas burns substantially cleaner than coal – emitting around half the CO2 per equivalent energy unit – and could thus provide a crucial mid-level step in the transition to a zero emission economy. However, critics remain unconvinced. Their response is to point out that a move to gas will only serve to lock in fossil fuel technology for decades to come. Natural gas, they claim, is a bridge to nowhere.
In some sense, the problem with these contrasting views is that they are emblematic of two sides talking passed each other. For instance, does it make any sense to talk of a bridge scenario if one automatically assumes that the expanded gas consumption will not be phased out? Similarly, what assumptions on greenhouse gas (GHG) emissions are truly reflective of the climatic impacts of gas?
Of methane leaks and climate models
An issue that has proved particularly contentious in all of this is the role that fugitive methane leakages might play in undermining gas’s climate credentials. Methane (CH4) is a short-lived, but potent GHG. Given that natural gas is predominantly methane in its natural form, any fugitive leakages that occur during the extraction and production process will clearly stand to impact the climate.
It is within this context that a study by Howarth et al. (2011) came to cause quite a stir, by suggesting that observed methane leakages – particularly from shale gas formations – were much higher than those typically assumed in the literature (3.6%-7.9%, as opposed to 1%-2%). This led the authors to conclude that gas confers little, if any, climate advantages over coal. The methodology and implications of the Howarth paper have subsequently been met with a fierce resistance. Some of the criticism may have been misplaced, but at the minimum it appears that the authors made unacceptably strong conclusions on the back of poor data.
Such misgivings are given scientific credence in the form of a more recent study by Michael Levi (2013). This paper fills an important gap in the literature by specifically constructing scenarios where gas does act as a bridge fuel. In other words, Levi considers the climate implications that follow in a world where natural gas first displaces coal, but is then itself phased out.[*] Levi’s results are illuminating, as they suggest both good and bad news for proponents of the bridge theory. Gas on its own has limited potential to contribute to the most ambitious climate targets (e.g. stabilization at 450 ppm). In the absence of complimentary technology like carbon capture and storage, the implied bridge is simply too short to make much of a difference. However, gas can play an important role by itself in the context of “more modest but still stringent objectives (550 ppm)”, since these are compatible with longer natural gas bridges – see Fig. 1. Levi goes on to show that these results are robust to various rates of assumed methane leakages, including those at the high end of the scale (e.g. as suggested by Howarth et al.).
Figure 1 – Simulated temperature outcomes relative to a delayed-transition scenario for stabilization near 450 and 550 ppm CO2, respectively. The solid line represents a “traditional” transition scenario where both gas and coal are displaced by zero-carbon energy sources; while the dotted line represents an imposed “bridge” scenario where gas first increases to displace coal, but is then itself phased out. (Note: Adapted from Levi (2013), Fig. 3.)
CCS and other breakthroughs
As Levi readily acknowledges, a key limitation of his study is that it doesn’t consider the economic or political viability of the imposed bridge scenarios. It is merely an analysis of predefined physical outcomes. There is clearly still much scope for future research to explore these socioeconomic aspects in more detail.
A case in point is the potentially decisive role played by complimentary technology like carbon capture and storage (CCS). A few demonstration plants notwithstanding, CCS remains technically contentious and unproven on a commercial scale. However, theoretical estimates suggest that it will add at least an extra 25% to a plant’s operating costs. This offers some insight into the economic difficulties that need to be overcome if CCS is to play a role in lengthening the span of any gas-fueled bridge. A new study by Bassi et al. (2013) underscores the point: “[E]xtensive deployment of gas-fired power stations would not be consistent with the UK’s carbon targets, unless it is accompanied by the widespread introduction of CCS technology.”
However, even this is not to say that natural gas lacks credibility as the most viable, climate-friendly alternative to coal. Unpalatable as it will be for some people, the unavoidable conclusion from my perspective is that achieving very stringent emissions targets will always depend on a hefty slice of fortune… It is certainly no accident that studies which demonstrate hypothetical pathways towards achieving such targets must inevitably make fairly heroic assumptions – whether that be in the form of changes to economic behaviour and institutional reform, or in the presumption of substantial technology breakthroughs.[**] In that light, it is not entirely obvious to me why CCS-enabled gas plants should be regarded as more unlikely than, say, thorium nuclear. And it certainly isn’t obvious to me that climate activists are best serving their cause by demonizing the one fuel source that has provably shaken coal’s grip on the global energy system.
To summarize: There are no panaceas for resolving the climate impasse, but gas does at least appear to offer a plausible hedge. To be sure, there is still a lot more research that needs to be done (and is currently being undertaken) before we can draw definitive conclusions. In the meantime, however, the least we can do is to make sure that we are debating the same issues. Agreeing on what constitutes a “bridge” is a good place to start.
Image: Climate Pollution via Shutterstock
[*] Previous studies (e.g. Wigley, 2011; Alvarez et al., 2012; Myhrvold and Caldeira, 2012) have typically assumed that gas consumption is greatly expanded and remains high into the indefinite future.
[**]That’s certainly not too say that such technological breakthroughs can’t happen. While I am somewhat skeptical of the tendency of people to extrapolate from current energy trends, I remain a big supporter of energy R&D funding.