The danger of overvaluing methane’s influence on future climate change
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- Shoemaker, J.K. & Schrag, D.P. Climatic Change (2013) 120: 903. doi:10.1007/s10584-013-0861-x
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Minimizing the future impacts of climate change requires reducing the greenhouse gas (GHG) load in the atmosphere. Anthropogenic emissions include many types of GHG’s as well as particulates such as black carbon and sulfate aerosols, each of which has a different effect on the atmosphere, and a different atmospheric lifetime. Several recent studies have advocated for the importance of short timescales when comparing the climate impact of different climate pollutants, placing a high relative value on short-lived pollutants, such as methane (CH4) and black carbon (BC) versus carbon dioxide (CO2). These studies have generated confusion over how to value changes in temperature that occur over short versus long timescales. We show the temperature changes that result from exchanging CO2 for CH4 using a variety of commonly suggested metrics to illustrate the trade-offs involved in potential carbon trading mechanisms that place a high value on CH4 emissions. Reducing CH4 emissions today would lead to a climate cooling of approximately ~0.5 °C, but this value will not change greatly if we delay reducing CH4 emissions by years or decades. This is not true for CO2, for which the climate is influenced by cumulative emissions. Any delay in reducing CO2 emissions is likely to lead to higher cumulative emissions, and more warming. The exact warming resulting from this delay depends on the trajectory of future CO2 emissions but using one business-as usual-projection we estimate an increase of 3/4 °C for every 15-year delay in CO2 mitigation. Overvaluing the influence of CH4 emissions on climate could easily result in our “locking” the earth into a warmer temperature trajectory, one that is temporarily masked by the short-term cooling effects of the CH4 reductions, but then persists for many generations.
Humans emit a wide variety of climate pollutants, each with different influences on Earth’s radiative balance and, often, greatly differing atmospheric lifetimes. Of these, carbon dioxide (CO2) is responsible for the most warming to date and has become the reference against which all other GHG’s are measured. However, the long atmospheric lifetime of CO2, in which 40 % of a given CO2 injection is removed within 10 to 50 years (Joos et al. 2013) while the remainder persists, some for centuries and some for millennia (Archer and Brovkin 2008), makes its status as the reference particularly problematic (Wigley 1998; Lashof and Ahuja 1990). Comparing the climate impact of emissions of different GHGs is therefore dependent on the timescale over which the analysis is carried out, particularly when considering short-lived gases, such as CH4 (Forster et al. 2007). The best choice from a climate perspective is the obvious one: reduce all GHG emissions as much as possible, as quickly as possible, eliminating any need to try and equate them. However, the real world is complicated; difficult choices will have to be made with limited political and economic capital, and certain GHGs will be reduced at the expense of others, creating a demand for a comparative metric. Effort must be undertaken to understand the possible impact of these choices over all timescales so that we understand the true costs and benefits.
The most widely used tool for comparing GHGs is the Global Warming Potential (GWP) (Derwent 1990; Fisher et al. 1990; Lashof and Ahuja 1990; Wuebbles 1989; see also Forster et al. 2007), a measure, not of “warming”, but of the integrated radiative forcing (RF) resulting from a pulse emission of a chosen GHG relative to a pulse emissions of CO2. GWP’s have most commonly been evaluated over 100 years, although the Intergovernmental Panel on Climate Change (IPCC) also publishes 20 and 500-year GWP’s. The GWP undervalues both the short- and long-term consequences of GHG emissions. With no discount rate, GWP places equal value on all time within the integration period. All RF effects beyond the integration period are valued at zero. The problem with this metric is that it is often used in ways that directly violate the assumptions on which it is based (O’Neill 2000). Understanding how a change in RF influences global temperature (or “warming”) requires a climate model and all its associated uncertainties, and using integrated RF requires an assumption of what timescales are important.
Several comprehensive literature reviews (Peters et al. 2011; Shine 2009; Fuglestvedt et al. 2003) have been published focusing on the GWP and the many alternative metrics that have been suggested. The Global Temperature Potential (GTP) (Shine et al. 2005, 2007) evaluates the relative impact on global temperature of a pulse emission of a GHG compared to CO2 at a chosen time in the future. The primary argument against temperature-based metrics has been that they require a climate model for evaluation, the results of which are non-transparent and potentially model-dependent. However, because this is a relative metric, the GTP has been shown to be somewhat independent of the climate sensitivity of the model used for its calculation (Shine et al. 2005), although other studies have shown considerable dependence for particular species (Fuglestvedt et al. 2010). Unlike the GWP, the GTP is an endpoint metric evaluating the relative differences between temperature trajectories at a single point in the future. The Mean Global Temperature Potential (MGTP) (Gillett and Matthews 2010) and the integrated GTP (iGTP) (Peters et al. 2011) are more directly comparable to the GWP, defined as the ratio of the temperature trajectories resulting from the emission of a GHG relative to that of CO2 integrated over the chosen time horizon (TH). Some recent studies have suggested that there may be only small differences between the MGTP/iGTPs and GWPs (Peters et al. 2011; Azar and Johansson 2012). In each case, as with the GWP, all climate impacts beyond the reference timescale are neglected. Although the need for climate models may translate into larger published uncertainties on temperature-based metrics than RF-based metrics, this can be viewed as an improvement. If the goal is to assess the relative impacts of various GHG emissions on climate and temperature, not on the global integrated radiative forcing balance, then these larger uncertainties are the correct ones and it is important that they are addressed overtly.
Some other approaches include the TEmperature Proxy Index (TEMP), the Economic Damage Index (EDI), the Forcing Equivalence Index (FEI), and Manne-Richels-type approaches. TEMP is defined in reverse, characterizing the influence that past GHG emissions have had on the temperature trajectory using paleo-temperature and atmospheric composition records (Tanaka et al. 2009). The Economic Damage Index (EDI) quantifies the reduction in CO2 emissions required to offset the “economic damage” accompanying an increased emission of a given GHG (Hammitt et al. 1996). Manne-Richels-type indices are defined as a function of time in which relative importance of various GHG’s constantly change as the chosen endpoint is approached (Manne and Richels 2001). The FEI is a timescale-independent approach that creates multi-gas emissions scenarios designed to maintain the same RF trajectory as a given reference scenario at all future timepoints (Wigley 1998).
These indices for comparing emissions of various GHG’s differ both in what they are quantifying: relative change in RF, temperature, or economic impact, and in how they account for time. A time-integrated metric is capable of valuing multiple timescales, from the present to the chosen end of the integration (Tf), with all longer timescales beyond Tf valued at zero. An endpoint metric most accurately captures the climate condition at Tf but at the cost of ignoring all other timescales, shorter and longer. Of these, the value of the FEI is the least dependent on the choice of timescale. Any single scaling factor (or normalized metric) used to equate a non-CO2 GHG with CO2 must be clearly associated with a timescale and some discussion provided about the tradeoffs inherent in this choice.
Recently, it has been shown that the value of CH4 relative to CO2 increases when additional interactions with aerosols are included, an effect that is particularly marked over short timescales, increasing the 20-yr GWP value from 70 to 105 (Shindell et al. 2009; Howarth et al. 2011), and the 100-yr GWP from 25 to 33. This revaluation has been used as a critical part of an analysis suggesting that natural gas consumption is worse for the climate than burning coal (Howarth et al. 2011). Additionally, it has been suggested that mitigation of CH4 and black carbon (BC) should be emphasized (Shindell et al. 2012). These arguments focus on the climate in the next 20 to 50 years, justified in part by the need to avoid what are referred to as dangerous “tipping points” in the earth’s climate system or a “threshold” 2 °C temperature increase (Howarth et al. 2012; Shindell et al. 2012).
Description of the model scenarios used
650 ppm stabilization
The 650WRE scenario
Using the 650WRE scenario as the reference, we increased the CH4 emissions by 5000 Tg CH4/yr for 10 years from 2015 to 2020.
CO2 pulse (GWP = 105)
Using the 650WRE scenario as the reference, the CO2 emissions were increased by 143 Pg CO2-C/yr for 10 years. This increase is equivalent to 105 times the CH4 mass added to the atmosphere in the CH4 pulse scenario.
CO2 pulse (GWP = 70)
Same as above with the increase in CO2-C equal to 70 times that of CH4 mass added to the atmosphere in the CH4 pulse scenario. (95 Pg CO2-C/yr)
CO2 pulse (GWP = 25)
Same as above with the increase in CO2-C equal to 25 times that of CH4 mass added to the atmosphere in the CH4 pulse scenario. (34 Pg CO2-C/yr)
CO2 pulse (GTP = 12)
Same as above with the increase in CO2-C equal to 12 times that of CH4 mass added to the atmosphere in the CH4 pulse scenario. (16 Pg CO2-C/yr)
Reference — 750 ppm stabilization
The 750WRE scenario with any decrease in CH4 emissions removed. Instead, CH4 emissions were allowed to stabilize at 600 Tg CH4/yr for the duration of the scenario.
The 750WRE scenario with CH4 emissions immediately reduced to 450 Tg CH4/yr and then allowed to continue decreasing when the CH4 emissions in the original 750WRE scenario fell below 450 Tg CH4/yr.
The 450Over (450 ppm stabilization w/overshoot) scenario with all CH4 emissions decreases removed. As in the “reference”, CH4 emissions were stabilized at 600 Tg CH4/yr.
The 450Over scenario with the same CH4 emissions trajectory used in the “CH4 measures” scenario.
Reference — 550 ppm stabilization
The 550WRE scenario was used as the reference scenario — a 550 ppm stabilization scenario with no overshoot.
15 years delay
Based from the 550WRE scenario, we substituted CO2 emissions from the A1 business as usual scenario from 2015 to 2030 after which we decreased CO2 emissions using the same percentage decreases used in the reference 550WRE scenario.
30 years delay
Same as above, but with the CO2 emissions from the AI BAU scenario from 2015 to 2045 after which CO2 emissions declined as above.
50 years delay
Same as above, but with the CO2 emissions from the A1 BAU scenario from 2015 to 2065, after which CO2 emissions declined as above.
We directly substitute CO2 emissions for CH4 emissions using a variety of commonly used metrics, and plot the climate response in Fig. 1. The values for defining “CO2-equivalence”, were derived from the IPCC (Forster et al. 2007) and several recent publications, (Shindell et al. 2009; Shine et al. 2005) based on both proposed Global Warming Potential (GWP) and Global Temperature Potential (GTP) values. This represents an extreme version of the carbon-trading case where CH4 emissions are exchanged for extra CO2 emissions at a variety of values, such that for every one Pg of CH4 emissions avoided, there is an additional X Pg of CO2 emitted, where X is a GWP- or GTP-based multiplier. These scenarios are meant to be illustrative only, as a future jump in anthropogenic CH4 emissions of this magnitude is neither proposed nor likely. Figure 1 shows that, using a multiplier of 70 accurately represents the climate response over the duration of the emission perturbation but, once this terminates, the trajectories quickly diverge. The temperature trajectory associated with replacing the CH4 pulse with a CO2 pulse of magnitude either 25 or 12 times the avoided CH4 pulse both result in cooler temperatures in the first 50 to 100 years respectively, but a warmer world thereafter. Plots of the emissions trajectories are available in the online supplemental material (Figs S1 and S2).
CO2 emissions and atmospheric concentrations resulting from delayed emissions reductions
Additional CO2 emitted (Pg C)
Δ [CO2] (ppm) at 2100
Δ [CO2] (ppm) at 2400
The climatic influence of CO2 is dominated by the “long tail” — the 25 to 40 % of cumulative CO2 emissions that remain in the atmosphere for thousands of years and the 10–20 % that will persist for tens of thousands of years (Archer and Brovkin 2008). Because almost 40 % of the emitted CO2 is removed in the first decades (Joos et al. 2013), the immediate impact of CO2 emissions is dampened. The atmospheric concentration of CH4 resulting from an emitted pulse follows an approximately standard exponential decay curve (Prather et al. 1994). Because of the different shapes and disparate lifetimes “there is no single scaling factor that can convert between CH4 and CO2 emissions” (Wigley 1998) and the same applies to all short-lived GHG’s — the value of the scaling factor is time-dependent. Unfortunately, the correct timescale over which to evaluate the relative impacts of different GHG emissions scenarios is not at all clear, and is likely to depend on the scientific or policy question being asked. What is certain is that in much of the recent debate, short timescales (≤50 years) have become increasingly emphasized, while the long timescale (>100 year) influences are often ignored (for example the 100-year GWP, by far the most commonly used metric, places a value of zero on all timescales longer than 100 years).
In Fig. 1, we show a direct comparison between increased CH4-emissions, and a case in which the CH4 pulse is “traded” for increased CO2 emissions using various equivalence factors. The purpose of Fig. 1 is to elucidate the trade-offs with time of allowing carbon exchanges between short- and long-lived GHGs such as CH4 and CO2. Each potential trading metric has embedded within it a value judgment over the relative importance of temperature changes over different timescales. We show how using a 20-yr GWP of 70 results in a temperature response that overlaps the response to the CH4 pulse scenario only for the duration of the prescribed pulse — after which the two temperature curves diverge with the CO2 emission resulting in much higher temperatures. Trading CH4 for CO2 using the 100-year GWP (25) leads to less warming for the first 50 years, but higher temperatures for the centuries and millennia that follow. Minimizing both short and long-term warming is critical, GHG mitigation policies that allow us to trade near-term (impermanent) warming for delayed but permanent warming is dangerous in a political arena where short-timescales dominate decision making; for this reason every attempt should be made to restrict carbon trading of CH4 (or any short-lived climate pollutant) with CO2 (Daniel et al. 2012; Humbert 2010).
A recent report by Shindell and colleagues (2012) highlighted the climate benefits of immediate reduction of short-lived climate pollutants. They showed that reductions in black carbon (BC) and CH4 would more effectively reduce climate warming in the near future than CO2 reductions, with concomitant human-health benefits that make such actions easier to implement, and even economically profitable for society. While many aspects of their argument are correct, some of which have been argued before (Hansen et al. 2000), there are also real climate concerns related to over-emphasizing reductions of short-lived gases. As Shindell and colleagues acknowledge, long-term (in this case decades to centuries) climate stabilization is only possible through CO2 reductions. Our concern is that by showing the reader only a timeline to 2070 (near the point at which the highlighted “CH4+BC tech” and “CO2 measures” trajectories cross), they avoided discussing the huge timescale tradeoffs inherent in the proposed emission reduction scenarios. In Fig. 2, we plot a similar array of scenarios as those published in Shindell et al.’s Fig. 1, but expand the timescale out to 2200. In doing so, we highlight the potential future impacts of today’s decisions should (a) we choose to devalue all longer timescales in comparing GHG reduction strategies or (b) we allow actions on short-lived gases such as CH4 to delay meaningful CO2 reductions.
Over all timescales the best scenario for the climate considered here is the “CH4+CO2 measures” scenario in which the CO2 emissions adhere to a 450 ppm stabilization scenario, and the CH4 emissions are greatly reduced (see SI for plots of emissions trajectories). Timeline only becomes important when discriminating between scenarios that reduce only short-lived gases (only CH4 in our study) or only CO2. If we restrict the picture to the short-term, it could be interpreted that the “CH4 measures” is preferable to, or at least approximately equal to, the “CO2 measures” scenario. However, for all times beyond approximately 50 years, the “CH4 measures” scenario falls only just below the “reference” scenario, in this case a 750 ppm stabilization scenario. We note that the reference scenario is roughly equivalent to the same emissions reductions required for the 450 ppm scenario, but with the start of emissions reductions delayed by ~50 years (see supplement Fig S7). This is extremely important because it means that at the point the trajectories cross, if we have chosen the “CH4 measures” instead of the “CO2 measures”, it is not possible to reverse course — we are now locked into the higher temperature trajectory.
Another important point is that the methodology employed by the Shindell analysis, and replicated here, exaggerates the benefits of reducing CH4 emissions in two ways: first, by underestimating the benefits of CO2 reductions; and second, by the timings of the reductions in the scenarios. First, consistent with the Shindell analysis, we removed any CH4 reductions in the 450 stabilization scenarios to approximate their “CO2 measures” scenario. However, CO2 and CH4 emissions are linked through fossil fuel extraction and transport; more than 60 % of the proposed reductions in the “CH4 measures” scenario are derived from the fossil fuel sector, and yet this is treated as independent of CO2 reductions. Accounting for these linkages would result in the “CO2 measures” temperature trajectory moving ~50 % closer to the “CO2+CH4 measures” trajectory (as they correctly point out in the SI) such that this methodological decision results in their undervaluing the influence of aggressive CO2 reductions on climate. In an analysis that was merely comparing the theoretical influence of reduced emissions of CH4 versus CO2 (such as our Fig. 1), enforcing independence might be fair, but in presenting potential real-world mitigation scenarios, exclusion of co-reductions from drastic alteration to the fossil fuel sector ignores an important aspect of such actions that has significant impact on the resulting temperature trajectories. Second, the SLCP reduction scenario implements the 40 % reduction in CH4 emissions and the 80 % reduction in BC emissions linearly between 2010 and 2030 while the “CO2 measures” (IEA’s 450 CO2-equivalent) scenario does not begin reducing CO2 emissions until 2020, reaching a 40 % reduction by ~2040. The delay in the CO2 reductions relative to the SLCP reductions further underestimates the climate benefits of immediate CO2 mitigation. Arguments about the feasibility of implementing these dramatic emissions reductions could be made for both the SLCP and CO2 measures.
Reducing emissions of CH4 and other short-lived climate pollutants such as BC, has real climate benefits, as well as co-benefits for human health and ozone (Ramanathan and Xu 2010). At the same time, there are legitimate concerns that taking strong actions to reduce these emissions could delay efforts to mitigate CO2, particularly if we overvalue the climate influence of CH4 and BC (or undervalue the influence of CO2 reductions as above). While the climate impacts of CH4 emissions are essentially reversible (over a decade or two), the climate impacts of CO2 emissions are not; CO2 persists and accumulates. If CO2 reductions are delayed and/or dis-incentivized by putting too high a focus (or price) on CH4, this will exacerbate the climate crisis. Current international policy that allows methane to be traded for CO2 at the100-yr-GWP-based price (25) could result in significantly more long-term warming (Fig. 1) depending on the volume of trading.
In Table 2 we show the “extra” CO2 emissions that could occur if focus on CH4 reductions results in remaining on a business-as-usual scenario for CO2 emissions for 15, 30, or 50 years, using a 550 ppm stabilization scenario as the comparison low CO2-emissions scenario and an A1 business as usual trajectory as the high-CO2 scenario. Here we find that, if reducing CH4 emissions were to result in a delay of just 15 years in addressing the growth of global CO2 emissions, this leads to an additional 100 ppm atmospheric CO2 that persists for thousands of years. If CO2 emissions continue on the business-as-usual scenario for 30 years, until 2045, this leads to atm [CO2] 230 ppm higher than the target 550 ppm of the base scenario.
The persistence of the climate system response to CO2 emissions, compared with the near-immediate benefits from reductions of short-lived GHG’s make prioritizing CH4 and BC reductions particularly attractive from a political perspective where election timescales are short. Combining the results from Fig. 2 and Table 2 we observe that, if we take actions to reduce short-lived GHG’s while remaining on either a 750 ppm trajectory, or a high-growth business-as-usual CO2 emission trajectory, by the time the warming begins to accelerate (15–30 years), we have already committed the earth to a much greater degree of warming regardless of the actions we take from that point forward. If we wait to reduce CO2 emissions until the warming from the CO2 begins to exceed the cooling from the CH4 reductions (2070 or approximately 50 years), we would already be “locked in” to more than 1.5 °C extra warming with no way to take those CO2 emissions back except through the very expensive and inefficient technological fix of capturing of CO2 out of the air (Socolow et al. 2011). However, if we ignore short-lived gases and focus only on reducing CO2 emissions, we can decide in 10 or 100 years that further reductions in CH4 (and BC) emissions are necessary and the coolest climate trajectory would still be nearly attainable. Because CH4 emissions have little cumulative impact on climate, reducing CH4 emissions now or in the future has essentially the same effect.
Beyond the timescales associated with GHG emissions and the climate response to them, there are also timescales associated with energy infrastructure. Implementing significant reductions in CO2 emissions requires huge changes to the fundamental structure of the global energy system. Even assuming great political will across all the world’s major economies, and future technological advances in CO2-free energy sources, these changes are likely to take decades to centuries to complete. This infrastructure timescale would further exacerbate the climate impact of turning attention away from reducing CO2 emissions, and must be considered when we search for the best path to a carbon-free energy future (Schrag 2012; Davis et al. 2010).
Another argument used to support immediate emissions reduction of short-lived GHG’s such as CH4 and BC is the necessity of avoiding a global temperature increase of >2 °C, which could trigger certain “tipping points” in the climate system (Shindell et al. 2012; Ramanathan and Xu 2010). This argument has 2 flaws: First, it misses the crucial point that it is only possible to use short-lived GHG emissions reductions to avoid a future “peak” in climate warming if we are already on the down-slope of the CO2 emissions trajectory (acknowledged in Ramanathan and Xu 2010). Otherwise, the only effect is to delay reaching this “tipping point” by a few years or decades, depending on the CO2 emissions trajectory. Second, there simply is not enough known about the exact nature of climate feedbacks to make a compelling scientific argument either that a line must be drawn at 2 °C or that any particular temperature threshold is going to tip us over the edge of a given binary feedback (Kriegler et al. 2009). The warmer the world, the greater the probability of catastrophic consequences and the only way to take the heater off is to reduce CO2 emissions.
As Allen et al. (2009) showed concisely in their “trillionth ton” analysis, climate responds to cumulative CO2 emissions over approximately 100 years. Short-lived gases also play an important role in contributing to climate change, and reducing these emissions could have a substantial cooling effect over the short-term. But if we overvalue the influence of CH4 on climate, this is likely to delay the imperative for CO2 reduction and lead to higher cumulative emissions and more long-term warming. Of course, methane emissions (and BC emissions) should be reduced, do not weaken efforts to reduce CO2 emissions. Otherwise, too much focus on reducing CH4 emissions will only delay for a short time the temperature peak we had hoped to avoid, while the extra CO2 emitted ensures that we remain above that temperature for a very, very long time.
We recognize the substantial contributions of three anonymous reviewers to the quality of this manuscript. Conversations with V. Ramanathan and M. Molina contributed to our understanding of these topics.