In contrast to the “green light” for coal-to-NG substitution for power generation, the authors suggest that climate benefits from vehicle fuel substitution are uncertain (gasoline, light-duty) or improbable (diesel, heavy-duty). However, they cautioned, those conclusions may undercount the benefits of natural gas, as both EPA GHGI methods and many regionally focused top-down studies attribute CH₄ emissions from co-producing NG systems to the NG sector, rather than to a mixture of oil and NG sources.

Modeling has shown climate benefits from coal to NG switching for power generation over all time periods (i.e., starting immediately) if the well-to-power-plant leakage rate is below 3.2%, while benefits are seen over a 100 year period if leakage is below 7.6%. Therefore, available evidence suggests climate benefits from NG substitution for coal in the power sector over a 100-year assessment period. Alvarez et al. found benefits from NG use in transport at leakage rates below 1.7% to 3.8% for 100 year assessment periods (gasoline and diesel substitution, respectively). Therefore, some scenarios appear to support use of NG in passenger vehicle gasoline displacement, but benefits from diesel substitution in heavy-duty trucking are less likely.

—Brandt et al., SM

While the authors conclude that there is a poor understanding of sources of excess CH₄ and point to areas where improved science would reduce uncertainty, they also note that hydraulic fracturing for NG is unlikely to be a dominant contributor to total emissions.

The team examined two basic types of studies:

• “Bottom-up” studies that measure emissions directly from devices or facilities and then compare results to emissions factors (EFs; e.g., emissions per device). Large-scale inventories are created by multiplying EFs by activity factors (e.g., number of devices).

• Atmospheric studies that estimate emissions after atmospheric mixing occurs. These typically compare measurements to emissions inventories, such as the US Environmental Protection Agency (EPA) national GHG inventory (GHGI). Atmospheric studies use aircraft, tower, and ground sampling, as well as remote sensing. All such studies observe atmospheric concentrations and must infer fluxes by accounting for atmospheric transport.

The researchers found that across years, scales, and methods, atmospheric studies systematically find larger CH₄ emissions than predicted by inventories. EFs were also found to underestimate bottom-up measured emissions. They also found that regional and multistate studies focusing on NG-producing and NG-consuming regions find larger excess CH₄ emissions than national-scale studies.

Inventories and emissions factors consistently underestimate actual measured CH4 emissions across scales. Ratios >1 indicate measured emissions are larger than expected from EFs or inventory. The main graph compares results to the EF or inventory estimate chosen by each study author. The inset compares results to a regionally scaled common denominator, scaled to region of study and (in some cases) the sector under examination. Multiple points for each study correspond to different device classes or different cases measured in a single study. Definitions of error bar bounds vary between studies. (US, United States; Can, Canada; SC, South Central; Petrol. and Pet., petroleum; SoCAB, South Coast Air Basin; LA, Los Angeles; DJ, Denver-Julesberg; UT, Utah; HF, hydraulic fracturing). See SM for figure construction details. A R Brandt et al. Science 2014; 343:733-735. Click to enlarge.

The authors suggest a number of causes for the consistent underpredicting of emissions inventories compared to what is observed in the atmosphere:

1. Devices sampled are not likely to be representative of current technologies and practices. Production techniques are being applied at scale (e.g., hydraulic fracturing and horizontal drilling) that were not widely used during sampling in the early 1990s, which underlies EPA EFs.

2. Measurements for generating EFs are expensive, which limits sample sizes and representativeness. Many EPA EFs have wide confidence intervals. There are also reasons to suspect sampling bias in EFs, as sampling has occurred at self-selected cooperating facilities.

3. If emissions distributions have “heavy tails” (e.g., more high-emissions sources than would be expected in a normal distribution), small sample sizes are likely to underrepresent high-consequence emissions sources. Studies suggest that emissions are dominated by a small fraction of “super-emitter” sources at well sites, gas-processing plants, co-produced liquids storage tanks, transmission compressor stations, and distribution systems. For example, one study measured ~75,000 components and found that 58% of emissions came from 0.06% of possible sources.

4. Activity and device counts used in inventories are contradictory, incomplete, and of unknown representativeness.

Improved science would aid in generating cost-effective policy responses. Given the cost of direct measurements, emissions inventories will remain useful for tracking trends, highlighting sources with large potential for reductions, and making policy decisions. However, improved inventory validation is crucial to ensure that supplied information is timely and accurate. Device-level measurements can be performed at facilities of a variety of designs, vintages, and management practices to find low-cost mitigation options. These studies must be paired with additional atmospheric science to close the gap between topdown and bottom-up studies. One such large study is under way, but more work is required.

If natural gas is to be a “bridge” to a more sustainable energy future, it is a bridge that must be traversed carefully: Diligence will be required to ensure that leakage rates are low enough to achieve sustainability goals.

—Brandt et al.

The research was funded by the nonprofit organization Novim through a grant from the Cynthia and George Mitchell Foundation. Novim was formed in late 2007 by a group of scientists and engineers associated with the Kavli Institute for Theoretical Physics at UC Santa Barbara to provide an independent, non-advocacy source of data, information and knowledge on important national and global issues in a manner that would help catalyze constructive debate and potentially lead to programs addressing these issues.

Resources

• A. R. Brandt, G. A. Heath, E. A. Kort, F. O’Sullivan, G. Pétron, S. M. Jordaan, P. Tans, J. Wilcox, A. M. Gopstein, D. Arent, S. Wofsy, N. J. Brown, R. Bradley, G. D. Stucky, D. Eardley, and R. Harriss (2014) “Methane Leaks from North American Natural Gas Systems,” Science 343 (6172), 733-735 doi: 10.1126/science.1247045