Supplemental Information Contributory Role of Atmospheric Methane and the Natural Gas Industry on Global Warming Radiative Forcing RADIATIVE FORCING AND ATMOSPHERIC METHANE The National Oceanic & Atmospheric Administration (NOAA) has compiled an Annual Greenhouse Gas Index (AGGI), starting with data from 1979. The NOAA AGGI is calculated by combining the total radiative forcing for 20 different greenhouse gases, the most significant being carbon dioxide (CO 2), methane (CH 4), and nitrous oxide (N 2O), followed by various other compounds. The NOAA AGGI database includes year-byyear changes in radiative forcing numbers for these twenty greenhouse gases, in units of watts per meter squared (W/m 2 ). These data can be found at: https://www.esrl.noaa.gov/gmd/aggi/aggi.html The following table provides a synopsis of this NOAA data. It is from this table that the value of 16.7% radiative forcing for methane is derived from the 2016 data. Table 1: NOAA AGGI Data (1979-2016) NOAA Annual Greenhouse Gas Index (AGGI, 1979-2016) CO2-eq (ppm) Total AGGI % AGGI change 1990 = 1 * CO2 % Total Methane % of Total CO2 CH4 N2O CFC12 CFC11 15-minor Total Annual Growth 1.8% 0.6% 1.7% 1.6% 1.0% 3.7% 1.6% Change (W/m 2 ) 0.958 0.101 0.089 0.072 0.018 0.090 1.328 0.721 0.076 Year Radiative Forcing (W/m2) 1979 1.027 0.406 0.104 0.092 0.039 0.031 1.699 382 0.785 60.4% 23.9% 1980 1.058 0.413 0.104 0.097 0.042 0.034 1.747 385 0.807 2.2 60.6% 23.6% 1981 1.077 0.42 0.107 0.102 0.044 0.036 1.786 388 0.825 1.8 60.3% 23.5% 1982 1.089 0.426 0.111 0.107 0.046 0.038 1.818 390 0.84 1.5 59.9% 23.4% 1983 1.115 0.429 0.113 0.113 0.048 0.041 1.859 394 0.859 1.9 60.0% 23.1% 1984 1.14 0.432 0.116 0.118 0.05 0.044 1.899 396 0.877 1.9 60.0% 22.7% 1985 1.162 0.437 0.118 0.123 0.053 0.047 1.94 400 0.896 1.9 59.9% 22.5% 1986 1.184 0.442 0.122 0.129 0.056 0.049 1.982 403 0.916 1.9 59.7% 22.3% 1987 1.211 0.447 0.12 0.136 0.059 0.053 2.025 406 0.935 2 59.8% 22.1% 1988 1.25 0.451 0.122 0.143 0.062 0.057 2.085 411 0.963 2.8 60.0% 21.6% 1989 1.274 0.455 0.126 0.149 0.064 0.061 2.13 414 0.984 2.1 59.8% 21.4% 1990 1.292 0.459 0.129 0.154 0.065 0.065 2.165 417 1 1.6 59.7% 21.2% 1991 1.312 0.463 0.131 0.158 0.067 0.069 2.2 419 1.016 1.6 59.6% 21.0% 1992 1.323 0.467 0.133 0.162 0.067 0.072 2.225 421 1.028 1.2 59.5% 21.0% 1993 1.334 0.467 0.133 0.164 0.068 0.074 2.24 423 1.035 0.7 59.6% 20.8% 1994 1.356 0.469 0.135 0.165 0.068 0.076 2.27 425 1.048 1.4 59.7% 20.7% 1995 1.383 0.472 0.137 0.168 0.067 0.077 2.303 428 1.064 1.6 60.1% 20.5% 1996 1.41 0.473 0.139 0.17 0.067 0.078 2.336 430 1.079 1.5 60.4% 20.2% 1997 1.426 0.474 0.142 0.171 0.067 0.079 2.359 432 1.09 1.1 60.4% 20.1% 1998 1.464 0.478 0.145 0.172 0.067 0.08 2.406 436 1.111 2.2 60.8% 19.9% 1999 1.495 0.481 0.148 0.173 0.066 0.082 2.444 439 1.129 1.8 61.2% 19.7% 2000 1.512 0.481 0.151 0.173 0.066 0.083 2.467 441 1.14 1.1 61.3% 19.5% 2001 1.535 0.48 0.153 0.174 0.065 0.085 2.492 443 1.151 1.2 61.6% 19.3% 2002 1.564 0.481 0.155 0.174 0.065 0.087 2.526 446 1.167 1.5 61.9% 19.0% 2003 1.6 0.483 0.157 0.174 0.064 0.088 2.567 449 1.186 1.9 62.3% 18.8% 2004 1.627 0.483 0.159 0.174 0.063 0.09 2.596 452 1.199 1.4 62.7% 18.6% 2005 1.655 0.481 0.162 0.173 0.063 0.092 2.627 454 1.213 1.4 63.0% 18.3% 2006 1.685 0.482 0.165 0.173 0.062 0.095 2.662 457 1.229 1.6 63.3% 18.1% 2007 1.709 0.484 0.167 0.172 0.062 0.097 2.692 460 1.243 1.4 63.5% 18.0% 2008 1.739 0.486 0.17 0.171 0.061 0.1 2.728 463 1.26 1.7 63.7% 17.8% 2009 1.76 0.489 0.172 0.171 0.061 0.103 2.755 465 1.273 1.2 63.9% 17.7% 2010 1.791 0.491 0.175 0.17 0.06 0.106 2.793 469 1.29 1.7 64.1% 17.6% 2011 1.817 0.492 0.178 0.169 0.06 0.109 2.825 471 1.305 1.5 64.3% 17.4% 2012 1.845 0.494 0.181 0.168 0.059 0.111 2.858 474 1.32 1.5 64.6% 17.3% 2013 1.882 0.496 0.184 0.167 0.059 0.114 2.901 478 1.34 2 64.9% 17.1% 2014 1.908 0.499 0.187 0.166 0.058 0.116 2.935 481 1.356 1.6 65.0% 17.0% 2015 1.939 0.504 0.19 0.165 0.058 0.118 2.974 485 1.374 1.8 65.2% 16.9% 2016 1.985 0.507 0.193 0.164 0.057 0.121 3.027 489 1.399 2.5 65.6% 16.7%
METHANE EMISSIONS INVENTORY ANALYSIS Table 2 provides values for estimated annual emissions of methane and the portion coming from different categorical sources. These data are from two primary sources: (1) The Global Carbon Project s (GCP) 2016 Global Methane Budget and (2) U.S. Environmental Protection Agency Inventory of U.S Greenhouse Gas Emissions and Sinks: 1990-2015. These data are applied to the NOAA AGGI radiative forcing for methane (16.7% of all radiative forcing in 2016, as discussed in the previous section) to determine the influence of various methane emission sources on radiative forcing (as shown previously in Figure 1). Table 2: Summary of Methane Emission Sources and Pro Rata Radiative Forcing Due to Methane Category Annual Emissions in Teragrams (Tg) % of Total % Total 2016 Radiative Forcing Based on NOAA AGGI - Natural Sources - Human Activity 1 265 293 47.5% 52.5% 7.9% 8.8% Total Worldwide Methane Emissions 1 558 100% 16.7% - Agriculture and Waste 1 - Fossil Fuels 1 188 105 33.7% 18.8% 5.63% 3.14% Total Human Activity Methane Emissions 293 52.5% 8.8% - Coal 1, 2 - Oil and Gas 1, 2 Total Fossil Fuels Methane Emissions 35.9 69.1 105 6.4% 12.4% 18.8% 1.07% 2.07% 3.14% - U.S. Natural Gas Production, Delivery, and 6.55 1.2% 0.20% Use 1, 3 Source: 1) Global Carbon Project, 2016 Global Methane Budget, Dec. 12, 2016. Source 1 refers to the Top-Down global mean values. 2) Global Carbon Project, 2016 Global Methane Budget, Dec. 12, 2016. Source 2 refers to using the Bottoms-Up global mean percentage value in a category to determine, for example, the fossil fuel split between oil and gas and coal. 3) U.S. EPA Inventory of U.S. Greenhouse Gas Emissions. U.S. natural gas industry emissions divided into the GCP total worldwide methane emissions to determine the percentage of total worldwide methane emissions. The Global Carbon Project analyses includes two different methodologies to estimate methane emissions: (1) Top-Down inventory estimates and (2) Bottoms-Up inventory estimates. For this analysis, we relied on the Top-Down inventories as a starting point. As will be shown, this is a more conservative assumption (i.e., it shows a larger radiative forcing for fossil fuel methane emissions). This link goes to an interactive graphic that provides details on the Top-Down and Bottoms-Up inventory data: http://lsce-datavisgroup.github.io/methanebudget 2
Figure 1 contains two screenshots that indicate the global mean values for (1) total mean worldwide methane emissions, (2) fossil fuel production and use, and (3) agriculture and waste. The second and third are combined into a category called Human Activity. Figure 1: Top-Down Budget total, Fossil Fuel, and Agriculture and Waste Methane Emissions 3
Figure 2 shows Bottoms-Up Budget values of 41 Tg for coal and 79 Tg for oil & gas. These two were summed and then used to determine an equivalent proportion of the Top-Down budget fossil fuel category to differentiate between methane from coal operations versus oil& gas industry segments. For example, the coal % of fossil fuel methane emissions equals 41 Tg/(41 Tg + 79 Tg) = 34.167% and the oil & gas methane percentage is one 100% minus this value, or 65.83%. Figure 2: Breakdown of Values Used to Apportion Fossil Fuel Methane Emissions to Coal and Oil & Gas from Bottoms-Up Budget The Top-Down total for fossil fuel methane emissions was 105 Tg. From this, we found the estimated Top- Down portion of coal as 105*34.167% = 35.9 Tg. The oil & gas amount is 105*65.83% = 69.1 Tg. From this, we are able to determine a percentage of total Top-Down methane emissions for these segments. 4
The final calculation is based on the US EPA report covering the inventory of U.S. greenhouse gas emissions. This can be found at: https://www.epa.gov/ghgemissions/inventory-us-greenhouse-gasemissions-and-sinks-1990-2015 The value of 6.55 Tg for U.S. natural gas methane emissions is a combination of data contained in (1) Table 3-2, which covers natural gas production, processing, transmission, storage, and distribution; this totals 6.497 Tg and (2) Table 3-10 which breaks down stationary fossil fuel methane from combustion by end use sector of electric power, industrial, commercial, and residential; this totals 0.056 Tg. Together, these total 6.553 which was rounded down to 6.55 Tg. Note that 1 Tg of methane is equal to 1000 kilotons (kt) of methane. Note that the data from Table 3-10 were converted from MMT CO 2 equivalent to Tg of methane (see below). Electric Power: 0.008 Tg CH 4 (0.2 MMT CO 2eq) Industrial: 0.008 Tg CH 4 (0.2 MMT CO 2eq) Commercial: 0.016 Tg CH 4 (0.4 MMT CO 2eq) Residential: 0.024 Tg CH 4 (0.6 MMT CO 2eq) 5
REFERENCES Global Carbon Project, 2016 Global Methane Budget. http://www.globalcarbonproject.org/methanebudget/ http://lsce-datavisgroup.github.io/methanebudget/ (interactive graphic) National Oceanic and Atmospheric Administration, The NOAA Annual Greenhouse Gas Index (AGGI), Spring, 2017. https://www.esrl.noaa.gov/gmd/aggi/aggi.html United States Environmental Protection Agency, Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2015, April 15, 2017. https://www.epa.gov/ghgemissions/inventory-us-greenhouse-gas-emissionsand-sinks-1990-2015 6 12-17