Executive Summary

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 IPCC. 5 Additionally, the calculated emissions and sinks in a given year for the United States are presented in a common manner in line with the UNFCCC reporting guidelines for the reporting of inventories under this international agreement. 6 The use of consistent methods to calculate emissions and sinks by all nations providing their inventories to the UNFCCC ensures that these reports are comparable. In this regard, U.S. emissions and sinks reported in this Inventory report are comparable to emissions and sinks reported by other countries. The report itself follows this standardized format, and provides an explanation of the IPCC methods used to calculate emissions and sinks, and the manner in which those calculations are conducted. On October 30, 2009, the U.S. Environmental Protection Agency (EPA) published a rule for the mandatory reporting of greenhouse gases from large greenhouse gas emissions sources in the United States. Implementation of 40 CFR Part 98 is referred to as the Greenhouse Gas Reporting Program (GHGRP). 40 CFR part 98 applies to direct greenhouse gas emitters, fossil fuel suppliers, industrial gas suppliers, and facilities that inject carbon dioxide (CO 2) underground for sequestration or other reasons. 7 Reporting is at the facility level, except for certain suppliers of fossil fuels and industrial greenhouse gases. The GHGRP dataset and the data presented in this Inventory report are complementary. The GHGRP data set continues to be an important resource for the Inventory, providing not only annual emissions information, but also other annual information, such as activity data and emissions factors that can improve and refine national emission estimates and trends over time. GHGRP data also allow EPA to disaggregate national inventory estimates in new ways that can highlight differences across regions and sub-categories of emissions, along with enhancing application of QA/QC procedures and assessment of uncertainties. EPA uses annual GHGRP data in a number of category estimates and continues to analyze the data on an annual basis, as applicable, for further use to improve the national estimates presented in this Inventory consistent with IPCC guidance. 8 23 24 25 26 27 28 29 30 31 32 33 34 35 36 ES.1 Background Information Greenhouse gases trap heat and make the planet warmer. The most important greenhouse gases directly emitted by humans include carbon dioxide (CO 2), methane (CH 4), nitrous oxide (N 2O), and several other fluorine-containing halogenated substances. Although the direct greenhouse gases CO 2, CH 4, and N 2O occur naturally in the atmosphere, human activities have changed their atmospheric concentrations. From the pre-industrial era (i.e., ending about 1750) to 2015, concentrations of these greenhouse gases have increased globally by 44, 162, and 21 percent, respectively (IPCC 2013 and NOAA/ESRL 2017). This annual report estimates the total national greenhouse gas emissions and removals associated with human activities across the United States. Global Warming Potentials Gases in the atmosphere can contribute to climate change both directly and indirectly. Direct effects occur when the gas itself absorbs radiation. Indirect radiative forcing occurs when chemical transformations of the substance produce other greenhouse gases, when a gas influences the atmospheric lifetimes of other gases, and/or when a gas affects atmospheric processes that alter the radiative balance of the earth (e.g., affect cloud formation or albedo). 9 5 See . 6 See . 7 See and . 8 See 9 Albedo is a measure of the Earth’s reflectivity, and is defined as the fraction of the total solar radiation incident on a body that is reflected by it. 2 DRAFT Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990–2015
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 The IPCC developed the Global Warming Potential (GWP) concept to compare the ability of each greenhouse gas to trap heat in the atmosphere relative to another gas. The GWP of a greenhouse gas is defined as the ratio of the accumulated radiative forcing within a specific time horizon caused by emitting 1 kilogram of the gas, relative to that of the reference gas CO 2 (IPCC 2014). Direct radiative effects occur when the gas itself is a greenhouse gas. The reference gas used is CO 2, and therefore GWPweighted emissions are measured in million metric tons of CO 2 equivalent (MMT CO 2 Eq.). 10,11 All gases in this Executive Summary are presented in units of MMT CO 2 Eq. Emissions by gas in unweighted mass tons are provided in the Trends chapter of this report. UNFCCC reporting guidelines for national inventories require the use of GWP values from the IPCC Fourth Assessment Report (AR4) (IPCC 2007). 12 All estimates are provided throughout the report in both CO 2 equivalents and unweighted units. A comparison of emission values using the AR4 GWP values versus the SAR (IPCC 1996), and the IPCC Fifth Assessment Report (AR5) (IPCC 2013) GWP values can be found in Chapter 1 and, in more detail, in Annex 6.1 of this report. The GWP values used in this report are listed below in Table ES-1. Table ES-1: Global Warming Potentials (100-Year Time Horizon) Used in this Report Gas GWP CO2 1 CH4 a 25 N2O 298 HFC-23 14,800 HFC-32 675 HFC-125 3,500 HFC-134a 1,430 HFC-143a 4,470 HFC-152a 124 HFC-227ea 3,220 HFC-236fa 9,810 HFC-4310mee 1,640 CF4 7,390 C2F6 12,200 C4F10 8,860 C6F14 9,300 SF6 22,800 NF3 17,200 Source: IPCC (2007) a The CH4 GWP includes the direct effects and those indirect effects due to the production of tropospheric ozone and stratospheric water vapor. The indirect effect due to production of CO2 is not included. 17 10 Carbon comprises 12/44 of carbon dioxide by weight. 11 One million metric ton is equal to 10 12 grams or one teragram. 12 See . Executive Summary ES-3
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