Why do we need GHGs?
GHGs are released through a variety of sources, which are grouped (within the United States) into 6 categories: transportation, agriculture, electricity, industry, residential and commercial, and land use/land use change. The size of each country, what industries they prioritize, and multiple political factors influence the breakdown of emissions between these categories, as well as how emissions are categorized overall. A breakdown of global emissions by several categories is visualized below (Ritchie et al., 2020). Electricity is the highest source of emissions, accounting for almost half of all global emissions, followed by transportation, agriculture, and then manufacturing and construction. We’ll be looking at a breakdown and trend of emissions from a US perspective, as well as looking at how adding an international component complicates emissions reporting and statistics.
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Electricity
When we think about emissions, some of us might naturally think of electricity, since it is such a common topic. We often talk about installing wind or solar power to reduce electricity based emissions, and for good reason. Globally, electricity(and heat) make up almost 50% of emissions, and within the US electricity generation alone is 25% of emissions. Electricity is also a complicated metric, as electricity is utilized within other emissions categories. Within the United States, the EPA tracks electricity emissions separately, as well as how end use electricity is categorized. This allows for the most wholesale view of what is driving emissions within the United States, and is pictured below (US EPA, 2015b).
Because it is such a major topic, electricity is also the category that has decarbonized the most in recent decades, driven by increases in wind and solar power, as well as battery storage in recent years. However, over 60% of US electricity production comes from fossil fuels such as coal and natural gas, as pictured below. Recent years have seen a switch from coal to natural gas as the primary generation source, while renewable sources continue to grow, and nuclear energy production has remained consistent, generating around 20% of US electricity for multiple decades (US EPA, 2025c). This chart also shows cleanly one of the two primary drivers of the reduction in emission intensity across the united states: a lack of electricity demand growth. Even as the population has increased, and technologies that require electricity have grown(such as the internet and more advanced computers), electricity usage across the United States has not grown significantly throughout the 21st century. This is because typical drivers of increased demand are offset by efficiency increases in heating and cooling, refrigeration, as well as within appliances. In addition, the increase in household solar panels over this same period has driven down total demand, especially in certain markets(more expensive, more electricity, more likely for solar?)
C. Chang – How one device could help transform our power grid. By Alexandra Witze, August 24, 2023, in en:Science News. Article lists data source on chart as en:U.S. Energy Information Administration., Public Domain, https://commons.wikimedia.org/w/index.php?curid=136502215
Transportation
Transportation is the other key emissions source that many think of when discussing climate change and GHGs, including many often comparing green projects in terms of ‘cars off of the road’. Within the United States, transportation makes up 28% of emissions, 29% if including electricity emissions within this sector. Emissions in this sector have also been flat over the 21st century within the United States, driven by increasing urban density and slightly higher mpg rates of vehicles. However, transportation is not just single family vehicles, but also includes trucks such as semis, all aircraft, boats, and trains! In fact, emissions from all cars (57%) and trucks (23%) make up 80% of the emissions in this category, while ALL boats and trains make up a combined 5% of emissions in transportation (US EPA, 2015a).
(US EPA, 2015a)
Agriculture
Agriculture may not be a sector that many attribute to high emissions, and yet this sector comprises 11% of US emissions as of 2022. Methane from livestock is a famous example of emissions, and comprises 25% of emissions in this sector. Other factors, such as farming equipment, manure, fertilizer use, and how the soil is managed are heavy contributors to emissions. In this sector, while CO2 is still a large component, nitrogen, including nitrous oxide, are vitally important to be managed properly. 75% of US emissions of nitrous oxide come from the agriculture sector, and as nitrous oxide is 273x stronger than CO2, any small emissions disproportionately affect total emissions (US EPA, 2016b)(SOURCE FOR 75%).
Emissions in this sector have increased by around 9% in the US since the 1990s, and around 10% globally since 2000. However, agriculture based emissions globally used to comprise 38% of total global emissions, and now represents over 30%, even as the global population has skyrocketed (Greenhouse Gas Emissions from Agrifood Systems. Global, Regional and Country Trends, 2000–2022, n.d.; Ritchie et al., 2023).
Industry
Emissions embedded in industry, both direct and indirect from electricity, accounted for over 30% of US emissions in 2022. However, emissions from this sector have decreased over the 21st century, declining by 22% since 1990 (US EPA, 2025d). Because industry is such a wide sector, emissions come from a variety of sources and in multiple GHGs. Most emissions in this sector come from energy or heat generation, while other emissions are produced from fuel usage and chemical reactions.
Globally, direct emissions from industry (including manufacturing and construction) have grown since the 1990s, and comprise 20% of emissions (Ritchie et al., 2020).
Residential and commercial
Emissions from the residential and commercial sector, including all homes and businesses, comprise 31% of US emissions(direct and indirect) as of 2022. However, emissions from this sector, especially indirect emissions, have decreased significantly since 2007. Indirect emissions in this sector have decreased from over 1600 million metric tons of CO2E in 2007 to less than 1100 million metric tons in 2022, driven by increases in renewable energy and energy efficiency increases of homes and appliances. However, direct emissions in this sector have been flat and increasing in recent years, driven by factors that many might not consider. Heating and cooking can often utilize gas and release GHGs, as well as food waste and wastewater. Most concerning is the recent resurgence of fluorinated gas in air conditioning and refrigeration. These gases are almost entirely man made, and were designed to replace ozone-depleting gases utilized throughout much of the 20th century. However, these gases can last hundreds of years in the atmosphere, and have GWPs that are several thousand times stronger than CO2, so even small rises in emissions can have massive effects (US EPA, 2025a, 2025b).
Land use and Land Use Change
The final major category utilized by the US is land use and land use change. Plants and soils store and release carbon and other elements, and human activity affects this heavily. The release of nitrogen from soil due to agriculture practices is also included in this sector. Due to the United States long clearing our old forests, and improving farming practices, the US land use is a land sink, as our areas absorb more carbon than it emits (US EPA, 2025e). Globally, many countries’ land uses are net emitters, especially in countries that are still currently cutting down forested areas, such as Brazil. In fact, in Brazil, land use change from deforestation accounts for over 40% of their total emissions (Dutra et al., 2024; SEEG Brazil – The Greenhouse Gas Emissions and Removals Estimation System (SEEG) Provides Real-Time Data That Supports the Transformation Needed to Address the Global Challenges Affecting All Regions of Brazil., n.d.)!