Greenhouse Gas Emissions

Greenhouse Gas Overview

Gases that trap heat in the atmosphere are often called greenhouse gases. This section of the EPA Climate Change Site provides information and data on emissions of greenhouse gases to Earth’s atmosphere, and also the removal of greenhouse gases from the atmosphere. For more information on the science of climate change, please visit EPA's climate change science home page.

Some greenhouse gases such as carbon dioxide occur naturally and are emitted to the atmosphere through natural processes and human activities. Other greenhouse gases (e.g., fluorinated gases) are created and emitted solely through human activities. The principal greenhouse gases that enter the atmosphere because of human activities are:

Carbon Dioxide (CO2): Carbon dioxide enters the atmosphere through the burning of fossil fuels (oil, natural gas, and coal), solid waste, trees and wood products, and also as a result of other chemical reactions (e.g., manufacture of cement). Carbon dioxide is also removed from the atmosphere (or “sequestered”) when it is absorbed by plants as part of the biological carbon cycle.
Methane (CH4): Methane is emitted during the production and transport of coal, natural gas, and oil. Methane emissions also result from livestock and other agricultural practices and by the decay of organic waste in municipal solid waste landfills.
Nitrous Oxide (N2O): Nitrous oxide is emitted during agricultural and industrial activities, as well as during combustion of fossil fuels and solid waste.
Fluorinated Gases: Hydrofluorocarbons, perfluorocarbons, and sulfur hexafluoride are synthetic, powerful greenhouse gases that are emitted from a variety of industrial processes. Fluorinated gases are sometimes used as substitutes for ozone-depleting substances (i.e., CFCs, HCFCs, and halons). These gases are typically emitted in smaller quantities, but because they are potent greenhouse gases, they are sometimes referred to as High Global Warming Potential gases (“High GWP gases”).

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Greenhouse Gas Inventories

A greenhouse gas inventory is an accounting of the amount of greenhouse gases emitted to or removed from the atmosphere over a specific period of time (e.g., one year). A greenhouse gas inventory also provides information on the activities that cause emissions and removals, as well as background on the methods used to make the calculations. Policy makers use greenhouse gas inventories to track emission trends, develop strategies and policies and assess progress. Scientists use greenhouse gas inventories as inputs to atmospheric and economic models.

To track the national trend in emissions and removals since 1990, EPA develops the official U.S. greenhouse gas inventory each year. The national greenhouse gas inventory is submitted to the United Nations in accordance with the Framework Convention on Climate Change .

In addition to the U.S. inventory, greenhouse gas emissions can be tracked at the global, state and local levels as well as by companies and individuals:

Many other countries also develop national greenhouse gas inventories, which can be compiled into global inventories. EPA works with developing and transition countries to improve the accuracy and sustainability of their greenhouse gas inventories. EPA has developed Greenhouse Gas Inventory Capacity Building templates and software tools targeting key sources, emissions factors, good practices, institutional infrastructure and use of the latest IPCC guidelines on greenhouse gas inventories.
Many state and local governments prepare greenhouse gas inventories, and EPA provides guidance and tools to assist them in their efforts.
Corporate greenhouse gas inventories provide information on the emissions associated with the operations of a company.
Individuals produce greenhouse gas emissions through everyday activities such as driving and using air conditioning or heating. EPA provides an online calculator for estimating personal emissions.

The Intergovernmental Panel on Climate Change (IPCC) publishes internationally accepted inventory methodologies that serve as a basis for all greenhouse gas inventories, ensuring that they are comparable and understandable. The 2006 IPCC Guidelines were completed and accepted by the IPCC in May 2006.

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Emission Trends & Projections

Estimates of future emissions and removals depend in part on assumptions about changes in underlying human activities. For example, the demand for fossil fuels such as gasoline and coal is expected to increase greatly with the predicted growth of the U.S. and global economies.

The Fifth U.S. Climate Action Report concluded, in assessing current trends, that greenhouse gas emissions increased by 17 percent from 1990-2007. Over that same time period, the U.S. GDP increased by 65 percent and population increased by 21 percent. The dominant factor affecting U.S. emissions trends is CO₂ emissions from fossil fuel combustion, which increased by 21.8 percent over the 17-year period, while methane and nitrous oxide emissions decreased by 5 percent and 1 percent, respectively. The declines in methane emissions are mostly due to increased collection and combustion of landfill gas, as well as improvements in technology and management practices at natural gas plants. The decline in nitrous oxide emissions is largely due to the installation of newer N₂O control technologies in motor vehicles throughout the past decade. Fluorinated substances (HFCs, PFCs, and SF₆) accounted for 2 percent of total U.S. GHG emissions in 2007. The increasing use of these compounds since 1995 as substitutes for ozone depleting substances has been largely responsible for their upward emissions trends. (Fifth U.S.Climate Action Report, 2010)

Many, but not all, human sources of greenhouse gas emissions are expected to rise in the future. This growth may be reduced by ongoing efforts to increase the use of newer, cleaner technologies and other measures. Additionally, our everyday choices about such things as commuting, housing, electricity use and recycling can influence the amount of greenhouse gases being emitted.

The United States government prepares projections of emissions and removals of all greenhouse gases. The following links provide more detailed information on projections:

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Project Methodologies

EPA has developed several Climate Leaders Offset Project Methodologies that use a standardized approach to determine project eligibility, address additionality, select and set the baseline, identify monitoring options, and quantify reductions. This approach seeks to ensure that the GHG emission reductions from offset projects meet four key accounting principles—they must be real, additional, permanent, and verifiable.

To be eligible as offsets, project activities must be surplus to regulation. Projects are also required to demonstrate additionality by achieving a level of performance with respect to emission reductions and/or removals that is significantly better than business-as-usual. Business-as-usual is determined from similar, recently undertaken or planned practices, activities or facilities in the same geographic region. This level of “performance” may be defined as an emissions rate, a technology standard or a practice standard. Data used in setting the performance standard is primarily collected from publicly available historic data (although planned or projected activities may be used in certain cases as well). The performance standard approach minimizes the risk of accepting a project that is not additional or rejecting a project that is additional. A performance standard approach also reduces the complexity, cost, and subjectivity of constructing individual project-specific reviews.

Performance standards are available for the following project types: