Understanding Global Warming Potentials | US EPA

Greenhouse gases (GHGs) warm the Earth by absorbing energy and slowing the rate at which the energy escapes to space; they act like a blanket insulating the Earth. Different GHGs can have different effects on the Earth's warming. Two key ways in which these gases differ from each other are their ability to absorb energy (their "radiative efficiency"), and how long they stay in the atmosphere (also known as their "lifetime").

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Starting in , the Intergovernmental Panel on Climate Change (IPCC) used the Global Warming Potential (GWP) to allow comparisons of the global warming impacts of different gases. Specifically, it is a measure of how much energy the emission of 1 ton of a gas will absorb over a given period of time, relative to the emission of 1 ton of carbon dioxide (CO2). The larger the GWP, the more that a given gas warms the Earth compared to CO2 over that time period. The time period usually used for GWPs is 100 years. GWPs provide a common unit of measure, which allows analysts to add up emissions estimates of different gases (e.g., to compile a national GHG inventory), and allows policymakers to compare emissions reduction opportunities across sectors and gases. 

• CO2, by definition, has a GWP of 1 regardless of the time period used, because it is the gas being used as the reference. CO2 remains in the climate system for a very long time: CO2 emissions cause increases in atmospheric concentrations of CO2 that will last thousands of years.
• Methane (CH4) is estimated to have a GWP of 27-30 over 100 years. CH4 emitted today lasts about a decade on average, which is much less time than CO2. But CH4 also absorbs much more energy than CO2. The net effect of the shorter lifetime and higher energy absorption is reflected in the GWP. The CH4 GWP also accounts for some indirect effects, such as the fact that CH4 is a precursor to ozone, and ozone is itself a GHG. 
• Nitrous Oxide (N2O) has a GWP 273 times that of CO2 for a 100-year timescale. N2O emitted today remains in the atmosphere for more than 100 years, on average. (Learn why EPA's U.S. Inventory of Greenhouse Gas Emissions and Sinks uses a different value.)
• Chlorofluorocarbons (CFCs), hydrofluorocarbons (HFCs), hydrochlorofluorocarbons (HCFCs), perfluorocarbons (PFCs), and sulfur hexafluoride (SF6) are sometimes called high-GWP gases because, for a given amount of mass, they trap substantially more heat than CO2. (The GWPs for these gases can be in the thousands or tens of thousands.)

Explore the questions and answers below to learn more about global warming potentials (GWPs).

Frequently Asked Questions

The International&#;Organization for Standardization (ISO) community differs in its definition and use of the term Global Warming Potential (GWP) from that used by IPCC. This ISO approach is applied in Environmental Production Declaration (EPD), Product Category Rules (PCR), Buy Clean Policies, and related programs. This definition and use are inconsistent with how GWP is defined by the IPCC and used in many international GHG accounting efforts, including national reporting by Parties to the UNFCCC and Paris Agreement.

The ISO and relevant communities use the term &#;GWP&#; as an impact category to refer to the embodied greenhouse gases of a specific product or product-level GHG emission intensities (see, e.g., ISO :). This specific use of GWP by the EPD community refers to the total greenhouse gas emissions directly associated with the production of a product, including the upstream activities of extraction and transport of raw materials. This type of calculation can also be described with terms such as &#;embodied GHG equivalent&#; or &#;GHG footprint.&#; The product GWP measure is reported in CO2-equivalents per functional unit in EPDs, PCRs, etc. However, the ISO calculation of CO2-equivalents requires the use of the original GWP as defined by IPCC, thereby making the EPD/ISO GWP inherently confusing as it uses both meanings of the term GWP simultaneously.

To reduce confusion, the use of the term &#;Global Warming Potential&#; or &#;GWP&#; that fall outside the IPCC definition or use&#;i.e., a measure of the relative climate impact of a given greenhouse gas relative to the impact of carbon dioxide (as defined on this page)&#;should include a definition of the non-IPCC usage of the term to distinguish it from the original established IPCC definition. In the case of how ISO and relevant communities use the term GWP, it should be clearly explained that the specific meaning in that context refers to &#;embodied GHG equivalent,&#; &#;embodied GHG emissions,&#; or &#;carbon equivalent footprint,&#; as applicable. This context is especially important if the document uses both different meanings of the term &#;GWP&#; such as in the ISO/EPD context.

EPA and other organizations will update the GWP values they use occasionally. This change can be due to updated scientific estimates of the energy absorption or lifetime of the gases or to changing atmospheric concentrations of GHGs that result in a change in the energy absorption of 1 additional ton of a gas relative to another.

In the most recent report by the Intergovernmental Panel on Climate Change (IPCC), multiple methods of calculating GWPs were presented based on how to account for the influence of future warming on the carbon cycle. For this Web page, we are presenting the range of the lowest to the highest values listed by the IPCC.

The EPA considers the GWP estimates presented in the most recent IPCC scientific assessment to reflect the state of the science. In science communications, the EPA will refer to the most recent GWPs. The GWPs listed above are from the IPCC's Sixth Assessment Report, published in .

The EPA's Inventory of U.S. Greenhouse Gas Emissions and Sinks (Inventory) complies with international GHG reporting standards under the United Nations Framework Convention on Climate Change (UNFCCC). UNFCCC guidelines now require the use of the GWP values from the IPCC's Fifth Assessment Report (AR5), published in . The Inventory also presents emissions by mass, so that CO2 equivalents can be calculated using any GWPs, and emission totals using more recent IPCC values are presented in the annexes of the Inventory report for informational purposes.

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The data collected by EPA's Greenhouse Gas Reporting Program is generally reported in mass units of greenhouse gas and is used in the Inventory. The Reporting Program, generally uses GWP values from the AR4 to determine whether facilities exceed reporting thresholds and to publish data in CO2 equivalent values. The Reporting Program collects data about some industrial gases that do not have GWPs listed in the AR4; for these gases, the Reporting Program uses GWP values from other sources, such as the AR5. 

EPA's CH4 reduction voluntary programs also use CH4 GWPs from the AR5 report for calculating CH4 emissions reductions through energy recovery projects, for consistency with the national emissions presented in the Inventory.

The United States primarily uses the 100-year GWP as a measure of the relative impact of different GHGs. However, the scientific community has developed a number of other metrics that could be used for comparing one GHG to another. These metrics may differ based on timeframe, the climate endpoint measured, or the method of calculation.

For example, the 20-year GWP is sometimes used as an alternative to the 100-year GWP. Just like the 100-year GWP is based on the energy absorbed by a gas over 100 years, the 20-year GWP is based on the energy absorbed over 20 years. This 20-year GWP prioritizes gases with shorter lifetimes, because it does not consider impacts that happen more than 20 years after the emissions occur. Because all GWPs are calculated relative to CO2, GWPs based on a shorter timeframe will be larger for gases with lifetimes shorter than that of CO2, and smaller for gases with lifetimes longer than CO2. For example, for CH4, which has a short lifetime, the 100-year GWP of 27&#;30 is much less than the 20-year GWP of 81&#;83. For CF4, with a lifetime of 50,000 years, the 100-year GWP of is larger than the 20-year GWP of .

Another alternate metric is the Global Temperature Potential (GTP). While the GWP is a measure of the heat absorbed over a given time period due to emissions of a gas, the GTP is a measure of the temperature change at the end of that time period (again, relative to CO2).The calculation of the GTP is more complicated than that for the GWP, as it requires modeling how much the climate system responds to increased concentrations of GHGs (the climate sensitivity) and how quickly the system responds (based in part on how the ocean absorbs heat).

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11-19- 15:-19- 16:08

Mona,

A couple more points to Allan's well put explanation if I may.

1) In D1.1 clause (formerly 'section') 5.15.4.3 (p. 195 of ed.) there is a stated roughness requirement.  That directs you to some ANSI numbers and then a reference put out by AWS that describes the roughness and you get a moulded sample to compare the product to for quality of finish.  AWS C4.1 is what you need to order.  Maybe a couple of them, one for the work station and one for QC person.

2) Having used most types of fuel gases on both hand and machine cutting operations through the years there is another point that is not made often.  There needs be almost NO slag nor clean up time with any of them especially with machines.  I don't even 'hammer' with a slag hammer.  Run it along the edge and any small debris falls off, ready for the next step.  Now having said that it takes a lot of practice and experience to produce to that level.  If the edge is not to be welded or hidden in some way some may want to hit the edge quick with a sander or grinder for a little better appearance but it is generally unneeded.  If you hear lots of hammering or grinding then something was not setup correctly.  Tip size, preheat or cutting pressures, speed, tip properly cleaned, distance from tip end to material, and possibly a couple I have missed.  I worked one machine that we had to rework the bearings and track system because it was so loose you could not get good cuts from all the jerking and bouncing that the unit did as it moved.

3) Since you asked about different techniques between the two gases, what thickness of material are we talking about and what grade (A-36 carbon steel or other) of material.  What is the intended use for most of the product after cut process (welded joints, individual complete part as cut, etc.)  There are things that can be done to improve quality of cut with a quick learning curve but there are various applications.

I think that about covers it from my point.  Good luck.

Have a Great Day,  Brent

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