Part 1 - GWP, GHG, and ODP – What Do They Mean?

Figure 1: Temperature Changes from 1971 to 1999, with Projected Future Temperature Changes. Source: Herring, David, “Climate Change: Global Temperature Projections”Figure 1: Temperature Changes from 1971 to 1999, with Projected Future Temperature Changes. Source: Herring, David, “Climate Change: Global Temperature Projections”While the current administration may choose to deny the existence of climate change to further their own agendas, most scientists agree that human impact has contributed to global warming, and the world must begin to control greenhouse gases (GHG). If the temperatures increase trends continue as seen in Figure 1, low lying areas in Florida will continue to be plagued by tidal flooding and storm or hurricane surges (see FEMA Flood Maps). The U.S. Climate Resilience Toolkit indicates that the risk of flooding has increased dramatically since 1900, and the risk is projected to grow even more over the next century with the projected sea level rising between 8 inches and 6.6 feet, depending on the model used. Higher temperatures can also cause more frequent severe and erratic weather patterns, such as increased hurricane generation, because the additional heat will evaporate more water from the oceans.

Even if you doubt that human activity impacts climate change, engineers are increasingly tasked with designing products for the global marketplace, and despite loosening regulations in the U.S., most industrial countries are attempting to limit or eliminate high global warming potential (GWP) gases as part of their commitment to the Kyoto Protocol Agreement. In the Kyoto agreement, 37 industrialized countries and the European Community committed to reduce GHG emissions to an average of five percent against 1990 levels by 2012. During the second commitment period, the U.S. and other countries committed to reduce GHG emissions by at least 18 percent below 1990 levels by 2020.

How Do Greenhouse Gases Trap Heat?

Approximately 29 percent of the sunlight, solar radiation, reaching the earth is reflected back into space. 23 percent of solar radiation is absorbed by the atmosphere by water vapor, dust, ozone and greenhouse gases. 48 percent of the sunlight passes through the atmosphere, and is absorbed by the earth’s surface (see Figure 2). Climate, or radiative, forcing is the difference between the absorbed energy from the sun versus the energy reflected back out into space. Positive forcing results in climate warming.

GWP of a gas is calculated by integrating the radiative forcing with the lifetime of the gas, and then normalizing to carbon dioxide (CO2), which gives CO2 a GWP of 1. GWP values are unitless except when expressed in terms of Carbon Dioxide Equivalent (CO2e). GWP value can change depending on time frame used. The Kyoto Protocol established a standardized GWP time frame of 100 years. The atmospheric lifetime or persistence in the environment has an impact on GWP.

Figure 3: Growth Rates of Greenhouse Gas Forcing. Source: NASA Scientific Visualization StudioFigure 3: Growth Rates of Greenhouse Gas Forcing. Source: NASA Scientific Visualization Studio

A 2014 article, "Recent and future trends in synthetic greenhouse gas radiative forcing" in Geophysical Letters, the authors stated that atmospheric measurements indicated hydrofluorocarbons (HFCs) and hydrochlorofluorocarbons (HCFCs) are now the primary drivers of the positive growth in synthetic greenhouse gas (SGHG) radiative forcing. Even though the amount of HFCs, HCFCs and perfluorinated compounds (PFCs) released to the environment in 2015 was small (3 percent) when compared to carbon dioxide (82 percent), methane (10 percent) and nitrous oxide (5 percent), fluorocarbon gases have very high radiative forcing or GWP values for several reasons:

  • HFC gases absorb infrared radiation at wavelengths that are normally transparent to infrared radiation when compared to natural GWP gases (CO2, methane, etc.).
  • Certain HFCs are very stable chemically and will remain in the environment for thousands of years. Naturally occurring nitrous oxide and methane gases decompose in much shorter time frames.
  • There is a higher amount of infrared radiation absorption by high GWP gases

The Rise of Low GWP Refrigerants

The European Union has mandated that residential air conditioning and refrigeration must use refrigerants of GWP of 150 or less by 2015. The same is true of automobiles by 2017. In the EU, low GWP refrigerants have a GWP of 150 or less. Medium GWP refrigerants are between 150 and 2,500. High GWP gases have GWP greater than 2,500.

In the U.S., certain refrigerants are being phased out and lower GWP gases are being transitioned in. The EPA's SNAP program, under authority from the Clean Air Act, evaluates substitute chemicals and technologies with low-global warming potential (GWP) and low ozone-depleting potential (ODP). The SNAP approved alternative low GWP refrigerants that range from 3 to 675 GWP that can replace older compounds with GWPs from 1,400 to 4,000. In addition, the EPA approved several low GWP hydrocarbon refrigerants with use restrictions that were already in use Europe and Asia including: propane, R-441A hydrocarbon blend and HFC-32 (difluoromethane).
Figure 4: ASHRAE Map of Refrigerant Evolution. Source: ASHRAE Position Document on Natural Refrigerants Figure 4: ASHRAE Map of Refrigerant Evolution. Source: ASHRAE Position Document on Natural Refrigerants

CFC refrigerants like R-12 were very common until being banned in 1994 due to their high ozone depleting potential (ODP). R-12 has an ODP of 1, a GWP of 10,900 and an atmospheric lifetime of 100 years (#very bad). CFCs were replaced by HCFCs such as R-22 and R 123.

HCFCs gave way to the common hydrofluorocarbons (HFCs) in use today, such as R-134a, R-404A R-407C, R-410A, R-404A and R-507A.

While the current HFCs have low or zero ODP values, many HFCs are high global warming contributors. HFCs released into the atmosphere also form trifluoroacetic acid (TFA), which is a strong acid with high toxicity to some organisms. TFA is highly persistent and there is no known degradation mechanism. Eventually, all HFCs will be replaced with natural or hydrocarbon refrigerants.

Certain HFCs are restricted for use as substitute refrigerants because they are volatile organic compounds (VOCs) or contain VOCs. The EPA often lists within their rulings or guidance certain refrigerants as acceptable subject to use conditions in new stand-alone equipment based on toxicity, flammability, VOC levels and other factors.

All hydrofluorocarbons (HFCs) used in refrigeration and air conditioning (AC) systems are eventually emitted to the atmosphere during equipment operation, repair, and disposal (unless the gas is recovered, recycled, and ultimately destroyed). Equipment is being retrofitted or replaced with redesigned equipment so the new low GWP refrigerants can be utilized.

Under the authority of the Clean Air Act and after receiving industry and environmental group recommendations, the EPA approved additional low-GWP hydrocarbon refrigerants, subject to use conditions. The EPA unacceptability of a refrigerant depends on the specific equipment end-use (e.g., air conditioner, refrigeration unit or chiller). Flammable refrigerants might be acceptable in a controlled commercial or residential refrigeration system, but not within a motor vehicle air conditioning system. For example, the table in Figure 5, extracted from the EPA Final Rule (FR-2016-12-01), lists new substitutes and prohibits certain high-GWP HFCs as alternatives under SNAP by equipment type.

Figure 5: Listing of new substitutes and prohibiting certain high-GWP HFCs as alternatives under SNAP according to EPA Final Rule (FR-2016-12-01)Figure 5: Listing of new substitutes and prohibiting certain high-GWP HFCs as alternatives under SNAP according to EPA Final Rule (FR-2016-12-01)

The Other Factors in Safe Refrigerant Use

Check back next week for Part 2 on the other factors involved in refrigerant selection and safe use. Expect to see more on flammability, toxicity, VOC levels, cost, and availability, in addition to information on the specific types of low GWP refrigerants available. Low GWP is important, but it is only one of many factors involved in refrigerant selection and safe use.

References and Further Reading

Intergovernmental Panel on Climate Change
What is a Global Warming Potential? And which one do I use?
The NOAA Annual Greenhouse Gas Index (AGGI)
Using GHG Inventory and GHGRP Data
Radiative Forcing of Climate Change (GRID-Arendal)
Radiative Forcing of Climate (IPCC)
Changes in Atmospheric Constituents and in Radiative Forcing (IPCC)
Center for Climate and Energy Solutions (C2Es)
The importance of the Montreal Protocol in protecting climate
The Climate Registry
Possible artifacts of data biases in the recent global surface warming hiatus, U.S. Global Change Research Program
National Climate Assessment Report
Climate Techbook
Toxicology Excellence for Risk Assessment (TERA)
ITER - International Toxicity Estimates for Risk
WLT – World Library of Toxicology
Occupational Alliance for Risk Science (OARS)– Workplace Environmental Exposure Levels (WEEL)™ Database
American Industrial Hygiene Association (AIHA)

Figure 1 Herring, David, “Climate Change: Global Temperature Projections”,

Figure 2 – Earth’s Energy Budget, NASA illustration by Robert Simmon. Astronaut photograph

Figure 3 – Growth Rates of Greenhouse Gas Forcing from NASA Scientific Visualization Studio

Figure 4 ASHRAE Map of Refrigerant Evolution from ASHRAE Position Document on Natural Refrigerants

Figure 5 EPA Final Rule (FR-2016-12-01)