GWP 101:
Understanding global warming potentials
This article examines the concept of global warming potential
(GWP) as it relates to environmental regulations and
the aerosol propellants market.
GWP: What is it?
No other issue related to global warming is perhaps more misunderstood
or misrepresented than the relationship between the
stated global warming potential (GWP) value of a substance and
its actual impact on global warming.
Global warming describes the long-term rise in the average
temperature of the Earth’s climate system. Gases that contribute
to global warming are known as greenhouse gases (GHGs) and
include carbon dioxide, methane and other hydrocarbons, nitrous
oxide and hydrofluorocarbons (HFCs).
The GWP value of a gas is simply a measure of how much a
given mass of a gas can contribute to global warming if that mass
is emitted to the atmosphere. GWP values are expressed relative
to carbon dioxide (CO2). For example, if a gas has a GWP value
of 100, the impact on global warming of releasing 1kg of that
gas is equivalent to the impact on global warming of releasing of
100kgs of CO2.
GWP value & the impact of a gas on global warming
An oft-encountered misinterpretation of GWP values is the
assertion that the higher the GWP value of a gas, the more any
emission of that gas will warm the earth (i.e., contribute to global
warming) compared to a gas of lower GWP. This misinterpretation
is based on a false assumption that the impact of a chemical
34 Spray December 2019
Mark L. robin, PhD & aLfreD ThornTon
The Chemours Co.
on global warming is determined solely by its GWP value, i.e., the
assumption that a high GWP value always equals a high impact
on global warming and, conversely, a low GWP value always
equals a low impact on global warming.
What are the facts?
It is important to note that the “P” in GWP stands for potential;
a gas not released into the atmosphere has no impact on global
warming. Hence, the GWP value by itself is not sufficient for an
accurate comparison of the impact of two different
gases on global warming. The impact on global
warming from the release of a gas depends on two
factors—the GWP value of the gas and the mass of
gas emitted. This is found by simply multiplying
the GWP value of the gas by the mass of the gas
emitted:
Impact on Global Warming = (GWP of gas) x
(mass of gas emitted)
If the mass is expressed in units of kilogram (kg),
the resulting impact value has the unit of kilograms
of CO2 equivalents (kg CO2 equivalents).
If the mass is expressed in units of million metric
ton (MMt), the resulting impact value has the unit
of million metric tons of CO2 equivalents (MMt
CO2 equivalents). For example, the GWP value
of the GHG sulfur hexafluoride (SF6) is 22,800.
The impact on global warming of releasing 50kg
of SF6 is thus 23,900 x 50 = 1,140,000kg CO2
equivalents. In other words, the release of 50kg of
SF6 has the same impact on global warming as the
release of 1,140,000kg of carbon dioxide.
Comparison of just the GWP values—or of just the mass emitted—
of two different gases does not provide a comparison of their
impact on global warming. Comparison of the relative impact on
global warming requires the comparison of CO2 equivalents (i.e.,
Impact = GWP x mass released).
Does a low GWP value always mean that the impact of that gas
on global warming is always low? Not always. Carbon dioxide has
a very low GWP of 1.0, but based on U.S. Environmental Protection
Agency (EPA) data,1 the impact of CO2 on global warming
is extremely high—accounting for 81.6 % of global warming! Data
from the European Environment Agency (EEA) for the EU-28
countries and Iceland is in agreement with the U.S. EPA data,
and indicates that CO2 emissions result in the largest impact on
global warming of all GHGs, accounting for a total of 81.3 % of
global warming.2
Clearly, a low GWP value does not always equate to a low
impact on global warming. In the case of CO2, the GWP value is
low, but the aggregate emissions of CO2 are enormous, stemming