Quantifying global warming potential

written by Pavel Makhnatch (under supervision of Rahmatollah Khodabandeh and Björn Palm)

Published Apr 23, 2014

GWP stands for Global Warming Potential and is an index used to compare the greenhouse effect of greenhouse gases over the single CO2 equivalent scale. As might appear at first sight the GWP index is a simple value that converts the contribution of the substance to global warming to that of CO2. In fact, science, and calculations behind values are relatively complex and relies on complicated interactions. In this article, we will try to sort things out and clarify some of the complex processes and calculations underlying the GWP values.

Many refrigerants are strong greenhouse gases (GHG) – gases that can absorb and emit infrared radiation and thus create greenhouse effect. As the rate at which various gases can absorb and emit infrared radiation varies from one substance to another, the Global Warming Potential (GWP) index has been introduced in order to put the contribution of all the non-CO2 GHGs gases to the climate change over the normalized CO2 scale.

An introduction to the global warming

Energy received from the Sun is reflected or absorbed by our planet. This energy is then emitted by the Earth’s atmosphere into the space in amount nearly equal to the absorbed energy amount. The balance between absorbed and radiated energy determines the average temperature of the Earth.

Greenhouse gases absorb certain wavelengths of outgoing radiation and reradiate it. Some of the reradiated energy is directed back towards the Earth. The balance between absorbed and radiated energy determines the average temperature of the Earth surface.

Radiative Forcing (RF) is an essential concept in the climate change science. It can be defined as the net change in the energy balance of the Earth system due to some imposed disturbance. It is usually expressed in W/m2 averaged over a particular period of time and quantifies the energy imbalance that occurs when the imposed change takes place. Due to the imbalance, the Earth system needs to find new equilibrium state and thus the global-mean surface temperature have to change.

Generally speaking, RF indicates the difference of radiant energy received by the Earth and radiated back to space (in watts per square meter of Earth’s surface) as a result of an emission of GHG into atmosphere, after allowing stratospheric temperatures to readjust to radiative equilibrium, while holding surface some other variables at undisturbed values. Intergovernmental Panel on Climate Change (IPCC) specify the radiative forcing as the change in global annual net flux relative to the year 1750, i.e. before the Industrial Revolution, when the RF set to 0 W/m2. The largest increase has occurred in recent decades: RF indicated by IPCC to have increased 0.57 W/m2 in 1950 to 2.29 W/m2 in 2011 [1]. To separate the effect of particular substance on RF, traditional radiative forcing is computed with all tropospheric properties held fixed at their undisturbed values. However, alternative definitions to RF are investigated in order to address rapid adjustments in the atmosphere to the emission of the gas, as well spatial distribution effects, particularly relevant to short-lived compounds (e.g. HFOs) that are not well mixed in the atmosphere [2].

How is the "atmospheric lifetime" defined?

Greenhouse gases contribute to global warming as long as they are located in the atmosphere. Ones emitted in the atmosphere, greenhouse gases are not there forever - they get gradually removed through chemical reactions with other reactive components or other processes. These processes characterize the lifetime of the gas in the atmosphere, defined as the time it takes for a concentration pulse to decrease by a factor of e (i.e. to 1/2.71 of the original concentration) [1].

Lifetime of different refrigerants varies over a wide range, from days to thousands of years. For example, HFO 1234yf have a lifetime of 10.5 days and HFC-23, produced as a byproduct during the manufacturing process of some refrigerants, of about 222 years. CO2 is more complicated as it is removed from the atmosphere through multiple physical and biogeochemical processes in the ocean and the land; all operating at different time scales. For an emission pulse of carbon, about half is removed within a few decades, but the remaining fraction stays in the atmosphere for much longer. About 15 to 40% of the CO2 pulse is still in the atmosphere after 1000 years [1].

Global Warming Potential

GWP stands for the Global Warming Potential. GWP is generally perceived as a metric that indicate the contribution of a substance to global warming in comparison to that of CO2. It defined as an index, based on radiative properties of greenhouse gases, measuring the radiative forcing following a pulse emission of a unit mass of a given greenhouse gas in the present day atmosphere integrated over a chosen time horizon, relative to that of carbon dioxide [1]. Taking in mind the definition of RF, GWP is an index of the total energy added to the climate system by a component in question relative to that added by CO2. It can be presented as the ratio of the time-integrated RF due to a pulse emission of a given component value (absolute GWP or AGWP) of some gas to the AGWP of CO2 (see Figure 1) 

Figure 1 – Global Warming Potential definition components for a number of gases [1].

The blue hatched field on Figure 1 represents the integrated RF from a pulse of CO2, whereas green and red fields represent example gases with lifetime of 1.5 and 13 years respectively. Based on the RF values distribution over the time it is easy to see, that the GWP values are very dependent on the chosen integration time horizon. For instance, for shorter time horizons the GWP of example gases are expected to be higher than their GWP values over the longer time horizons.

IPCC has usually presented GWP for 20, 100 and 500 years and the Kyoto Protocol has adopted GWPs for a time horizon of 100 years (referred as GWP100 in this article). Although initially these three time horizons were presented as “candidates for discussion that should not be considered as having any special significance” and considering that the choice of time horizon has a strong effect on the GWP values, the 100 years time horizon has practically become a standard for the policy makers. However, there is no scientific argument for selecting 100 years compared with other choices [1].

Great uncertainty in tabulated GWP values

IPCC assessment reports have been listed updated values of GWP over the time since the first IPCC report from 1990. The values are updated due to new scientific knowledge about various properties, but also due to changes in lifetimes and radiative efficiencies caused by changing atmospheric background conditions. Table 1 summarizes the GWP values listed in the IPCC reports for some common refrigerants over the years. Not only are the reported GWP values not constant, each of the reported GWP value is uncertain. For instance, the uncertainty for gases with lifetimes of a few decades is estimated to be of the order of +-35% for 100 years GWP. In other words, GWP of R134a (GWP100=1300) can be anywhere in the range from 845 to 1755.

The variation in the listed GWP values arise from improved knowledge and new estimates of the parameters important for GWP estimation: lifetime and radiative forcing values. Even if only the AGWP value for the reference CO2 is updated, it automatically affects all the other GWP values.

Table 1 – GWP100 values development of some refrigerants over the years

Refrigerant/Year

1995 [3]

2001 [4]

2007 [5]

2013 [1]

R32

650

550

675

677

R125

2800

3400

3500

3170

R134a

1300

1300

1430

1300

R152a

140

120

124

138

Taking into account that GWP values differ from year to year, it is important not to mix up the values from different years in a single comparison. Policies that rely on GWP values normally refer to certain source of GWP values. Thus, for instance the European MAC Directive refers to the GWP100 values published in the IPCC third assessment report adopted in 2001.

There are other suggestions on how to compare carbon footprint of refrigerants. In the following article we, among others, review the concept of GTP, Global Temperature change Potential, proposed as an alternative to GWP.

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Works Cited

[1]

IPCC, "Fifth Assessment Report," Cambridge University Press, Cambridge, Great Britain, New York, NY, USA and Melbourne, Australia, 2013.

[2]

Hodnebrog et al., "Global Warming Potentials and Radiative Efficiencies of Halocarbons and Related Compounds: A Comprehensive Review.," Reviews of Geophysics, vol. 51, no. 2, pp. 300-378, 2013.

[3]

IPCC, "Second Assessment Report," Cambridge University Press, Cambridge, Great Britain, New York, NY, USA and Melbourne, Australia, 1995.

[4]

IPCC, "Third Assessment Report," Cambridge University Press, Cambridge, Great Britain, New York, NY, USA and Melbourne, Australia, 2001.

[5]

IPCC, "Fourth Assessment Report," Cambridge University Press, Cambridge, Great Britain, New York, NY, USA and Melbourne, Australia, 2007.

[6]

IPCC, "First Assessment Report," Cambridge University Press, Cambridge, Great Britain, New York, NY, USA and Melbourne, Australia, 1990.

Page responsible:bpalm@energy.kth.se
Belongs to: Department of Energy Technology
Last changed: Apr 23, 2014

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