1-14: Global Warming Potential

1-14: En Espaņol

1-14: Em Portuguêus

Eugene S. Takle
© 1997, 2002, 2006

Introduction

We have discussed how the impact of increased greenhouse gas concentrations relates to increased radiative forcing, or heating, of the atmosphere. To calculate the magnitude of future global warming due to increasing greenhouse-gas concentrations, we need two items: (1) projections of future atmospheric concentrations of greenhouse gases and (2) a global climate model to translate increased radiative forcing into changes in surface temperatures. In this unit, we focus on the projections of future concentrations of greenhouse gases. The topic of global climate models will be covered in the second block of this course.

Global Warming Potential (GWP)

Figure 1 sketches the impact over time of adding one unit of a greenhouse gas to the atmosphere.

Lifetime of the gas is the primary factor in determining the overall warming effect of the gas. Carbon dioxide, for instance, which has an atmospheric lifetime of about 120 years, continues to contribute to radiative forcing, although with decreasing impact, for many decades. And other species, like some CFCs that have very long lifetimes, may contribute to global warming for many centuries.

We define the "global warming potential" (GWP) as the total impact over time of adding a unit of a greenhouse gas to the atmosphere. It is calculated by multiplying the effect of the instantaneous radiative forcing by the concentration of gas added and integrating over time from 0 to some arbitrary time period, T. Carbon dioxide, for instance, has relatively low radiative forcing but a very high volume of gas annually added to the atmosphere and a long atmospheric lifetime, so it has a very high GWP. The CFCs on the other hand have low concentrations but very high radiative forcing factors and very large lifetimes, so they also have very high GWPs.

Atmospheric Lifetime and GWP Compared to CO₂

Figure 2, from the IPCC report, gives the atmospheric lifetime and GWP compared to carbon dioxide for various gases over several different time horizons.

From this table you can see that methane, a short-lived species, contributes 62 times as much as carbon dioxide to the Global Warming potential over twenty years, but over longer periods of time its impact decreases because of its shorter lifetime (relative to carbon dioxide). Contrast this with CFC-13, which has a lifetime of 400 years: its GWP increases as the time horizon of consideration increases. This points out the importance of knowing the lifetime of these chemicals that we put into the atmosphere.

A byproduct of increased methane in the stratosphere is an increase in stratospheric water vapor, which arises due to oxidation of methane. So even after stratospheric methane is destroyed, its contribution to global warming continues through the presence of water vapor.

Global Warming Contribution

Figure 3 gives the relative contribution to global warming caused by different chemical species, with an assumed 100-year time horizon and 1990 emissions estimates.

Carbon dioxide, with its enormous annual increase in concentration, contributes most, at 61%. Methane is second in importance, at 15%, CFC-12 is third, contributing 7%, and nitrous oxide fourth with 4% of the warming under these assumptions. If we took the 500-year horizon, the percentages would change, with longer-lived species contributing larger percentages.

The question now is how do we estimate future emissions of greenhouse gases? Past trends can be used as a starting point, but future emissions will be determined by a complex combination of economic, regulatory, and societal factors. Some, like the CFCs, can be changed relatively quickly by switching to different chemicals for certain industrial and manufacturing processes and consumer products (e.g., eliminating Styrofoam cups that use CFCs). Others like the burning of fossil fuels cannot be changed very quickly, even if society chooses to do so (it takes many years to put a nuclear power plant in operation to replace a coal-fired plant). Since no one can predict the future with any certainty, we resort to considering several different scenarios, each of which gives a specific set of assumptions on economic, political, and social factors.

Different Future Socio-Economic Development Paths

Assumptions of future economic conditions have two dimensions:

1. Will future societies be more environmentally oriented or continue on a high consumption path?
2. Will future societies look globally or locally for solutions to economic, social, and environmental sustainability?

Figure 4 shows four families of scenarios depending on these behavioral patterns of future societies as described in the Special Report on Emissions Scenarios (SRES). Each of these four families has a "storyline" that describes the global population and energy consumption patterns and the associated greenhouse gas emissions:

Scenario A1 represents high economic growth and global perspectives to economic and environmental issues. It is further subdivided into a scenario continuing to emphasize intensive use of fossil fuels (A1FI), one being energy intensive but with emphasis on use of non-fossil energy (A1T), and a scenario with a balance of fuel sources between fossil and non-fossil (A1B). Global population peaks about 2050 and then declines.

Scenario A2 assumes self-reliance and preservation of local identities. Developing regions are less influenced by developed countries, so that, for instance, fertility follows local historical traditions rather than patterns of developing countries. Global population does not peak in mid-century. Economic development is linked to regional rather than global patterns.

Scenario B1 has global population peaking around 2050 and declining thereafter. Economic growth is more globally linked but with introduction of clean and resource-efficient technologies. Social equity is emphasized with global attention to economic, social and environmental problems. However, there are no global restrictions on emissions of greenhouse gases.

Scenario B2 has an increasing global population, but somewhat less than A2, which does not peak in mid-century. Emphasis is on a local approaches to addressing economic, social, and environmental sustainability. Emphasis is on environmental protection and social equity through local approaches. Economic development is not as rapid as in B1 and A1 but with more diverse technological change.

Resulting Emissions Scenarios and Atmospheric Greenhouse Gas Concentrations

Each of these scenarios creates a somewhat different pattern of greenhouse gas emissions. The following figures show the emissions corresponding to each of these scenarios and the time record of atmospheric concentrations of greenhouse gases that will result from each scenario.

CO₂ Emissions and Concentration*
N₂O Emissions and Concentration*
CH₄ Emissions and Concentration*
SO₄ Emissions and Concentration*

* Adapted from IPCC, 2001: Climate Change 2001: Synthesis Report. A contribution of Working Groups I, II, and III to the Third Assessment Report of the Intergovernmental Panel on Climate Change [Watson, R.T. and the Core Writing Team (eds.)]. Cambridge University Press, Cambridge, United Kingdom, and New York, NY, USA, 398 pp.