Global Warming Potential
Introduction
Global Warming Potential (GWP) is a measure of how much energy the emissions of one ton of a gas will absorb over a given period of time, relative to the emissions of one ton of carbon dioxide (CO2). It is a critical concept in understanding the impact of greenhouse gases on global warming and climate change. The GWP provides a common platform for comparing different gases and their potential impact on global warming.
Understanding Global Warming Potential
The GWP is a relative scale which compares the amount of heat trapped by a certain mass of the gas in question to the amount of heat trapped by a similar mass of carbon dioxide. As such, it is a unitless measure. The GWP is calculated over specific time intervals, typically 20, 100 and 500 years. GWP values are often used in discussions and analyses of trade-offs between emissions of different gases, particularly in the context of international agreements to limit greenhouse gas emissions.
Calculation of Global Warming Potential
The calculation of GWP involves the integration of the radiative forcing (the change in energy in the atmosphere due to the influence of a factor or factors) caused by the instantaneous release of one kilogram of a trace gas, relative to that of one kilogram of a reference gas, over a specified time horizon. The reference gas used is CO2, and it has a GWP of 1 regardless of the time horizon used for calculation.
Factors Influencing Global Warming Potential
The GWP of a gas depends on several factors, including the gas's lifespan in the atmosphere, the wavelength of radiation the gas absorbs, and the strength of the radiation at those wavelengths. The longer a gas stays in the atmosphere, the more it contributes to warming. Similarly, gases that absorb radiation at wavelengths where other gases do not absorb much contribute more to warming.
GWP of Common Greenhouse Gases
The GWP varies widely for different gases. For example, over a 100-year period, methane (CH4) has a GWP of 28-36, meaning it is 28-36 times more potent at trapping heat in the atmosphere than CO2. Nitrous oxide (N2O) has a GWP 265-298 times that of CO2 for the same time period. Some hydrofluorocarbons (HFCs) and perfluorocarbons (PFCs) have GWPs that are thousands of times larger than CO2.
Use of GWP in Policy and Regulation
GWP values are used in formulating policies and regulations for reducing greenhouse gas emissions. For example, in the Kyoto Protocol, countries are required to reduce emissions of a basket of six greenhouse gases. The GWP values are used to convert emissions of these gases into carbon dioxide equivalents (CO2e), which can then be summed to give a single figure for total greenhouse gas emissions.
Criticisms and Limitations of GWP
While GWP is a useful measure, it has its limitations. It does not take into account the spatial distribution of the emissions, which can influence the climate system. Also, it does not consider the indirect effects of the emissions, such as the impact on ozone levels. Some critics also argue that the choice of time horizon is arbitrary and can influence the GWP values significantly.