If you haven’t read it already, you may want to read this post about GWP first.
From the moment it was created more than 30 years ago, climate scientists have known that the concept of Global Warming Potential had some major drawbacks. Since different greenhouse gases have different radiative efficiencies and different atmospheric lifetimes, there is no simple way to compare their effects. Here let’s talk about an alternative metric for comparing greenhouse gases called Global Temperature Potential, or GTP.
You can see the values for GTP-50 and GTP-100 in table 7.15 from AR6 at the bottom of this post. Note that just like GWP, the GTP values depend on time: in this case they have been calculated for 50 years and 100 years respectively. However, unlike GWP, which reflects cumulative radiative forcing over time, GTP depends only on the global average surface temperature value at the time in question. Whatever happened in the past does not matter. To put it another way, GWP reflects the area under the curve, while GTP reflects the value of the line at one point in time.
Note that GTP values are also unitless—they simply compare the temperature effects of a gas to the reference gas, which is CO₂. In other words, you might say that if you release 1 tonne of CH₄-non fossil, the resulting change in the earth’s temperature after 100 years will be 4.7 times higher than if you released 1 tonne of CO₂.
How did the IPCC derive these unitless GTP values? They first had to calculate something called Absolute GTP (AGTP). Unlike GTP, AGTP does have units—the change in global average surface temperature that will result from an emission.
To calculate AGTP, researchers used computer models to simulate the release of a greenhouse gas and then tracked the effects on global average surface temperatures over time. They used the results to create and parameterize the equation that is used in the interactive graphic here. Once you have AGTP values, it is simple to calculate GTP. You just divide the AGTP value of the gas in question by the AGTP value for the reference gas, which is CO₂.
Now let’s put these equations to work. Set the values in the interactive visualizations below to:
- GtCO₂: 1
- GtCH₄: 1
- GtN₂O: 0
- Years: 100
See how potent CH₄ is in the first few years after its release? See how its effects diminish rapidly after about 12 years? That’s because it has such a short atmospheric lifetime. Carbon dioxide, on the other hand continues to exert an effect on temperature for a very long time after its release.
Next let’s consider the implications for the carbon offset market. Carbon markets typically use GWP-100 to compare emissions. Suppose I was going to emit one tonne of CH₄ and I wanted to offset those emissions so that I could claim my operations were carbon neutral. According to the table above, I could pay somebody else to remove 29.8 tonnes of CO₂. Try setting the values in the visualizer to:
- GtCO₂: -29.8
- GtCH₄: 1
- GtN₂O: 0
- Years: 100
Logically, you might think that if something is carbon neutral, it will have no effect on temperatures. It should be as though there were no emissions at all. But from the combined line in the graph, clearly that is not the case. At first, thanks to my trade, temperatures on the earth will be hotter than they would have been if there had been no emissions at all. Then, after about 35 years, temperatures will actually be lower than they would have been in a true no-emissions scenario—a result of the different lifetimes and radiative efficiencies of the two gases. Flip that trade around and you’ll get the opposite result.
In the end, GTP suffers from many of the same drawbacks as GWP. Different greenhouse gases differ in both radiative efficiency and atmospheric lifetime. You have to set a time period of interest, and that requires judgments of value as well as science.
Or perhaps the very short life of methane just makes it fundamentally different from carbon dioxide and the topic demands a completely rethink. This post explores the another recently developed metric called GWP*.
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