Note: Posts here, here, here, and here, discuss some of the different metrics like GWP, GTP, and GWP* that have been proposed for comparing the different greenhouse gases. If you aren’t already familiar with those metrics, you should probably read those posts first.
Ever since the Kyoto Protocol was drafted in 1997, GWP-100 has been the commonly agreed conversion metric for reporting and comparing greenhouse gas emissions. This is the metric, for example, that is used for assessing nationally determined contributions (NDCs) under the 2015 Paris Agreement.
Since GWP-100 has such deep flaws, other conversion metrics like GTP have been proposed. Some places, like the state of New York, now use GWP-20. More recently, agricultural interests in countries like New Zealand have pushed their governments and the UN to use GWP*.
The interactive charts on the side of this page show how the use of different conversion metrics would affect reported emissions. In the top chart you can pick any country. The bars show how many megatonnes of greenhouse gas emissions the country would be responsible for in CO₂ equivalents. First, note that the amount of CO₂ doesn’t change—that’s because CO₂ is the reference gas and has a value of one for metrics like GWP-20, GWP-100, GWP-500, GTP-50, and GTP-100. GWP* is only applicable to short-lived gases like methane, so it doesn’t change the CO₂ value either.
Next let’s take a look at the values for methane. For a really graphic example, select New Zealand from the drop-down list. Recall that methane is a very potent greenhouse but that it has a relatively short atmospheric lifetime. For this reason, it has high GWP and GTP values in the short term and lower values over the long term.
New Zealand has about 25 million sheep and about 4 million cows. These ruminants emit a lot of methane! That means if New Zealand were required to report its total GHG emissions using a metric like GWP-20, they would be almost twice as high as they are with GWP-100.
On the flip side, if New Zealand were to use GWP* to report its total GHG emissions, they would only be about half as large as they are with GWP-100.
An even more dramatic example of the power of GWP* is just a short hop over the Tasman Sea. Go to the country drop-down list and select Australia. Notice that using GWP*, Australia’s methane emissions are actually negative. This is because GWP* is calculated by comparing a country’s current methane emissions to its emissions 20 years ago. In part because of a steep decline in the number of sheep, Australia’s absolute methane emissions were a lot lower in 2021 (5.1 Mt) than they were in 2001 (6.6 Mt).
Now compare Australia to Ethiopia. In absolute terms, Ethiopia emitted about 3.4 Mt of methane in 2021. That’s a lot less than Australia’s 5.1 Mt. And it is reflected in the GWP-100 values.
But if Ethiopia was required to use GWP* to report its emissions, it would have to report the equivalent of 200 Mt of CO₂. In other words, even though it emitted less methane, Ethiopia would have to report a much higher amount than Australia!
Here’s an interactive map of the world showing by what amount each country’s reported emissions would change if they used GWP* instead of GWP-100. Hover over a country to see a popup showing the percentage by which reported emissions would increase or decrease.
Open this graphic in a new window
Notice how richer countries like those in Europe and Australia and New Zealand would see significant reductions in their reported emissions. Meanwhile, many poorer countries including several in Africa would have to report much higher emissions.
This is one of the main criticisms of GWP*: it would reward countries that have been emitting methane for a long time and punish other, often poorer, countries that are now growing their economy. It doesn’t seem fair.
Next look to the right side again and take a look at the bottom chart, which shows a comparison for five countries with very different types of emissions. Because emissions values are so different in scale, the Y axis shows values as a percent of their values under GWP-100. Select a metric at the top to look at how it would change these countries’ reported emissions compared to GWP-100.
As a result of its extensive tropical rainforest, Costa Rica absorbs a lot of carbon dioxide, enough to make up for almost all of its CO₂ emissions. A metric that discounts the importance of methane, like GWP-500 substantially reduces its emissions. Heavily industrialized Japan, on the other hand, emits a lot of CO₂. Changing metrics hardly changes its emissions at all.
In sum, the choice of metric can have an enormous impact on a country’s reported emissions. And while I haven’t shown it here, the same thing applies to any company or organization that reports emissions. Want to claim your company or country is climate neutral? Try switching your metric.
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