Chapter IV: The role of CO2 equivalent in climate change

Imagine you are back from Drupa (good old days) visit and you find one dollar, Rs 5,000, and 250 euros in your wallet. What’s the combined value? You can’t add the three different currencies together. So, you first convert all three values into a common denomination - let’s assume USD using exchange rates, only then can a single value be derived from the currencies. The same concept applies to greenhouse gases.

Similar to the currency conversion, to make meaningful comparisons between different gases and calculate their combined effect on global warming, we need to come up with a common unit of measurement - tonnes of carbon dioxide equivalent (commonly expressed as CO2e).

Before exploring this CO2e, let’s revisit a basic physics lesson on greenhouse gases. As per the Encyclopædia Britannica, greenhouse gas is any gas that has the property to absorb infrared radiation (net heat energy) emitted and reradiated back to the earth’s surface, contributing to the greenhouse effect. The primary greenhouse gases are carbon dioxide, methane and nitrous oxide among others. These gases can be emitted by both natural and anthropogenic sources such as fossil fuel combustion in cars, agriculture, landfills and air conditioners, among others.

In terms of its characteristics, each greenhouse gas traps heat in the atmosphere at different rates and has different atmospheric lifetimes. For instance, methane has a lifetime of around 12 years, while nitrous oxide stays in the atmosphere for 10 times longer, at 121 years. This leaves us with the classic problem of comparing apples to peaches. Here’s where CO2e comes in!

Scientists use carbon dioxide as a benchmark for measuring the heat-trapping ability of other greenhouse gases. To convert the other greenhouse gases into carbon dioxide equivalents, we multiply the mass of emissions by the appropriate global warming potential (GWP), which represents the relative warming effect of a unit mass of the gas when compared with the same mass of carbon dioxide over a specific period. Usually, this is 20 or 100 years - the period is important to address the different atmospheric lifetimes of the different greenhouse gases.

Source: IPCC fifth assessment report (2014)

The GWP is published by the Intergovernmental Panel on Climate Change and recognised by the United Nations Framework Convention on Climate Change. Here, you can see the GWP of several gases for 100 years. For instance, methane has a GWP of 28. That means one tonne of methane is equivalent to 28 tonnes of carbon dioxide. So, if 1,000 tonnes of methane are generated annually by a landfill site that equals 28,000 tonnes of CO2e. So, we’ve learned that not all greenhouse gases are created equal. But, by using tonnes of CO2e we have a common unit, which enables us to measure and compare different greenhouse gases and understand their combined impact on climate change.

More importantly, with CO2e, GWP and IPCC, we understand the scope and range of assessments that can be done to mitigate climate change and global warming. With few simple steps in the right direction and the available resources at hand, tackling global warming may not seem like a distant goal. Let’s explore more in the coming weeks…