How are greenhouse gas emissions calculated?

Discover 4 different ways to know the volume of gases that vehicles, ships and even crops release into the atmosphere

The gases from greenhouse effect —such as carbon dioxide (CO2), O methane (CH4) and nitrous oxide (N2O)— are always mentioned when we talk about global warming. To avoid excessive increases in the Earth's temperature, experts recommend reducing emissions. At this point, a question arises: if we can't even see these gases, how is it possible to measure their presence in the atmosphere accurately?

This was the question raised by the Short Wave podcast, from NPR (National Public Radio, public radio network in the United States). Science reporter Rebecca Hersher went after experts to find the answers and discovered four different ways to measure emissions from different sources, such as vehicles, factories, ships and farms — find out what they are below.

Sensors in concentrated sources

The best way to measure emissions from concentrated sources, such as vehicles, factories and even barbecue grills, is to connect a sensor to the gas exhaust pipe and measure what comes out of there, explains to the program Kent Johnson, professor at the University of California and researcher of the theme. “Everything that burns and generates emissions, we measure,” he says.

But, to know how much each activity emits as a whole, it is necessary to use computer models that make the calculation taking into account estimates of how many cars and trucks are on the road, for example.

Fuel consumption calculation

Placing a sensor in the exhaust is a solution that is not suitable, for example, for measuring how much carbon dioxide is released by transport ships that make long journeys. “The ship's environment is very challenging: it's hot, it's at sea and it's rocking,” explains Bryan Comer, who studies emissions at the International Council on Clean Transportation.

The solution is simple: find out how much fuel is supplied to ships and calculate the CO emissions2 from this volume. “By knowing how much fuel the ship is consuming, it is possible to say, with great precision, the amount of carbon dioxide released,” says Bryan. “Marine fuels emit 3.1 to 3.2 tons of carbon dioxide per ton. In total, the global shipping industry is emitting around one billion tons of carbon dioxide each year.”

“Baldinho” on the ground

Farms are also sources of greenhouse gases: when turning the soil for cultivation or cutting down trees, carbon dioxide is released into the atmosphere — the removed plants, in fact, reduce the capacity of that area to remove CO2 of the environment in photosynthesis. 

“With fertilizer application, husbandry or livestock practices, and land clearing, agriculture contributes about 25% of global greenhouse gas emissions,” says Ben Houlton, environmental scientist and dean of the Faculty of Agriculture and Life Sciences at Cornell University.

How is this measured? “Imagine that you have a 7.5 liter bucket and you tip it over the ground and you are trying to capture the gases emitted by microscopic creatures and plants”, proposes Ben. “Then you take what you collected and measure how much carbon dioxide or other greenhouse gases were released,” he adds. With this number in hand, you can calculate how much was released in one square meter and in the entire area of the farm.

Towers and satellites

In more extensive plantations, there is more sophisticated technology to make this calculation. In this case, towers can measure emissions from larger areas of land using infrared sensors that are capable of covering a space the size of ten football fields, Ben points out. And, with the use of satellites, it is possible to have a global image of emissions from large plantations.

From space, satellites can have an even broader view of the entire Earth. These devices are capable of visualizing the area covered by trees, for example, while an instrument on the ground indicates how much carbon dioxide is being emitted or absorbed by plants.

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