Carbon dioxide concentrations range from three parts per million below background levels (dark blue) to three parts per million above (pale yellow). The high emissions captured over Poland and Germany (top centre) and Iraq and Kuwait (right) are mostly due to burning fossil fuels. The high emissions over sub-Saharan Africa (bottom) are mostly due to fires. Finnish Meteorological Institute

Carbon dioxide emissions have been mapped in high resolution by NASA’s Orbiting Carbon Observatory-2 (OCO-2) satellite, showing widespread carbon dioxide across major urban areas and smaller pockets of high emissions emanating from individual cities.

In Geophysical Research Letters, a trio from the Finnish Meteorological Institute in Helsinki led by Janne Hakkarainen provides the first direct observation of anthropogenic carbon dioxide from OCO-2 over three main pollution regions: eastern US, central Europe and East Asia.

Launched in 2014, OCO-2’s path covers most of the globe every 16 days. It tracks carbon dioxide by measuring sunlight bouncing off the planet’s surface.

As the light filters through the atmosphere, carbon dioxide molecules absorb some of the infrared light. OCO-2 detects these infrared differences.

But the biggest hurdle, the researchers say, was untangling recent carbon dioxide emissions from older ones.

The gas hangs around in our atmosphere for a century or so, meaning recent human output is only a tiny fraction of the total carbon dioxide that OCO-2 records as it looks down at Earth’s surface.

“Currently, the background level of carbon dioxide in the atmosphere is about 400 parts per million, and human emissions within the past year may add only something like three parts per million to that total,” Hakkarainen says.

So the team put together a new data processing technique. It accounts for seasonal changes in carbon dioxide, the result of plant growth and dormancy, as well as the background carbon dioxide level.



To be sure their method was correct, they compared the results with measurements of nitrogen dioxide – another gas emitted by burning fossil fuels – from the Ozone Monitoring Instrument, a Dutch-Finnish instrument on NASA’s Aura satellite.

Aura and OCO-2 cover the same area of Earth and are separated by only 15 minutes. The two measurements correlated well, giving the researchers confidence that their new technique produced reliable results.