How do we test oxygen and carbon dioxide levels, and derive that carbon dioxide is emitted at night?

1..2..3 breathe in with Mat Disney, walking us through the emissions of the forest. 1..2..3 breathe out.

Trees use carbon dioxide (CO2) from the air as fuel for photosynthesis and as part of that process they also release oxygen (O2). [That's why forests are important in fighting climate change - they take up some of the CO2 we put into the atmosphere when we burn fossil fuels.] But plants also take in oxygen and give out carbon dioxide as part of just staying alive - this is called respiration. At night, photosynthesis stops but respiration carries on - like we carry on breathing even when we're asleep and not doing anything.

We can measure this directly by studying individual plants in a sealed space e.g. inside a glass jar where we can capture exactly what goes in and out. By shining a special infrared light through the air in the jar, we can measure how much of that light is absorbed (CO2 is a very good absorber of infrared, hence the greenhouse effect of CO2). That tells us exactly what the concentrations of O2 and CO2 are in there. And if we do it day and night we see how that balance changes.

To measure forests in the real world we use this kind of sensor mounted on towers, on aircraft and even on satellites. We see the O2 over the forest drops at night and the CO2 rises, as photosynthesis stops and respiration takes over. As the sun comes up, the balance shifts back,  the CO2 drops over the day and the O2 rises. If we do this over time and add up the differences between day and night, we get the uptake of CO2 by the forest over that time.