Plants fluoresce. A small part of the light a leaf absorbs is not used for photosynthesis and not turned into heat, but re-emitted at slightly longer wavelengths. This is solar induced chlorophyll fluorescence, SIF.

Chlorophyll absorbs very efficiently in the blue and the red, and much less beyond about 700 nm. That drop (the red edge), as well as cell structure, is why leaves look green and why they are so bright (reflective) in the near infrared. A small fraction of what is absorbed (often ~1%) is emitted as fluorescence, in a broad band with two peaks, one in the red around 685 nm and one further into the near infra red around 740 nm.

OCO-2 and OCO-3 fit SIF in narrow spectral windows near 757 and 771 nm, in the band where the O₂ A-band is detected. In addition to O₂ absorption, the spectra contain solar Fraunhofer lines - absorption lines arising from the presence of elements and molecules in the solar atmosphere. Unlike reflected sunlight, emission from chlorophyll does not have these features and so the emission can be separated from the reflected sunlight.

The OCO instruments are extremely stable by design, and that stability is why the OCO SIF products are the world's reference standard even though fluorescence was never a baseline goal of the mission.

To Learn More about SIF and the OCO retrieval: A tutorial written by Caltech Professor Christian Frankenberg

The Solar Induced chlorophyll Fluorescence signal from every land pixel is determined from separating the 'offset' of solar Fraunhofer lines from what is expected from reflection of sunlight - this offset arises from emission from the surface. The data are averaged over a year of OCO-2 soundings and aggregated onto a 1°×1° grid. The pattern is the biosphere: the Amazon, the Congo, the maritime continent and the summer croplands of the US Midwest, the North China Plain and Europe stand out, while deserts are dark. Nothing about land cover went into this map; it is a direct measurement of where photosynthesis is happening. Blank areas are cells with too few retrievals to average — persistent cloud, low sun, or open ocean.