Trade-offs and compromises are inevitable when designing something new. It is a challenge to make a laptop durable and lightweight, for example, or a smartphone small and have all of the desired features. Or there is the classic trade-off: high quality or low cost.
The ubiquity of trade-offs makes it remarkable when innovators find win-win scenarios. This is the case with the NASA Orbiting Carbon Observatory-2 satellite (OCO-2). OCO-2 was designed to monitor global atmospheric carbon dioxide levels, which it does by measuring sunlight absorption by the greenhouse gas (Figure 1).
But OCO-2 can also measure solar-induced chlorophyll fluorescence (SIF), a critical piece of information for understanding carbon cycling. SIF is light that is given off by chlorophyll as a byproduct of photosynthesis. It just so happens that the equipment on OCO-2 can be tuned to measure SIF in addition to carbon dioxide. This is a win-win, described as “serendipitous” by NASA scientists, because the SIF measurement adds so much value to the OCO-2 observations yet it didn’t cost anything to include the measurements on the satellite. A paper by Ying Sun and colleagues that describes OCO-2’s SIF measurements was recently published in the journal Science.
The other half of the carbon cycle
The level of SIF depends on the amount of sunlight absorbed by chlorophyll molecules in plant cells, as does photosynthesis. Because of the link between SIF and photosynthesis, SIF measurements tell us about plant functioning and, importantly, the amount of carbon plants take up during photosynthesis. This is important because it would be very handy to have global measurements of plant carbon uptake. Currently, only half of the carbon dioxide emitted by human activities stays in the atmosphere; the other half is taken up by vegetation (25%) or the ocean (25%). Scientists who study this uptake by natural ecosystems call it the “other half” of the carbon cycle. This terminology is a shorthand way of distinguishing between the “first half” of the carbon cycle — human emissions of carbon dioxide to the atmosphere — and the land and ocean sinks of carbon dioxide.
We have a pretty good estimate of the first half of the carbon cycle, because it is tied to economic activities. The other half is more of a mystery. Furthermore, there is a lot of uncertainty about the future of these carbon sinks. Annmarie Eldering, the deputy project scientist for OCO-2, put it this way in a NASA video: “A huge question is, in the future as carbon dioxide builds up, will the land and the ocean continue to take up that 50%? Do they get saturated? Are they full and they quit at some point? Or do they always just take up more and more?” The answer to this question, whether the land and oceans will quit or take up more and more, has big implications for the “first half” of the carbon cycle: how much humans can emit. If the land or ocean sink slows down as more greenhouse gases build up, that would be a climate feedback, accelerating the rate of climate change.
An eye in the sky for photosynthesis: verifying the link between SIF and carbon uptake
Global measurements of SIF by the OCO-2 satellite are helping answer the question of how uptake of carbon dioxide by vegetation will respond to a changing climate (Figure 2). But first, scientists need to establish a relationship between SIF and photosynthetic carbon dioxide uptake. Although SIF is linked to photosynthesis, how much of the SIF light reaches the satellite depends on the canopy structure, that is, leaf angles, leaf clumping, and chlorophyll profiles of different vegetation types.
An advantage of OCO-2 for this purpose is that it was designed with a fine enough spatial resolution that it’s measurements can be compared to ground-based measurements. The spatial resolution of OCO-2 is less than three square kilometers, meaning that the satellite measurements can tell the difference between SIF levels in two neighborhoods or ecosystems that are just several kilometers apart.
The study authors used carbon dioxide uptake data from several ground-based sites that matched up geographically up with OCO-2 overpasses to check the SIF measurements. They found that not only was there a strong relationship between SIF and carbon dioxide uptake, the relationship was similar between forests, grasslands, and cropland. This suggests that SIF could be used as a “shortcut” to estimate carbon dioxide uptake via photosynthesis.
Putting this information to use
Now that we’ve established how to test the SIF measurements, and we’ve seen that the SIF measurements have passed the initial tests, how can we use this information to answer the big question about climate feedback? How might the “other half” of the carbon cycle, uptake by vegetation, change in the future?
The study authors propose combining SIF measurements with OCO-2 carbon dioxide concentration measurements. Carbon dioxide levels are the net result of emissions and uptake. Having information on two of the components enables researchers to disentangle the carbon dioxide signal, shedding light on the processes that control the carbon budget. Furthermore, continued monitoring covering periods across different seasons, temperatures, and levels of water stress will give further insight into how these climate conditions affect vegetation carbon uptake across the globe.
Source: Sun, Y., C. Frankenberg, J.D. Wood, D.S. Schimel, M. Jung, L. Guanter, D.T. Drewry, M. Verma, A. Porcar-Castell, T.J. Griffis, L. Gu, T.S. Magney, P. Kohler, B. Evans, and K. Yuen, 2017. OCO-2 advances photosynthesis observation from space via solar-induced chlorophyll fluorescence. Science 358 (189). http://dx.doi.org/10.1126/science.aam5747