For the last 25 years, scientists have been able to monitor the "greenness" of trees from space and use that as a tool for evaluating plant health. The problem is that greenness isn't a good indicator for stresses—such as drought—because some trees (think pines) continue to be a lovely green until they're dead. Researchers are thus turning to a new indictor: the way plants glow.
It turns out that a signature of photosynthesis is something known as "solar-induced chlorophyll fluorescence". During photosynthesis, while the plant is converting sunlight into chemical energy, its chlorophyll is absorbing some of the incoming radiation. Some of that incoming radiation is re-emitted at a longer wavelength, which manifests as fluorescence.
This phenomenon was first observed from space via the Japanese Greenhouse gases Observing SATellite (GOSAT) starting in 2009. In July of this year, however, NASA will launch its own satellite called the Orbiting Carbon Observatory-2 (OCO-2). The instruments on board the OCO-2 will be taking precise measurements of carbon dioxide in the atmosphere in addition to peeping large-scale plant fluorescence. The upshot? The OCO-2 can record observations a very speedy 24 times per second. Compare that to the observations made only once every four seconds by the GOSAT, and we're looking at almost a 100-fold increase in data density.
So why haven't we always been doing this? It turns out that fluorescent light is pretty hard to see when there's a lot of ambient sunlight blasting back at you. To counter this, these satellites use spectrometers—which can look at a selected, very narrow channel of light—in order to disentangle the fluorescent light from the reflected sunlight. It basically acts as a filter that blocks everything other than what needs to be seen.
This is a big win for researchers, because it will give them a better look at how plants are taking up carbon dioxide. "Everybody that's using fossil fuels right now is being subsidised by the biosphere," says Joseph Berry, a researcher in the Department of Global Ecology at Carnegie Institution for Science in Stanford, CA. "But one of the key unknowns is—what's going to be happening in the long term? Is it going to continue to subsidise us?"
NASA calculates that about half of carbon dioxide remains in the atmosphere while the other half is absorbed in the ocean or taken up by plants and animals in what is known as "carbon reservoirs" or "sinks." There is the potential that climate change could disrupt the amount of carbon that plants can take up, greatly upsetting our already tenuous carbonic balancing act.
Water is needed for photosynthesis, which is the mechanism plants use to pull carbon dioxide out of the air. As periods of drought become more common, that means plants have less water to use, and will be less able to clear our air. This becomes a vicious circle. The increased carbon in the air plays a role in climate change —> climate change contributes to droughts —> droughts lead to less photosynthesis among plants —> less photosynthesis among plants leaves more carbon in the air. Or so the theory goes.
"We really don't understand the quantitative relationship between climate and photosynthesis very well, because we've only been able to study it at very small scales," said David Schimel, lead scientist for the Carbon and Ecosystems research program at NASA's Jet Propulsion Laboratory in Pasadena. "Measuring plant fluorescence from space may be an important addition to the set of techniques available to us."
Basically, we're talking about more and better data, which will lead to better and more accurate models of climate change, which will give us a better picture of just what the hell is happening on this planet. OCO-2 is currently slated to launch in July of this year. [NASA JPL]