Way up in the sky and sprinkled across the seas, two of the littlest yet most influential things in the world have stubbornly guarded their secrets: aerosols and phytoplankton. Today, NASA launched its Plankton, Aerosol, Cloud, Ocean Ecosystem mission, or PACE, to unravel their mysteries. The mission’s findings could be a key to understanding how drastically the world is changing as it warms.
Aerosols are little bits of dust, wildfire smoke, and fossil fuel pollution floating around the atmosphere, which both absorb and reflect the sun’s energy and help build clouds—wildly complex dynamics that climate models still struggle to account for. And phytoplankton are the microscopic, plant-like marine organisms that form the foundation of the food web. They also sequester carbon, keeping Earth’s climate from warming even further. “Phytoplankton are basically moving carbon around, and we need to understand how that changes with time,” says Jeremy Werdell of NASA’s Goddard Space Flight Center.
PACE is a satellite observatory that’ll provide scientists with unprecedented views of these ultra-important denizens of the skies and seas, to help them try to predict how our world will evolve. “The warming atmosphere and warming oceans have a cost, and that cost from a biological point of view is that the base of the food chain will unequivocally change,” says Werdell, who is the project scientist of PACE.
Though phytoplankton are minuscule, they bloom in such numbers that they smear great green streaks across the oceans. That’s been easy enough to monitor by satellite, sure, but up until now what’s been observed has been more or less a uniform streak of green. But PACE is equipped with an extremely sensitive instrument that can see in high resolution across the electromagnetic spectrum, from ultraviolet to the near infrared. (The visible spectrum, which we can see, is in between the two.) The effect is that PACE can see all kinds of different greens.
Think about what you see staring into a forest. “All the leaves on the various trees are green, but they’re very subtly different greens, which means they’re different plants,” says Werdell. “Really what we’re searching for are these very, very subtle changes in color.”
That’ll allow scientists to determine not just where phytoplankton are blooming and why, but what kind of community that creates. There are thousands upon thousands of phytoplankton species—some that act as food for tiny animals known as zooplankton, others that are highly toxic, some that sequester carbon better than others. What modern satellites can see from space is like drawing with a box of eight crayons, but the species will look different to PACE’s eye. “What we’re getting with PACE is a box of 128,” says Werdell.
Better understanding these phytoplanktonic communities is critical because of how rapidly the oceans are transforming. They’ve absorbed something like 90 percent of the excess heat humanity has added to the atmosphere, and over the past year or so in particular, sea surface temperatures have soared to record highs and stayed there. The high temperatures themselves might adversely affect the growth of some phytoplankton species, but might actually benefit others that thrive as the mercury climbs.
More subtly, warm water acts like a kind of cap at the ocean surface, with cooler waters swirling below. “It’s kind of like drinking a half and half at your favorite Irish pub: Guinness floating on top of Harp,” says Werdell. “That creates a barrier in this huge stretch of real estate in the upper ocean, where nutrients in the cold water underneath this layer of warm water can’t penetrate.”
Phytoplankton need those nutrients to grow, so if the cap of warm water persists in a given area, that’ll further shake up the community of photosynthesizing species. If there’s less of the species that zooplankton need for food, their numbers may decline too. And then the larger predators like fish that eat the zooplankton will be impacted, on up the food chain. That could eventually affect the food species that humans rely on for protein.
Some phytoplankton species also produce toxins that can kill marine mammals like sea lions and accumulate in seafoods like shellfish that we humans eat. With PACE, scientists can get a better idea of whether a given algal species is proliferating in a given phytoplankton bloom, and what conditions precipitated that. “There's always been harmful algal blooms, but with changing ocean conditions, we’re seeing them change in different ways,” says oceanographer Tom Bell, who uses satellite imagery to study phytoplankton at the Woods Hole Oceanographic Institution in Massachusetts. “There might be higher concentrations of harmful algal species at different times a year than we’re used to, or at a different frequency or duration.”
The PACE team has developed algorithms that automatically parse this data. “That takes basically this ocean color signal and converts it, let’s say, to the concentration of harmful algal species,” says Dariusz Stramski of California’s Scripps Institution of Oceanography, who is a principal investigator for PACE and worked on its algorithms. “So perhaps a species starts growing, and this is the onset of a harmful algal bloom—and this can happen right away, because the data are almost in real time.”
As phytoplankton photosynthesize and grow, they also absorb carbon. When they die and sink, or get eaten by zooplankton and packaged in a fecal pellet, some reach the seafloor, locking that carbon in the depths of the oceans for potentially thousands of years. We’re talking a lot of carbon here: Last month, scientists calculated that bottom-trawling boats are churning up so much of the seafloor and releasing so much CO2 stored in this manner, it adds up to double the fossil fuel emissions of the world’s whole fishing fleet—all 4 million vessels.
With PACE, scientists can get a better handle on which species of phytoplankton might be winning or losing in rapidly changing ocean conditions, and how that’ll go on to influence the carbon cycle. They need to be able to see which species are recycling carbon in the upper, sun-lit part of the ocean—and trapping it there—versus which plankton communities are helping take carbon down to the deep ocean.
At the same time, PACE will use two other instruments, known as multi-angle polarimeters, to take snapshots of the atmosphere from different angles. Think of previous techniques as being like looking at aerosols in 2D, whereas this is more 3D. “These instruments effectively have polarized sunglasses on, and it offers a view of the world that is completely different,” says Werdell. “By looking through the atmosphere at different angles, you have more information about thicknesses of an aerosol plume, or clouds, or their location vertically in the column. So when you put these together—the polarization information with the multi-angle view of all of this—now you have this real quantum leap forward.”
Aerosols remain a major climate wildcard. Depending on the material, aerosols can both absorb and reflect the sun’s energy, so they can have a cooling or heating effect. An unfortunate side effect of decarbonization currently under study is that by burning fossil fuels less, we could be emitting less of the aerosols that help cool the climate. So while we absolutely must stop putting carbon into the atmosphere, in doing so we may get some additional warming without those additional aerosols. (Which, by the way, are also catastrophic for human health—yet another reason to rapidly decarbonize.)
But modeling the extraordinary complexity of an atmosphere swirling with aerosols remains difficult. In addition, aerosols act as nuclei for water vapor to glom onto, thus forming clouds. Depending on the variety, those clouds can either trap heat against the Earth or help cool it by bouncing the sun’s energy back into space, adding still more complexity. “There’s this entire interplay between aerosols and clouds, pollution, or dust from the Sahara Desert, or volcanic eruptions, or marine haze, that really fuels this whole system,” says Werdell. “If we can take steps forward in understanding the clouds—their brightnesses and their thicknesses and their response to aerosols—we’ll know more to beat down these uncertainties.”
