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Remote Sensing

When will global warming change the color of the ocean?

Source: Dutkiewicz, S., Hickman, A. E., Jahn, O., Henson, S., Beaulieu, C., & Monier, E. (2019). Ocean colour signature of climate change. Nature communications, 10(1), 578. DOI:10.1038/s41467-019-08457

While many people muse over the color of the sky, Dr. Stephanie Dutkiewicz of the Massachusetts Institute of Technology wonders about the color or the ocean. With all sorts of changes predicted to happen in the ocean due to global warming, she wanted to know how these changes would impact ocean color, and when they would cause a detectable change in the color of the water.

The color of the ocean is influenced by a number of factors, including the amounts and types of phytoplankton, decaying matter and other particulates suspended in the water. Among these, phytoplankton are primarily responsible for the greenness of seawater. Scientists often measure the population of phytoplankton in the ocean with satellite images of ocean color, using the amount of green in these images to infer the amount of chlorophyll in the ocean, and therefore how many phytoplankton are there. Phytoplankton are hugely important for the ocean ecosystem; they help  sequester carbon to the deep ocean and they form the base of the marine food chain.

But as our planet and ocean warm, ocean mixing is expected to slow down because it’s harder for warmer water, which is less dense, to mix with cool, denser water. Decreased ocean mixing means fewer nutrients brought to the surface for phytoplankton to consume. This spells trouble for larger phytoplankton, which are less competitive than smaller ones in low-nutrient settings (their lower surface area-to-volume ratio means they have a harder time pulling in the nutrients they need).

However, ice melting in cooler regions will create more areas with livable conditions for phytoplankton, and warmer temperatures will speed up their growth rates across the globe, further affecting the size and composition of phytoplankton populations in the future.

Phytoplankton communities are made up of a number of different types of phytoplankton, such as diatoms (Figure 1), dinoflagellates and algae. These species all have their own unique collection of pigments and therefore influence ocean color differently. Changes in ocean temperature, mixing and nutrient availability are all expected to alter phytoplankton community composition, leading to changes in ocean color as inferred from RRS.

Figure 1. A pennate diatom. Source: Derek Keats via Wikimedia Commons

Other factors that determine the color of the ocean, including colored dissolved organic matter (CDOM, very small organic material that absorbs light) and detritus (e.g. dead organisms, silt), will also be impacted by a changing climate. If ocean mixing slows, CDOM will spend more time at the sea surface where bleaching by sunlight changes its color. And if the number of large phytoplankton in the water diminishes, there will be fewer large organisms to die, break apart, and add to the collection of detrital matter suspended in the water.

Recognizing the numerous predicted changes to phytoplankton and other material that influence ocean color, Dutkiewicz and her colleagues wondered how global warming would change the color of the ocean. Further, they wanted to know which measurement of ocean color would show the earliest global warming-driven change.

Remotely-sensed estimates of chlorophyll are used for inferring phytoplankton populations (Figure 2), and the scientists started by modeling changes in chlorophyll. They found that chlorophyll was predicted to decrease over wide swaths of the ocean, meaning that there the ocean would become less green, though the shifts would be imperceptible to the human eye. However, Dutkiewicz wanted to examine other measurements that were affected not only by phytoplankton populations but also by other determinants of ocean color, such as CDOM and detritus to get a fuller picture of changing ocean optics.

 

Figure 2. An image of average sea surface chlorophyll concentrations (light green) during the Northern Hemisphere spring. The data are average remotely-sensed concentrations taken by the SeaWIFS satellite between 1998 and 2004. Source: NASA via Wikimedia Commons.

 

The satellite estimates of chlorophyll are derived from remotely-sensed measurements of light reflectance at the ocean surface. The ratio of light leaving the water to the light striking the water is referred to as remote sensing reflectance (RRS) and is influenced by anything that changes the optics of seawater. Dutkiewicz and her colleagues looked specifically at RRS in the in the blue-green range, between 467-510 nm.

To attribute a shift in RRS to climate change means that the shift has to be discernable from the naturally occurring variation in RRS, and the time at which this shift becomes detectable is called the “time of emergence.” Running their model simulations to 2100, Dutkiewicz and her colleagues found a significant (discernible from natural variability) trend in RRS (measured at 500 nm) over 63% of the ocean, whereas chlorophyll showed a change over only 31% of the ocean (Figure 3).

 

Figure 3. a, b) Modeled percent changes in chlorophyll and RRS. c, d) Time of emergence of changes in chlorophyll and RRS, or, in other words, the year when a change in either measurement can be attributed to climate change. Source: Dutkiewicz et al. (2019)

 

Next, the scientists wanted to find out which of the climate-driven changes played a biggest role in changing RRS. By running their model, they found that changes in the amount of ocean detritus would impact RRS less than changes in CDOM would, with changes in CDOM being detectable over 36% of the ocean by 2100. However, both of these were overshadowed by the modeled impact phytoplankton, specifically changes in phytoplankton community structure, would have on RRS­, yielding a signal in 50% of the ocean by 2100 (Figure 4).

Figure 4. Percent of ocean area (darker red = higher percentage) showing a significant trend in a) chlorophyll, b) detritus, c) CDOM, and d) dis (the Bray-Curtis Dissimilarity Index, which measures changes in phytoplankton community composition). Source: Dutkiewicz et al. (2019)

Dutkiewicz and her colleagues point out that their model isn’t perfect; it overestimates the amount of phytoplankton in high-latitude waters, and underestimates the natural variability in RRS for certain colors, suggesting that some key processes are missing in the model. They also note that RRS isn’t the perfect proxy for ocean color, since it doesn’t capture changes in other factors that affect ocean optics, including the presence of viruses, minerals, and salts.

However, RRS can provide information on changes in the ocean at the ecosystem-level, and the scientists’ study suggests that phytoplankton community structure will be changing perceptibly over the coming century. Dutkiewicz and her colleagues emphasize the importance of satellites measuring RRS in the blue-green range to monitor ocean ecosystem changes, and point out that broad measurements of phytoplankton, like remotely-sensed chlorophyll, don’t capture more subtle changes in phytoplankton community composition, or other factors that influence ocean color such as CDOM and detritus.

Even before the end of the century, many changes are predicted to occur in the ocean as our planet warms. Dutkiewicz and her colleagues showed that measurements of ocean color hold information about an amalgam of phytoplankton and non-living particles suspended in the water. While these shifts in ocean color are unlikely to be perceptible to the human eye, these findings show that measurements of ocean color are a useful tool for tracking, monitoring, and understanding changes in the ocean as our planet warms.

 

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