Climate change is causing significant changes in the state of phytoplankton in the oceans, and a new MIT study has shown that these changes will greatly affect the color of the ocean in the coming decades, intensifying its blue and green areas. Satellites should detect these hue changes and give early warning of widespread changes in marine ecosystems.
In the journal Nature Communications, scientists report that they have developed a global model that mimics the growth and interaction of different types of phytoplankton, or algae, and how the mixture of species in different places will change as temperatures rise around the world. The researchers also modeled how phytoplankton absorbs and reflects light, and how the ocean's color changes as global warming affects the composition of phytoplankton communities.
Researchers tested their model by running it to the end of the 21st century and found that by 2100 more than 50% of the world's oceans will change color due to climate change.
The study suggests that blue areas, such as the subtropics, will become even bluer as there is less phytoplankton - and life in general - in these waters when compared to the current state of affairs. Some regions that are greener today, such as those near the poles, may become even greener, as higher temperatures lead to the proliferation of a variety of phytoplankton..
"This model assumes that these changes will not be easy for the naked eye to notice, and the ocean will still look like it has blue areas in the subtropics and greener areas near the equator and poles," says lead author Stephanie Datkiewicz of the Science Division. Earth, Atmosphere, and Planet of the Massachusetts Institute of Technology. “This basic scheme will remain the same. However, the depth changes will be significant enough to affect the rest of the phytoplankton food chain."
The color of the oceans depends on the amount of chlorophyll
The color of the ocean depends on how sunlight interacts with what is in the water. Only water molecules absorb almost all sunlight, except for the blue part of the spectrum - it is reflected. Consequently, the relatively barren areas of the open ocean appear deep blue from space. If there are any organisms in the ocean, they can absorb and reflect light waves of different lengths, depending on their individual properties.
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Phytoplankton, for example, contains chlorophyll, a pigment that absorbs mostly blue parts of sunlight, producing carbon for photosynthesis, and to a lesser extent green parts. As a result, more green light is reflected off the ocean, which gives the algae-rich areas a greenish tint.
Since the late 1990s, satellites have continuously measured the color of the ocean. Scientists used these measurements to get the amount of chlorophyll and, by extension, phytoplankton in a specific area of the ocean. But Datkevich says chlorophyll will not necessarily reflect a sensitive signal of climate change. Any significant fluctuations in chlorophyll may well be caused by global warming, but also by "natural variability", normal periodic jumps in chlorophyll due to natural weather-related phenomena.
“An El Niño or La Niña event will cause very large changes in chlorophyll because it changes the amount of nutrients entering the system,” says Datkevich. "With these big, natural changes that happen every few years, it's hard to see if climate change will change just by looking at chlorophyll."
Ocean light simulation
Rather than looking at the estimates of chlorophyll obtained, the team wondered if there was a clear signal about the impact of climate change on phytoplankton if one looked only at satellite measurements of reflected light.
The team has refined a computer model that it has used in the past to predict phytoplankton changes with rising temperatures and ocean acidification. This model takes information about phytoplankton, such as its food intake and how it grows, and incorporates this information into a physical model that demonstrates ocean currents and mixing.
This time, however, scientists have added a new element to the model that has not been incorporated into other ocean modeling methods: the ability to estimate specific wavelengths of light that are absorbed and reflected by the ocean, depending on the number and type of organisms in a particular region.
“Sunlight hits the ocean, and everything in the ocean absorbs it like chlorophyll,” Datkevich says. “Other things will absorb it or dissipate it. So determining how light will reflect off the ocean and give it color is difficult.
It turned out that the scientists' model can be used to predict the color of the ocean when environmental conditions change in the future. And the best thing about it is that it can be used in the laboratory.
Signal in blue-green tones
When scientists added global temperatures to the model and raised them by 3 degrees by 2100 - most scientists predict if no action is taken to reduce greenhouse gas emissions - they found that the wavelengths of light in the blue and green parts of the spectrum responded the fastest. …
What's more, this blue-green waveform shows a very clear signal, or shift, associated with climate change: the shift occurs earlier than expected when scientists looked at chlorophyll.
“Chlorophyll changes, but you can't see it because of the incredible natural variability,” says Datkevich. “However, you can see significant climate change in some of these wavebands in the signal sent to the satellites. So this is where we should look for the real signal of changes in satellite measurements.”
According to the scientists' model, climate change is already changing the composition of phytoplankton, and hence the color of the oceans. By the end of the century, our blue planet will change dramatically.
“By the end of the 21st century, there will be a noticeable change in the color of 50% of the oceans. The change will be pretty big. Different types of phytoplankton absorb light in different ways, and if climate change displaces one phytoplankton community to another, it will also change the types of food webs they can support.”
Ilya Khel