The ocean's color is changing as a consequence of climate change
The color changes reflect significant shifts in essential marine
ecosystems.

Date:
July 12, 2023
Source:
Massachusetts Institute of Technology
Summary:
The ocean's color has changed significantly in 20 years, and the
trend is likely a consequence of human-induced climate change,
report scientists.


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FULL STORY ==========================================================================
The ocean's color has changed significantly over the last 20 years, and
the global trend is likely a consequence of human-induced climate change, report scientists at MIT, the National Oceanography Center in the U.K.,
and elsewhere.

In a study appearing today in Nature,the team writes that they have
detected changes in ocean color over the past two decades that cannot
be explained by natural, year-to-year variability alone. These color
shifts, though subtle to the human eye, have occurred over 56 percent
of the world's oceans -- an expanse that is larger than the total land
area on Earth.

In particular, the researchers found that tropical ocean regions near the equator have become steadily greener over time. The shift in ocean color indicates that ecosystems within the surface ocean must also be changing,
as the color of the ocean is a literal reflection of the organisms and materials in its waters.

At this point, the researchers cannot say how exactly marine ecosystems
are changing to reflect the shifting color. But they are pretty sure of
one thing: Human-induced climate change is likely the driver.

"I've been running simulations that have been telling me for years that
these changes in ocean color are going to happen," says study co-author Stephanie Dutkiewicz, senior research scientist in MIT's Department
of Earth, Atmospheric and Planetary Sciences and the Center for Global
Change Science. "To actually see it happening for real is not surprising,
but frightening. And these changes are consistent with man-induced
changes to our climate." "This gives additional evidence of how human activities are affecting life on Earth over a huge spatial extent,"
adds lead author B. B. Cael PhD '19 of the National Oceanography Center
in Southampton, U.K. "It's another way that humans are affecting the biosphere." The study's co-authors also include Stephanie Henson of the National Oceanography Center, Kelsey Bisson at Oregon State University,
and Emmanuel Boss of the University of Maine.

Above the noise The ocean's color is a visual product of whatever lies
within its upper layers.

Generally, waters that are deep blue reflect very little life,
whereas greener waters indicate the presence of ecosystems, and mainly phytoplankton -- plant- like microbes that are abundant in upper ocean and
that contain the green pigment chlorophyll. The pigment helps plankton
harvest sunlight, which they use to capture carbon dioxide from the
atmosphere and convert it into sugars.

Phytoplankton are the foundation of the marine food web that sustains progressively more complex organisms, on up to krill, fish, and seabirds
and marine mammals. Phytoplankton are also a powerful muscle in the
ocean's ability to capture and store carbon dioxide. Scientists are
therefore keen to monitor phytoplankton across the surface oceans and to
see how these essential communities might respond to climate change. To
do so, scientists have tracked changes in chlorophyll, based on the ratio
of how much blue versus green light is reflected from the ocean surface,
which can be monitored from space But around a decade ago, Henson, who
is a co-author of the current study, published a paper with others,
which showed that, if scientists were tracking chlorophyll alone, it
would take at least 30 years of continuous monitoring to detect any
trend that was driven specifically by climate change. The reason, the
team argued, was that the large, natural variations in chlorophyll from
year to year would overwhelm any anthropogenic influence on chlorophyll concentrations. It would therefore take several decades to pick out a meaningful, climate-change-driven signal amid the normal noise.

In 2019, Dutkiewicz and her colleagues published a separate paper, showing through a new model that the natural variation in other ocean colors
is much smaller compared to that of chlorophyll. Therefore, any signal
of climate- change-driven changes should be easier to detect over the
smaller, normal variations of other ocean colors. They predicted that such changes should be apparent within 20, rather than 30 years of monitoring.

"So I thought, doesn't it make sense to look for a trend in all these
other colors, rather than in chlorophyll alone?" Cael says. "It's worth
looking at the whole spectrum, rather than just trying to estimate one
number from bits of the spectrum." The power of seven In the current
study, Cael and the team analyzed measurements of ocean color taken by
the Moderate Resolution Imaging Spectroradiometer (MODIS) aboard the
Aqua satellite, which has been monitoring ocean color for 21 years. MODIS
takes measurements in seven visible wavelengths, including the two colors researchers traditionally use to estimate chlorophyll.

The differences in color that the satellite picks up are too subtle for
human eyes to differentiate. Much of the ocean appears blue to our eye,
whereas the true color may contain a mix of subtler wavelengths, from
blue to green and even red.

Cael carried out a statistical analysis using all seven ocean colors
measured by the satellite from 2002 to 2022 together. He first looked at
how much the seven colors changed from region to region during a given
year, which gave him an idea of their natural variations. He then zoomed
out to see how these annual variations in ocean color changed over a
longer stretch of two decades. This analysis turned up a clear trend,
above the normal year-to-year variability.

To see whether this trend is related to climate change, he then looked
to Dutkiewicz's model from 2019. This model simulated the Earth's oceans
under two scenarios: one with the addition of greenhouse gases, and the
other without it.

The greenhouse-gas model predicted that a significant trend should show up within 20 years and that this trend should cause changes to ocean color
in about 50 percent of the world's surface oceans -- almost exactly what
Cael found in his analysis of real-world satellite data.

"This suggests that the trends we observe are not a random variation
in the Earth system," Cael says. "This is consistent with anthropogenic
climate change." The team's results show that monitoring ocean colors
beyond chlorophyll could give scientists a clearer, faster way to detect climate-change-driven changes to marine ecosystems.

"The color of the oceans has changed," Dutkiewicz says. "And we can't
say how.

But we can say that changes in color reflect changes in plankton
communities, that will impact everything that feeds on plankton. It will
also change how much the ocean will take up carbon, because different
types of plankton have different abilities to do that. So, we hope
people take this seriously. It's not only models that are predicting
these changes will happen. We can now see it happening, and the ocean
is changing." This research was supported, in part, by NASA.

* RELATED_TOPICS
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# Environmental_Awareness # Global_Warming # Oceanography
# Climate # Ecology # Ecosystems # Geography #
Earth_Science
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o IPCC_Report_on_Climate_Change_-_2007 o Climate_model

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Story Source: Materials provided by
Massachusetts_Institute_of_Technology. Original written by Jennifer
Chu. Note: Content may be edited for style and length.


========================================================================== Related Multimedia:
* Ocean_color ========================================================================== Journal Reference:
1. B. B. Cael, Kelsey Bisson, Emmanuel Boss, Stephanie Dutkiewicz,
Stephanie
Henson. Global climate-change trends detected in indicators of
ocean ecology. Nature, 2023; DOI: 10.1038/s41586-023-06321-z ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2023/07/230712123442.htm

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