Environment
Study: Much of the
surface ocean will shift
in color by end of 21st
century
Jennifer Chu | MIT News Office
C
limate-driven changes in phytoplankton
communities will intensify the blue and
green regions of the world’s oceans.
Climate change is causing significant
changes to phytoplankton in the world’s
oceans, and a new MIT study finds that over
the coming decades these changes will
affect the ocean’s color, intensifying its blue
regions and its green ones. Satellites should
detect these changes in hue, providing early warning of wide-scale
changes to marine ecosystems.
Writing in Nature Communications, researchers report that they
have developed a global model that simulates the growth and
interaction of different species of phytoplankton, or algae, and how
the mix of species in various locations will change as temperatures
rise around the world. The researchers also simulated the way
phytoplankton absorb and reflect light, and how the ocean’s color
changes as global warming affects the makeup of phytoplankton
communities.
The researchers ran the model through the end of the 21st
century and found that, by the year 2100, more than 50 percent of
the world’s oceans will shift in color, due to climate change.
The study suggests that blue regions, such as the subtropics, will
become even more blue, reflecting even less phytoplankton —
and life in general — in those waters, compared with today. Some
regions that are greener today, such as near the poles, may turn
even deeper green, as warmer temperatures brew up larger blooms
of more diverse phytoplankton.
“The model suggests the changes won’t appear huge to the
naked eye, and the ocean will still look like it has blue regions in the
subtropics and greener regions near the equator and poles,” says
lead author Stephanie Dutkiewicz, a principal research scientist at
MIT’s Department of Earth, Atmospheric, and Planetary Sciences
and the Joint Program on the Science and Policy of Global Change.
“That basic pattern will still be there. But it’ll be enough different
that it will affect the rest of the food web that phytoplankton
supports.”
Dutkiewicz’s co-authors include Oliver Jahn of MIT, Anna Hickman
56
of the University of Southhampton, Stephanie Henson of the
National Oceanography Centre Southampton, Claudie Beaulieu
of the University of California at Santa Cruz, and Erwan Monier,
former principal research scientist at the MIT Center for Global
Change Science, and currently assistant professor at the University
of California at Davis, in the Department of Land, Air and Water
Resources.
Chlorophyll count
The ocean’s color depends on how sunlight interacts with
whatever is in the water. Water molecules alone absorb almost all
sunlight except for the blue part of the spectrum, which is reflected
back out. Hence, relatively barren open-ocean regions appear as
deep blue from space. If there are any organisms in the ocean, they
can absorb and reflect different wavelengths of light, depending on
their individual properties.
Phytoplankton, for instance, contain chlorophyll, a pigment which
absorbs mostly in the blue portions of sunlight to produce carbon
for photosynthesis, and less in the green portions. As a result, more
green light is reflected back out of the ocean, giving algae-rich
regions a greenish hue.
Since the late 1990s, satellites have taken continuous
measurements of the ocean’s color. Scientists have used these
measurements to derive the amount of chlorophyll, and by
extension, phytoplankton, in a given ocean region. But Dutkiewicz
says chlorophyll doesn’t necessarily reflect the sensitive signal of
climate change. Any significant swings in chlorophyll could very
well be due to global warming, but they could also be due to
“natural variability” — normal, periodic upticks in chlorophyll due to
natural, weather-related phenomena.
“An El Niño or La Niña event will throw up a very large change
in chlorophyll because it’s changing the amount of nutrients that
are coming into the system,” Dutkiewicz says. “Because of these
big, natural changes that happen every few years, it’s hard to see if
things are changing due to climate change, if you’re just looking at
chlorophyll.”
Modeling ocean light
Instead of looking to derived estimates of chlorophyll, the team
wondered whether they could see a clear signal of climate change’s
effect on phytoplankton by looking at satellite measurements of
reflected light alone.
The group tweaked 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 what they consume and how they grow,
and incorporates this information into a physical model that
simulates the ocean’s currents and mixing.