Oceans are no longer blue: Study finds 56% of water has become more green due to climate change
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More than half of the world’s oceans have turned green in the past two decades, a study shows.
Measurements of ocean surface color taken via satellite over the past 20 years have revealed a global explosion in the growth of phytoplankton, the plant-like microbes found in the upper ocean.
While many of these microscopic organisms, including green algae, absorb carbon dioxide as they harvest solar energy, their “pond scum”-like population explosion has contributed to suffocating, low-oxygen “dead zones” worldwide.
The color shift is subtle to the human eye, but NASA-run satellite equipment has confirmed that more than 56 percent of the world’s oceans — a vast area larger than Earth’s total landmass — have turned greener.
“It’s not surprising, but frightening,” says study co-author Stephanie Dutkiewicz, a senior researcher and climate modeller at the Massachusetts Institute of Technology (MIT), “to actually see it happen in real life.”
“These changes,” she said, “are consistent with human-induced changes in our climate.”
NASA-controlled satellites have confirmed that more than 56 percent of the world’s oceans — an expanse larger than Earth’s total land mass — have turned green. This ‘pond scum’-like population growth in phytoplankton has contributed to low-oxygen ‘dead zones’ worldwide
MIT and Britain’s National Oceanography Center analyzed decades of ocean color data from NASA’s Aqua satellite. Purple areas (above) indicate greener oceans detected twice or more above the signal-to-noise ratio. Black dots mark areas of heavy chlorophyll elevations
The number of ocean “dead zones” – areas deprived of oxygen in which animal life suffocates and dies – has quadrupled in 50 years from 1963 to 2013 due to global warming. Pictured are coastal (red) and open ocean (blue) dead zones across from NOAA’s 2013 World Ocean Atlas
The MIT team, in collaboration with the UK’s National Oceanography Centre, analyzed decades of ocean color data collected by the MRI Spectroradiometer (MODIS) aboard NASA’s Aqua satellite.
The color data, collected from low Earth orbit, showed that the hotter tropical oceans near the equator have become the most consistently greener over time.
An increase in phytoplankton, the base of the marine food chain that sustains krill, fish, and seabirds and marine mammals, can usually be interpreted as a sign of ocean health.
But the overgrowth and oxygen-sucking decay caused by the large masses of these microbes has been consistently associated for more than a decade with an increase in ocean dead zones and massive sea migrations.
“I’ve been running simulations that have been telling me for years that these changes in ocean color are going to happen,” Dutkiewicz said. “So we hope people take this seriously.”
“It’s not just models that predict these changes will happen,” she noted. “We’re seeing it happen now, and the ocean is changing.”
The greening discovered by the researchers’ analysis of NASA’s MODIS-Aqua data from July 2002 to June 2022, published today in the journal Naturemade every effort to become twice as green as the expected signal-to-noise ratio.
TThe result, they said, is that this greening cannot be explained by natural, seasonal or annual variations in phytoplankton blooms alone.
In the ‘dead zones’ of the oceans, oxygen levels drop to levels so low that many animals suffocate and die. Low oxygen caused the death of the corals and crabs pictured here in Bocas del Toro, Panama
“This provides additional evidence of how human activities affect life on Earth over a vast spatial extent,” said lead author BB Cael of the UK’s National Oceanography Center in Southampton.
“It’s another way humans affect the biosphere.”
The researchers tracked seven wavelengths of colored light from the ocean’s surface via the MODIS system aboard NASA’s Aqua satellite.
While the ocean appears uniformly blue to the unaided eye, its true color contains a mixture of these wavelengths, from blue and green to even red, some of which vary much less from year to year and thus provide much more signal than noise.
Cael and his team performed a statistical analysis using all seven wavelengths measured by the Aqua satellite, not just the two typically used to measure the changes in green chlorophyll pigment due to phytoplankton activity.
“I thought, wouldn’t it make sense to look for a trend in all these other colors,” Dr. Cael said, “rather than just chlorophyll?”
“It’s worth looking at the whole spectrum, rather than trying to estimate just one number from bits of the spectrum.”
Cael’s group was able to compare these results to a predictive model created by Dutkiewicz at MIT in 2019.
Dutkiewicz’s model simulated the changes in ocean color based on two scenarios: one with additional greenhouse gases added to the atmosphere, and another scenario without.
The greenhouse gas model predicted that about 50 percent of the world’s ocean surface would become detectably greener within 20 years — exactly what Cael found in the real-world MODIS-Aqua data.
“This suggests that the trends we observe are not a random variation in the Earth system,” Cael said in a press statement.
“This is consistent with anthropogenic climate change.”
But Cael noted that more detailed study than just color changes would be needed to understand exactly how all of these marine ecosystems around the world are changing individually as a result of a hotter planet.
“The ecosystem is changing – although it’s hard to say exactly how with the current state of our knowledge of planktonic ecosystems,” Cael said. Shame.
‘These color changes could mean, among other things, a shift to smaller or larger plankton, more or fewer predators or prey, different species of plankton that affect carbon storage or fishing differently.’