MSGID: 1:317/3 64af7e12   
   PID: hpt/lnx 1.9.0-cur 2019-01-08   
   TID: hpt/lnx 1.9.0-cur 2019-01-08   
    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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   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.   
      
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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.   
      
      
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       * Ocean_color   
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   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   
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   Link to news story:   
   https://www.sciencedaily.com/releases/2023/07/230712123442.htm   
      
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