I've tried to research this, and this is the best I can find. It suggests there have probably always been "low productivity regions" of the ocean, but they are expanding rapidly due to global warming, which inhibits water rising up with nutrients from below.
(NB. I speculate that plastics sinking down from the "great ocean garbage patches…
I've tried to research this, and this is the best I can find. It suggests there have probably always been "low productivity regions" of the ocean, but they are expanding rapidly due to global warming, which inhibits water rising up with nutrients from below.
(NB. I speculate that plastics sinking down from the "great ocean garbage patches" may also inhibit that rising water. They are created by corresponding ocean gyres or whirlpools: https://www.mdpi.com/2077-1312/9/11/1289 )
'The mixing of different layers of water is one reason that high productivity areas exist in the world’s oceans. One of the signatures of global warming is rising ocean temperatures. As the surface layer of the ocean warms, the water becomes less dense and stays on top rather than mixing down to allow cooler, nutrient-rich water to well up. Over time, areas with less mixing show reduced productivity, less phytoplankton, and so less chlorophyll. Polovina said, “Regions that have the lowest level of chlorophyll are akin to biological deserts; there’s less energy propagating up through the food web.”
“We saw that the low-productivity area of the west Pacific was expanding, and we wondered if it was unique or if it was happening globally.” Models predict that the warmest portions of the world’s oceans will become less productive because of climate change. Some people call these low productivity regions “biological deserts.” Polovina said, “The climate models are on century timescales and suggest that the rate of expansion of these expected low-productivity areas will be slow.”
Over nearly the past decade, regions with low surface chlorophyll were expanding into nearby ocean basins. The total area lost was quite enormous.” The area of new global ocean desert added up to 6.6 million square kilometers (2.5 million square miles), representing about a 15 percent expansion in the area of the least productive waters between 1998 and 2006.
...this step-by-step desertification has led to a 1 to 4 percent loss in productive waters per year.
“The actual rate of expansion was much bigger than the models predicted.”
A recent study using six climate models suggested that between the beginning of the Industrial Revolution and 2050, the total growth in low-productivity areas in the Northern and Southern Hemispheres would range from 0.7 percent to 8.1 percent, depending on various parameters. Polovina said, “We’ve measured more than even the high range in only nine years.” '
4-8% p.a. growth over several decades is massive! These desert areas may well have increased in size many fold due to human impacts.
The "new global ocean desert" of 6.6 million km2 over a decade equals 4% of the total area of the Pacific.
Note this article seems to refer to separate, smaller "dead zones" of ocean closer to coasts or in seas:
It refers to "ocean deserts" as "dead zones" that are "most often found close to places with large amounts of polluted water... caused by large amounts of nitrogen that end up in the ocean from fertilizers used in agriculture, or through emissions from automobiles or factories. Nitrogen is what causes the plankton to explode and deplete the oxygen from the water.
These deserts have been rapidly growing for the past 50 years. It is believed that climate change is affecting the appearance and growth of these dead zones as well. Dead zones can be found in many places all over the oceanic bottom, but are most often found close to places with large amounts of polluted water. For example, one large ocean desert can be found in the Gulf of Mexico, and it has grown considerably during the last decade. The dead zone now covers an area of 18,000 square kilometers, all thanks to the huge amounts of polluted water dropped there by the Mississippi River."
The same theme is reflected in this & other articles:
"The has been a staggering increase in the number of dead zones worldwide over the past 60 years, from just 42 in 1950, to 405 in 2008. Dead zones now cover 95,000 square miles; this is the size of the United Kingdom.
The largest dead zone worldwide is the Baltic Sea. Overfishing of Baltic cod has greatly intensified the problem.
The second largest dead zone is the northern Gulf of Mexico, surrounding the outflow of the Mississippi River".
I've tried to research this, and this is the best I can find. It suggests there have probably always been "low productivity regions" of the ocean, but they are expanding rapidly due to global warming, which inhibits water rising up with nutrients from below.
(NB. I speculate that plastics sinking down from the "great ocean garbage patches" may also inhibit that rising water. They are created by corresponding ocean gyres or whirlpools: https://www.mdpi.com/2077-1312/9/11/1289 )
From:
https://www.earthdata.nasa.gov/learn/sensing-our-planet/an-ocean-full-of-deserts
'The mixing of different layers of water is one reason that high productivity areas exist in the world’s oceans. One of the signatures of global warming is rising ocean temperatures. As the surface layer of the ocean warms, the water becomes less dense and stays on top rather than mixing down to allow cooler, nutrient-rich water to well up. Over time, areas with less mixing show reduced productivity, less phytoplankton, and so less chlorophyll. Polovina said, “Regions that have the lowest level of chlorophyll are akin to biological deserts; there’s less energy propagating up through the food web.”
“We saw that the low-productivity area of the west Pacific was expanding, and we wondered if it was unique or if it was happening globally.” Models predict that the warmest portions of the world’s oceans will become less productive because of climate change. Some people call these low productivity regions “biological deserts.” Polovina said, “The climate models are on century timescales and suggest that the rate of expansion of these expected low-productivity areas will be slow.”
Over nearly the past decade, regions with low surface chlorophyll were expanding into nearby ocean basins. The total area lost was quite enormous.” The area of new global ocean desert added up to 6.6 million square kilometers (2.5 million square miles), representing about a 15 percent expansion in the area of the least productive waters between 1998 and 2006.
...this step-by-step desertification has led to a 1 to 4 percent loss in productive waters per year.
“The actual rate of expansion was much bigger than the models predicted.”
A recent study using six climate models suggested that between the beginning of the Industrial Revolution and 2050, the total growth in low-productivity areas in the Northern and Southern Hemispheres would range from 0.7 percent to 8.1 percent, depending on various parameters. Polovina said, “We’ve measured more than even the high range in only nine years.” '
4-8% p.a. growth over several decades is massive! These desert areas may well have increased in size many fold due to human impacts.
The "new global ocean desert" of 6.6 million km2 over a decade equals 4% of the total area of the Pacific.
Note this article seems to refer to separate, smaller "dead zones" of ocean closer to coasts or in seas:
https://www.worldatlas.com/articles/what-are-ocean-deserts.html
It refers to "ocean deserts" as "dead zones" that are "most often found close to places with large amounts of polluted water... caused by large amounts of nitrogen that end up in the ocean from fertilizers used in agriculture, or through emissions from automobiles or factories. Nitrogen is what causes the plankton to explode and deplete the oxygen from the water.
These deserts have been rapidly growing for the past 50 years. It is believed that climate change is affecting the appearance and growth of these dead zones as well. Dead zones can be found in many places all over the oceanic bottom, but are most often found close to places with large amounts of polluted water. For example, one large ocean desert can be found in the Gulf of Mexico, and it has grown considerably during the last decade. The dead zone now covers an area of 18,000 square kilometers, all thanks to the huge amounts of polluted water dropped there by the Mississippi River."
The same theme is reflected in this & other articles:
https://www.sailorsforthesea.org/programs/ocean-watch/ocean-dead-zones
"The has been a staggering increase in the number of dead zones worldwide over the past 60 years, from just 42 in 1950, to 405 in 2008. Dead zones now cover 95,000 square miles; this is the size of the United Kingdom.
The largest dead zone worldwide is the Baltic Sea. Overfishing of Baltic cod has greatly intensified the problem.
The second largest dead zone is the northern Gulf of Mexico, surrounding the outflow of the Mississippi River".