- The microscopic plants that underpin all life in the
oceans are likely to be destroyed by global warming, a study has found.
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- Scientists have discovered a way that the vital plankton
of the oceans can be starved of nutrients as a result of the seas getting
warmer. They believe the findings have catastrophic implications for the
entire marine habitat, which ultimately relies on plankton at the base
of the food chain.
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- The study is also potentially devastating because it
has thrown up a new "positive feedback" mechanism that could
result in more carbon dioxide ending up in the atmosphere to cause a runaway
greenhouse effect.
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- Scientists led by Jef Huisman of the University of Amsterdam
have calculated that global warming, which is causing the temperature of
the sea surface to rise, will also interfere with the vital upward movement
of nutrients from the deep sea.
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- These nutrients, containing nitrogen, phosphorus and
iron, are vital food for phytoplankton. If the supply is interrupted the
plants die off, which prevents them from absorbing carbon dioxide from
the atmosphere.
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- "Global warming of the surface layers of the oceans
reduces the upward transport of nutrients into the surface layers. This
generates chaos among the plankton," the professor said.
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- The sea is one of nature's "carbon sinks",
which removes carbon dioxide from the atmosphere and deposits the carbon
in a long-term store - dissolved in the ocean or deposited as organic waste
on the seabed. The vast quantities of phytoplankton in the oceans absorb
huge amounts of carbon dioxide. When the organisms die they fall to the
seabed, carrying their store of carbon with them, where it stays for many
thousands of years - thereby helping to counter global warming.
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- "Plankton... forms the basis of the marine food
web. Moreover, phytoplankton consumes the greenhouse gas carbon dioxide
during photosynthesis," Professor Huisman said. "Uptake of carbon
dioxide by phytoplankton across the vast expanses of the oceans reduces
the rising carbon dioxide levels in the atmosphere."
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- Warmer surface water caused by global warming causes
greater temperature stratification, with warm surface layers sitting on
deeper, colder layers, to prevent mixing of nutrients.
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- Professor Huisman shows in a study published in Nature
that warmer sea surfaces will deliver a potentially devastating blow to
the supply of deep-sea nutrients for phytoplankton.
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- His computer model of the impact was tested on real measurements
made in the Pacific Ocean, where sea surface temperatures tend to be higher
than in other parts of the world. He found that his computer predictions
of how nutrient movement would be interrupted were accurate.
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- "A larger temperature difference between two water
layers implies less mixing of chemicals between these water layers,"
he said. "Global warming of the surface layers of the oceans, owing
to climate change, strengthens the stratification and thereby reduces the
upward mixing of nutrients."
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- Scientists had believed phytoplankton, which survives
best at depths of about 100 metres, is largely stable and immune from the
impact of global warming. "This model prediction was rather unexpected,"
Professor Huisman said.
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- "Reduced stability of the plankton, caused by global
warming of the oceans, may result in a decline of oceanic production and
reduced sequestration of the greenhouse gas carbon dioxide into the oceans."
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- Vital link in the food chain
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- Microscopic plankton comes in animal and plant forms.
The plants are known as phytoplankton. They lie at the base of the marine
food chain because they convert sunlight and carbon dioxide into organic
carbon - food for everything else.
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- Smaller animals such as shrimp-like krill feed on plankton
and are themselves eaten by larger organisms, from small fish to the biggest
whales. Without phytoplankton, the oceans would soon because marine deserts.
Phytoplankton are also important because of the role they play in the carbon
cycle, which determines how much carbon dioxide - the most important greenhouse
gas - ends up in the atmosphere to cause global warming. Huge amounts of
carbon dioxide from the atmosphere, which dissolves in the oceans, are
absorbed by phytoplankton and converted to organic carbon. When the phytoplankton
die, their shells and bodies sink to the seabed, carrying this carbon with
them.
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- Phytoplankton therefore acts as a carbon "sink"
which takes carbon dioxide from the atmosphere and deposits the carbon
in long-term stores that can remain undisturbed for thousands of years.
If the growth of phytoplankton is interrupted by global warming, this ability
to act as a buffer against global warming is also affected - leading to
a much-feared positive feedback.
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