Tropical seas are threatened by famine as warming quickens, satellite survey shows
Pastor Dale Morgan
Tropical seas are threatened by famine as warming quickens, satellite
survey shows*
· Phytoplankton change may wreck food chain
· Fish stocks predicted to decline significantly
James Randerson, science correspondent
Thursday December 7, 2006
The Guardian
Coral feeding on plankton at night
Coral feeding on plankton at night. Scientists believe warmer ocean
temperatures will drastically reduce the amount of plankton available
for sea creatures to eat. Photograph: Jeffery L Rotman/Corbis
Global warming is creating an ocean famine in swaths of tropical and
sub-tropical seas, according to research using nearly a decade of
satellite data.
The finding, which has long been predicted by computer models, suggests
that as warming continues, fish stocks in tropical and sub-tropical
regions will drop significantly. The study showed that in some ocean
regions microscopic plants in the plankton, known as phytoplankton,
respond to rising temperatures by scaling down their productivity by 30%
or more. With less production at the bottom of the food chain, fish and
other large ocean creatures have less to eat.
Commenting on the study in the journal Nature, Scott Doney of Woods Hole
Oceanographic Institution in Massachusetts said that if the trends
observed in the satellite observations continued "the future suggests
that marine biological productivity in the tropics and mid-latitudes
will decline substantially".
The changes occur because warmer surface temperatures lead to changes in
the flow of ocean currents that deliver nutrient-rich water from the
cooler depths to the surface. About half of the production generated by
the world's living organisms is done by phytoplankton, microscopic green
plants which operate in the top 100 to 200 metres of the ocean where
light levels are high enough for photosynthesis.
Each day they pull in more than 100 million tonnes of CO2. "Almost the
whole food chain of the open ocean depends on these plants," said Duncan
Purdie, a plankton expert at the National Oceanography centre in
Southampton.
The tropical and sub-tropical open oceans are deserts by comparison with
higher latitudes despite receiving more intense sunlight.
The problem is that the growth of microscopic plants here is limited by
a dearth of nutrients such as phosphorus, nitrogen, iron and silicon at
the surface.
To investigate how temperature changes affect these tiny plants, Michael
Behrenfeld at Oregon State University and his team examined data
collected between 1997 and 2006 by a Nasa satellite which records the
colour of the ocean, with the intensity of green corresponding to the
concentration of microscopic plants in the water and hence the
productivity of that part of the ocean.
They report that in tropical and sub-tropical regions with an average
surface temperature of 15C, plant production closely matched temperature
changes.
"This clearly showed that overall ocean productivity decreases when the
climate warms," said Professor Behrenfeld.
The oceans are currently warming at about 0.2C per decade on average and
scientists expect that trend to accelerate.
The drop in production occurs because rising surface temperatures result
in less mixing with nutrient-rich water below. The hot water on top is
lighter and essentially floats on the colder water below. This already
occurs in tropical and sub-tropical regions, but the satellite data show
that the effect is intensifying as the surface gets hotter.
Another global climate effect that impacts on ocean productivity and
hence fish stocks is el Niño, meaning "the Christ child" - a period of
low fish stocks that was so named by Latin American fishermen because it
often occurs around Christmas. Under normal conditions, the trade winds
blow westwards across the Pacific so pushing surface water away from the
west coast of South America. Colder water wells up from below to replace
it, creating a plankton bonanza as the tiny plants grab nutrients from
the depths. This also means a bumper food crop for fish.
But in el Niño years the trade winds slow or even reverse, which dampens
the upwelling and leads to a crash in fish stocks.
Prof Purdie said el Niño events, which currently occur every four to
seven years, may become more frequent as a result of global warming,
depressing ocean productivity and fish stocks further. "They are
extremely important for fish production," he said.
Dr Doney said that it was only possible to do such a global analysis of
ocean productivity using satellite data.
"The ocean is vast, and the limited number of research ships move at
about the speed of a bicycle," he wrote in Nature. "By contrast a
satellite can observe the entire globe, at least the cloud-free areas,
in a few days."
Prof Behrenfeld said that the rapid turnover of microscopic plants in
the ocean meant that it was possible to watch changes as they happened.
"This very fast turnover, along with the fact that phytoplankton are
limited to just a thin veneer of the ocean surface where there is enough
sunlight to sustain photosynthesis, makes them very responsive to
changes in climate," he said.
"This was why we could relate productivity change to climate variability
in only a 10-year record. Such connections would be much harder to
detect from space for terrestrial plant biomass." The reduced
productivity in tropical oceans may be counter-balanced to some degree
by changes in the far north and south, where the growth of phytoplankton
is limited by the amount of light, so reduced mixing will keep them
closer to the surface and allow them to grow faster."
Extinctions in the future
Rising global temperatures are already changing the oceans, plants and
animals that live in them. Hurricanes, for example, are becoming more
intense so the southern states of the US will have to brace themselves
for more catastrophic events such as hurricane Katrina.
Coral reefs are also extremely vulnerable to small temperature rises.
When put under stress they lose the symbiotic algae that supply them
with nutrients and although they can recover if the water cools again, a
permanent temperature rise would kill vast swathes of reef. Scientists
say if global warming projections are correct, the Great Barrier Reef
will lose 95% of its living coral by 2050.
Rising CO2 concentrations in the atmosphere are also causing oceans to
acidify. Since the beginning of the industrial revolution a third of the
CO2 released by fossil fuel burning has been absorbed by the oceans.
A study last year of fossil deposits going back 55m years found that a
mass extinction of ocean creatures was caused by ocean acidification
linked to the release of 4,500bn tonnes of carbon.
It took more than 100,000 years for the oceans to recover.