By James Randerson
    The Guardian

    Thursday 07 December 2006

    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.