Climate change is killing the oceans' microscopic 'lungs'

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Global warming has begun to change the way microscopic plant life in the oceans absorbs carbon dioxide from the atmosphere - a trend that could lead to a dramatic increase in the heating power of the greenhouse effect.

Satellite data gathered over the past 10 years has shown for the first time that the growth of marine phytoplankton - the basis of the entire ocean food chain - is being adversely affected by rising sea temperatures.

Scientists have found that as the oceans become warmer, they are less able to support the phytoplankton that have been an important influence on moderating climate change.

The fear is that as sea temperatures continue to rise as a result of global warming, the loss of phytoplankton will lead to a positive-feedback cycle, where increases in carbon dioxide concentrations in the atmosphere leads to warmer oceans, and warmer oceans lead to increasing carbon dioxide concentrations.

A team of American scientists used a Nasa satellite to study global concentrations of phytoplankton over the past two decades to see how ocean productivity - as measured by the density of chlorophyll, the pigment of photosynthesis - changes with sea temperatures.

The result was a clear link between warmer oceans and decreases in ocean productivity, said Michael Behrenfeld, professor of botany at Oregon State University and lead author of the study published in the journal Nature.

"Rising levels of carbon dioxide in the atmosphere are a key part of global warming. This study shows that as the climate warms, phytoplankton production goes down, but this also means that carbon dioxide uptake by the ocean plants will decrease," Professor Behrenfeld said. "That would allow carbon dioxide to accumulate more rapidly in the atmosphere, making the problem worse."

As average global sea temperatures increase, the growing season in some parts of the ocean at high latitudes is extended. This has the effect of boosting phytoplankton growth in these regions. But for most other areas of the world, the opposite occurs, because phytoplankton are starved of nutrients when sea temperatures rise too high.

"There is regional variability. But on a global basis,increased temperatures causes decreased phytoplankton production," Professor Behrenfeld said.

Phytoplankton needs nutrients and sunlight to grow and in warmer, tropical oceans, the levels of nutrients become the limiting factor because there are few "upwelling" currents from the sea bed to bring nitrogen, phosphorus and iron to the surface layers.

In tropical seas, the water forms layers, with warm water sitting on top of cooler water. When seawater becomes layered or stratified in this way, nutrients are prevented from rising to the surface where the phytoplankton live.

This is why cooler oceans at higher latitudes in the north and south are more productive than tropical oceans near the equator.

Global warming is having the effect of extending the range of nutrient-poor regions of the ocean to include areas that were richer in phytoplankton "blooms" on which all other marine life depends.

Despite their small size, phytoplankton account for about half of the photosynthesis carried out by all plants on Earth. And phytoplankton have a high turnover because they are quickly eaten by small marine animals - making them even more vulnerable to climate change.

"This fast turnover and the fact that phytoplankton are limited to a thin veneer of the ocean surface, where there is enough sunlight to sustain photosynthesis, makes them very responsive to climate change," Professor Behrenfeld said. "This was why we could relate productivity changes to climate variability in only a 10-year record. Such connections would be much harder to detect from space for terrestrial plant biomass."