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."