A recent study by scientists from MIT, Woods Hole Oceanographic Institution (WHOI), and other institutions has challenged the prevailing notion that the North Atlantic Ocean's productivity has dropped 10% during the industrial era due to rising surface temperatures.
The study, published in Nature, used ice cores from Greenland to reconstruct the history of a chemical compound that is produced by phytoplankton, the microscopic organisms that form the base of the marine food web.
The researchers found that the levels of this compound, known as methanesulfonic acid (MSA), have not changed significantly over the past 8,000 years, suggesting that phytoplankton's productivity has been relatively stable in the North Atlantic.
Phytoplankton and the ocean's carbon cycle
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Phytoplankton are responsible for absorbing carbon dioxide from the atmosphere and converting it into organic carbon and oxygen through photosynthesis.
Some of this carbon is consumed by other marine organisms, while some sinks to the deeper ocean, where it is stored for long periods of time.
This process, known as the biological pump, plays a crucial role in regulating the global carbon cycle and climate.
Phytoplankton also produces dimethylsulfide (DMS), a gas that is released into the atmosphere and breaks down into sulfate aerosol or MSA.
These particles can act as cloud condensation nuclei, increasing the reflectivity of clouds and cooling the planet.
Therefore, phytoplankton can influence the climate not only through the carbon cycle, but also through the sulfur cycle.
Ice cores as a proxy for phytoplankton productivity
Previous studies have used satellite observations and ocean measurements to estimate phytoplankton's productivity in the North Atlantic, one of the world's most productive marine basins.
These studies have shown that phytoplankton's productivity has declined by about 10% since the mid-19th century, coinciding with rising surface temperatures.
However, these data are limited to the past few decades, and do not capture the natural variability and long-term trends of phytoplankton's productivity.
To overcome this limitation, the researchers in the new study used ice cores from Greenland as a proxy for phytoplankton productivity in the North Atlantic.
Ice cores are cylinders of ice that are drilled from glaciers and ice sheets, and contain layers of snow that have accumulated over thousands of years.
By analyzing the chemical composition of these layers, scientists can reconstruct the past climate and environmental conditions.
The researchers focused on MSA, which is deposited on the ice surface by winds after it is produced by phytoplankton in the ocean.
They analyzed ice cores from four different sites in Greenland, covering a period of 8,000 years, from the early Holocene to the present day.
They compared the MSA levels in the ice cores with other indicators of past climate, such as temperature, precipitation, and atmospheric circulation.
Phytoplankton productivity has been stable over the past 8,000 years
The results of the analysis showed that MSA levels in the ice cores have not changed significantly over the past 8,000 years, indicating that phytoplankton productivity in the North Atlantic has been relatively stable over this period.
The researchers also found that MSA levels were not correlated with surface temperature, but rather with changes in atmospheric circulation and precipitation patterns, which affect the transport and deposition of MSA on the ice surface.
The researchers suggested that the apparent decline in phytoplankton productivity observed in the satellite and ocean data may be due to natural variability, rather than a long-term trend driven by climate change.
They also proposed that phytoplankton may have adapted to the warming temperatures by changing their physiology or community composition, maintaining their productivity despite the changing environment.
