Climate change impacts marine productivity
Increasing sea temperatures from the current 0.2 degrees per decade have reduced the biomass and growth of marine phytoplankton. As a result, an important store of carbon dioxide is disappearing and the entire food web of the oceans is in danger, reports researchers led by Michael Behrenfeld from Oregon State University [1].
The scientists evaluated nine years of data from the Sea-viewing Wide Field-of-View Sensor (SeaWiFS) on board the OrbView-2 satellite. The biomass of the phytoplankton and the photosynthetic activity can be calculated based on the ocean colour. According to this, productivity initially increased from 1997 until it reached a maximum in 1999 – parallel to a global cooling after a very pronounced El Niño. However, with the onset of global warming since 2000, productivity began to fall, by as much as thirty percent in some regions.
The reason behind this is that warmer surface temperatures make it more difficult for the different layers of water to mix, and the tiny seaweed therefore does not get enough nutrients that they need for their growth. In the high latitudes, on the other hand, the greater stratification would prevent the algae from being torn down in large quantities here at the overturning points of the oceanic conveyor belts - productivity could therefore increase here. However, this would probably not compensate for the losses in the temperate and tropical latitudes.
Overall, marine phytoplankton account for half of global photosynthesis. In the event of a further decrease, the researchers fear a fatal feedback loop: If the marine organisms absorb less carbon dioxide from the atmosphere because the oceans are getting too warm for them, the greenhouse effect increases - and the surface temperatures climb even more. In addition, the broad basis of the marine food chain narrowed, with unforeseeable consequences for wildlife and ultimately also for human nutrition.
While the consequences of the massive greenhouse effect 55 million years ago are known, the source of the greenhouse gases remains unclear, conclude Yale University's Mark Pagani and his colleagues. In view of the unimaginable amounts of carbon dioxide or methane that would have to have gotten into the air for this to happen, the researchers conclude that at least at that time the climate reacted far more sensitively to the increase in greenhouse gases than previously thought [2].
Currently, if CO2 doubles as expected by mid-century, a temperature increase of 1.5 to 4.5 degrees Celsius is expected. Applied to the warming at the end of the Paleocene, this would equate to a release of 5,400 to 112,000 billion tons of carbon. For comparison, the total fossil fuel resources currently available are 5 trillion tons. So if such an immense emission had not taken place, then the temperatures must have climbed by more than 2.5 degrees for every doubling of carbon dioxide. If the main culprit is methane, the researchers even speak of 5.6 degrees more. As part of the warming at the Paleocene/Eocene boundary, global temperatures had risen by five degrees in just 10,000 years and remained at this level for 170,000 years. (af)
