The latest issue of Nature contained a paper by Daniel G. Boyce, Marlon R. Lewis & Boris Worm called Global phytoplankton decline over the past century. This research describes a planetary catastrophe which, on a scale of 1 to 10, ranks about 8.5 on the disaster scale. This post should be viewed as a follow-up to How We Wrecked The Oceans (DOTE, May 17, 2010).

Here's the Nature abstract—

In the oceans, ubiquitous microscopic phototrophs (phytoplankton) account for approximately half the production of organic matter on Earth. Analyses of satellite-derived phytoplankton concentration (available since 1979) have suggested decadal-scale fluctuations linked to climate forcing, but the length of this record is insufficient to resolve longer-term trends. Here we combine available ocean transparency measurements and in situ chlorophyll observations to estimate the time dependence of phytoplankton biomass at local, regional and global scales since 1899. We observe declines in eight out of ten ocean regions, and estimate a global rate of decline of ~1% of the global median per year. Our analyses further reveal interannual to decadal phytoplankton fluctuations superimposed on long-term trends. These fluctuations are strongly correlated with basin-scale climate indices, whereas long-term declining trends are related to increasing sea surface temperatures. We conclude that global phytoplankton concentration has declined over the past century; this decline will need to be considered in future studies of marine ecosystems, geochemical cycling, ocean circulation and fisheries.

Phytoplankton are microscopic photosynthesizers, which means that they use light energy from the sun and take in carbon dioxide (CO₂) to produce oxygen. These tiny plants, along with cyanobacteria, do a lot of the work that keeps the biosphere stable. From the press release—

The findings contribute to a growing body of scientific evidence indicating that global warming is altering the fundamentals of marine ecosystems. Says co-author Marlon Lewis, "Climate-driven phytoplankton declines are another important dimension of global change in the oceans, which are already stressed by the effects of fishing and pollution. Better observational tools and scientific understanding are needed to enable accurate forecasts of the future health of the ocean." Explains co-author Boris Worm, "Phytoplankton are a critical part of our planetary life support system. They produce half of the oxygen we breathe, draw down surface CO₂, and ultimately support all of our fisheries. An ocean with less phytoplankton will function differently, and this has to be accounted for in our management efforts."

Doing the math, the researchers estimate that an astonishing 40% of ocean's phytoplankton population has disappeared since 1950! The fewer microscopic plants there are living in the ocean surface waters, the less CO₂ is drawn down from the atmosphere. Thus, the Earth's carbon cycle is being fundamentally altered, with uncertain but surely deleterious effects.

As the press release indicates, the researchers found that warmer surface waters caused by global warming are the main suspect in the decline—

The scientists report that most phytoplankton declines occurred in polar and tropical regions and in the open oceans where most phytoplankton production occurs. Rising sea surface temperatures were negatively correlated with phytoplankton growth over most of the globe, especially close to the equator. Phytoplankton need both sunlight and nutrients to grow; warm oceans are strongly stratified, which limits the amount of nutrients that are delivered from deeper waters to the surface ocean. Rising temperatures may contribute to making the tropical oceans even more stratified, leading to increasing nutrient limitation and phytoplankton declines. The scientists also found that large-scale climate fluctuations, such as the El-Niño Southern Oscillation (ENSO), affect phytoplankton on a year-to-year basis, by changing short-term oceanographic conditions.

In other words, warmer oceans are not well mixed at the surface, with warmer water sitting atop colder deeper water. Photosynthesizers must live in the surface waters where there is access to sunlight, but do not get the nutrients from upwelling colder water required by their metabolism. So they die off.

It is clear that we have a disastrous positive feedback loop at work here, in which warmer surface water supports fewer phytoplankton, which then take up less CO₂ from the atmosphere, which causes the surface water to warm some more due to the greenhouse effect, etc. In fact, one of the crazier geo-engineering solutions to global warming is to seed the ocean surface waters with iron filings to stimulate phytoplankton growth!

Up to this point, I have written up this disaster without much emotion. That's probably because it's so depressing. If this trend continues, we Earthlings are surely fucked. And maybe on relatively short time scales (a few decades). In fact, if we have indeed lost 40%  of the phytoplankton in the oceans since 1950, I do not understand why we have not felt the terrible effects already. Perhaps the Earth's biosphere (primary productivity) and nutrient recycling (as with carbon) are more resilient than they appear. None of this is well understood, but our uncertainty cuts both ways.

I think there's a certain point—perhaps we have just now passed it—where all you can do is throw your hands up in the air and shout out what the fuck do we do now? Frankly, I hope this phytoplankton decline result is wrong, but I fear it is right.