Scientists may have found the most devastating impact yet of human-caused global warming — a 40% decline in phytoplankton since 1950 linked to the rise in ocean sea surface temperatures. If confirmed, it may represent the single most important finding of the year in climate science.

The headlines above are from an appropriately blunt article in The Independent about the new study in Nature, “Global phytoplankton decline over the past century” (subs. req’d). Even the Wall Street Journal warned, “Vital Marine Plants in Steep Decline.” Seth Borenstein of the AP explains, “plant plankton found in the world’s oceans are crucial to much of life on Earth. They are the foundation of the bountiful marine food web, produce half the world’s oxygen and suck up harmful carbon dioxide.”

We’ve known for a while that we are poisoning the oceans and that human emissions of carbon dioxide, left unchecked, would likely have devastating consequences — see “2010 Nature Geoscience study: Oceans are acidifying 10 times faster today than 55 million years ago when a mass extinction of marine species occurred.” And we’ve known those impacts might last a long, long time — see 2009 Nature Geoscience study concludes ocean dead zones “devoid of fish and seafood” are poised to expand and “remain for thousands of years.”

But until now, conventional wisdom has been that the biggest impacts might not be until the second half of the century. This new research in Nature suggests we may have much less time to act than we thought if we want to save marine life — and ourselves. The study concludes:

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.

The WSJ explains, “The data are more reliable for recent decades, translating into a 40% decline since 1950.” It points out:

The team investigated several factors that could have caused the decline. “We found that temperature had the best power to explain the changes,” said Boris Worm, a marine biologist at Dalhousie and co-author of the study.

Marine algae live in the upper layers of the ocean but rely on nutrients that circulate up from lower layers. Rising temperatures mean the different layers mix less with each other, so fewer nutrients reach the algae. However, Dr. Worm noted that algal abundance can be affected by other factors, such as shifts in predator-prey populations.

Mike Behrenfeld, an expert on phytoplankton at Oregon State University, said the paper was similar to a 1992 study that used Secchi data to show a long-term decline in marine algae in the north Pacific. “But this paper covers the globe,” he said. “And the scientists also took the next step of relating the [algal decline] to sea temperatures.”

Yes, I know, the marine biologist is named Boris Worm. Readers may recall that last year, Worm was lead author on a major study on fisheries in Science, and the WashPost quoted him predicting that “if fishing continued at the same rate, all the world’s seafood stocks would collapse by 2048” (see “What’s in a name? For the slimehead and toothfish, the extreme makeover leads to rampant overfishing“). And people think I’m a pessimist!

The Independent also catches a quote from Worm:

“If this holds up, something really serious is underway and has been underway for decades. I’ve been trying to think of a biological change that’s bigger than this and I can’t think of one,” said marine biologist Boris Worm of Canada’s Dalhousie University in Halifax, Nova Scotia. He said: “If real, it means that the marine ecosystem today looks very different to what it was a few decades ago and a lot of this change is happening way out in the open, blue ocean where we cannot see it. I’m concerned about this finding.”

… “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 fishes.” he said.

Certainly, scientists are going to have to verify this finding in the coming years, but as AP reports:

Previous plankton research has mostly relied on satellite data that only goes back to 1978. But Worm and colleagues used a low-tech technology — disks devised by Vatican scientist Pietro Angelo Secchi, in the 19th century. These disks measure the murkiness of the ocean. The murkier the waters, the more plankton.

It’s a proxy the scientific community has long accepted as legitimate, said Paul Falkowski of Rutgers University, who has used Secchi disk data for his work.

He and other independent scientists said the methods and conclusions of the new study made sense.

Recognizing the importance of the article, Nature published a second piece by two leading ocean scientists, that discussed the methodology and findings, calling the work “an impressive synthesis of the relevant data”:

Taking great care, they created time series of phytoplankton biomass in the pelagic ocean, quantified as surface chlorophyll concentrations. They find a strong correspondence between this chlorophyll record and changes in both leading climate indices and ocean thermal conditions. They also show statistically significant long-term decreases in chlorophyll concentrations for eight of the ten ocean basins, and for the global aggregate.

We ignore these results at our gravest peril.