Why’s the Ocean Less Greenby Bill Chameides | August 4th, 2010
posted by Erica Rowell (Editor)
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Is climate change causing an oceanic famine?
When I was a kid there was this commercial on television for encyclopedias in which a kid asks his dad, “Why’s the sky blue?” The dad isn’t able to answer until he goes and buys the encyclopedia — and then all is well, both dad and kid all smiles. Now the kid spends his free time reading the encyclopedia, and he’s going to grow up and be president or something. Meanwhile I’m just sitting there watching the stupid ad. Who wants to be president, anyway? Look at poor Barack.
Today it’s a different world and the dad would probably tell the kid to Google it, and since hope springs eternal, maybe his discoveries about the sky would pique his curiosity about other aspects of the earth. Maybe next he’d query cyberspace about the ocean — why is it green? He’d quickly learn that the ocean is green because of phytoplankton. If he keeps digging, he might come across a scientific paper that concludes the ocean is becoming less green. It could happen. In fact, that last bit, finding a paper stating that the ocean is becoming less green? That’s what happened to me over the weekend.
Microscopic Organisms Known As Phytoplankton Are Key to Underwater Life
When it comes to ocean life, phytoplankton are key — they exist at the bottom of the ocean’s food chain; the organic carbon they make through photosynthesis supports essentially all other life in the sea including the seafood we eat. To a first approximation, the less production from phytoplankton, the less fish there will be for us to eat.
There’s another reason to care about our green friends in the ocean. Microscopic phytoplankton help to transfer carbon dioxide (CO2) from the atmosphere to the deep ocean through something called the biological pump. (More on that in this post.) Fewer phytoplankton doing their thing in the ocean means more heat-trapping, global-warming CO2 in the atmosphere.
For those reasons the new paper by Daniel Boyce of Dalhousie University in Nova Scotia and colleagues published last week in the journal Nature is a bit of a shocker. The authors combined data from as far back as 1899 on the amount of phytoplankton in the ocean to determine if there has been any change in phytoplankton over the past 100+ years.
Chlorophyll Levels Used to Determine Amount of Phytoplankton
Marine biologists often use the amount of chlorophyll pigment (the thing that makes green plants green and allows them to be little photosynthesis machines) in the water column as a metric for the amount of phytoplankton there.
Since the 1950s scientists have quantified chlorophyll by collecting and analyzing water samples to measure concentrations of chlorophyll pigment. More recently (i.e., starting in the 1970s), we have also used satellite measurements of the intensity of the ocean’s green color to do the same with much greater spatial coverage.
While back in 1899 neither method was available to folks curious about the ocean, clever nineteenth century minds found clever, low-tech ways to study chlorophyll. Pietro Angelo Secchi, an Italian scientist and Jesuit priest, is credited with developing a simple but accurate method of determining chlorophyll levels: by lowering a white disk into the ocean until it disappeared, the water’s transparency can be determined and hence the amount of phytoplankton in the water column.
Scientists in the modern era have determined the specific quantitative relationship between the depth the white Secchi disk is no longer visible and the amount of chlorophyll by calibrating the results from the Secchi method with laboratory analysis of water from the same column.
And so, voila, we have more than 100 years of chlorophyll data and by extension data on the amount of chlorophyll in the ocean. Assembling such a record is no small feat — it requires carefully combining sundry data sets and making sure they are all appropriately calibrated to the same standard so they can be consistently compared. This Boyce and his colleagues did, and then examined the data for evidence of a long-term trend.
Declines in Phytoplankton Found to Be Extensive and Troubling
And evidence of a trend they did find. The authors found that oceanic productivity has declined; they found significant declines in the data spanning the period since 1899 to the present and, if you are suspicious of data from the Secchi method, they also found significant declines for the period from 1950 to the present. The declines were found for all the world’s oceans with the exception of the North and South Indian Oceans.
And by the way, we’re not talking about small changes. Globally the rate of decline was estimated at one percent per year since 1899. That might not seem like a lot but over 100 years it is; it adds up to a factor-of-two decrease.
What could have caused the decline? The authors found that chlorophyll declines were most closely correlated with increases in sea surface temperatures, and, as I’m sure you know, sea surface temperatures have been on the rise because of global warming. Such a result is not all that surprising.
I don’t find it all that surprising that phytoplankton would be unhappy with global warming. As the surface ocean heats up, the ocean becomes more stratified; there is less upwelling or mixing of nutrient-rich deep ocean water with surface waters and this tends to suppress photosynthesis. What I do find surprising, indeed astounding, is the size of the chlorophyll decline. If Boyce et al’s result is applicable to the total productivity of the ocean, it would imply that the productivity has decreased by a factor of two over the past 100 years. And that’s a huge decrease, more than enough to have had a large impact on ocean fisheries and CO2 uptake by the ocean.
In all likelihood the overall decrease was not quite so large. Ocean productivity varies spatially over the ocean. Much of the productivity occurs in specialized regions of the ocean such as coastal and shallow water areas — areas that Boyce et al excluded from their analysis. Much of the open oceans, where the authors found the largest decreases in chlorophyll, are veritable deserts with very low chlorophyll concentrations and productivity. Perhaps a careful weighting of the regional decreases with the amount of productivity in the regions would yield a more modest overall decrease.
So like the kid in the commercial this paper leaves with me some questions unanswered. I doubt if the answers can be found in an encyclopedia or on the Web. I’ll have to wait for additional research.filed under: carbon dioxide, climate change, faculty, global warming, oceans, Planetary Watch
and: chlorophyll, photosynthesis, phytoplankton, Pietro Angelo Secchi, seafood, Secchi disk