|Plankton blooms swirl north of Norway (ESA)|
That's the lead from a press-release accompanying an interesting new paper in Nature. The paper's authors have found that melting ice sheets in Greenland and Antartica aren't just cranking up sea-levels, inch-by-worrying-inch. They're also pumping out iron - an important nutrient - into the seas surrounding them. Given that a little bit of extra iron has been shown to boost the growth of blooms of phytoplankton - the tiny micro-organisms that are the basis of nearly all marine life - and that phytoplankton absorbs CO2 - the headache gas of global warming - it's easy enough to spot the kernel of a good-news story.
|Nanoparticulate ferrihyrite in all its glory|
The fact that melting glaciers are a major source of this 'bioavailable' iron is something of a novel concept. It may go some way towards explaining the large blooms of phytoplankton that colour the Arctic ocean in spring and summer. So this paper is major step forward in understanding the complex interweave between climate, chemistry and life in the coldest spots on the planet. But does this new research also point the way towards phytoplankton blooms as a force for good in the battle against climate change?
That's where the good news starts to falter. Ignore for the moment the rather perverse fact that, in order for the plankton blooms to start pulling CO2 out the atmosphere, we'd have to see a continued and substantial melting of the polar ice-sheets. Put on hold, too, the question as to how any benefit from the mighty plankton's chipping away at CO2 would balance against the extra warming caused by the loss of all that pristine, reflective white ice.
Would a change in plankton blooms actually have the projected effects in the first place? Dig into recent literature, and the answer is far from unambiguous. The benefits of plankton blooms, for taking the edge off of global warming, arise because plankton near the surface absorb CO2 from the water. That in turn drives its absorption from the atmosphere, helping lower CO2 levels. When the plankton die, they sink to bottom of the ocean, taking their carbon with them. That's the theory.
|The dazzling diversity of the phytoplanton community (PNAS)|
In practice, that plankton-lowering of CO2 also has some rather unsavory knock-ons further down the line. The increased levels of decomposing plankton causes more oxygen to be sucked out of the sea, resulting in plumes of oxygen-starved water which can kill off other wildlife.
These sinking waters will also reappear at the surface eventually. Because they are also rich in the nutrient by-products of the dead bloom, once at the surface they are likely to release methane and nitrous oxide to the atmosphere. Those are 2 powerful greenhouse gases, with a more powerful kick (per molecule) than CO2.
Then there's the fact that blooms themselves heat the surrounding water. Where there's more plankton capturing the sunlight for photosynthesis, there's more heat gathered at the sea surface. Worse, warming surface waters hold less CO2, and are likely to turn-over less, reducing the chances of absorbing CO2 from the atmosphere. How would these warming effects balance out against the cooling effect of the plankton pumping carbon from the atmosphere into the ocean's deep sediments? The truth is, no-one knows.
It all adds up to big messy pile of complications to what sounded like a nice, positive wrinkle to the climate change story. It's true that global warming is melting ice-sheets faster. And that might end up producing more iron to feed more plankton blooms. But the final consequences of all the subsequent complex interactions of biology and chemistry, of sun and sea and air - these are likely to remain a big unknown. So rather than acting as a possible 'buffer' for global warming, those boosted blooms just add to the uncertainty. And so to the risk.