Geoengineering research should look at nutrient-poor waters
1 November 2013, by Tom Marshall
A new study suggests that plans to lock up carbon and slow climate change by adding nutrients to the ocean to encourage plankton growth might have been directed at the wrong places.
So far, researchers' attention has tended to focus on waters that have plenty of the major plant nutrients – nitrates and phosphates – and where the growth of phytoplankton – tiny marine algae – is held back by a shortage of elements that plants need only in tiny quantities. Most commonly the limiting element is iron.
The idea is that adding nutrients could create blooms of algae that would absorb carbon from the atmosphere. This is a type of geoengineering – using technology to slow down climate change caused by human activities.
But experiments that added iron to these waters suggested that the plankton blooms are usually short-lived and that most of the carbon absorbed by the marine ecosystem soon ended up back in the atmosphere, so there was little lasting impact on the climate.
A new paper published in Journal of The Royal Society Interface suggests would-be geoengineers might be better off looking instead at the 'macronutrient-limited' parts of the ocean, known as the subtropical ocean gyres.
These are permanently stratified, separated into distinct layers with little mixing between them. Because of this, nutrients from deeper water can't get to the layers near the surface, which therefore lack the nutrients that plants need lots of – most importantly, nitrogen. They make up around 70 per cent of the oceans' total area, but because they're so poor in nutrients, they have very little plant life.
Professor Jim Aiken, an Honorary Fellow of Plymouth Marine Laboratory (PML) and one of the study's authors, says that if we could somehow increase plankton growth even a little across these vast marine deserts, the overall effect on carbon absorption would be enormous. That wouldn't be the only advantage. 'The gyres are so huge that if we could get them growing productively, there would be major benefits not just in terms of carbon but also in other areas like bigger fish populations,' he says.
The authors used a well-established model of plankton growth and carbon absorption at different depth zones in the water column to examine the effects of two different potential approaches to geoengineering – nutrient addition and solar shading by adding particles to the upper atmosphere – on the productivity of plankton in macronutrient-limited (or 'oligotrophic') waters.
Satellite image showing plankton activity in the Atlantic in January and July - click to enlarge.
Adding nutrients generated more activity from tiny algal plankton, which led to more of the bigger kinds of plankton too. The overall effect was more dead plankton sinking to the seabed, and so more carbon being taken out of circulation for the long term.
The study also suggests that adding particles to the atmosphere to reflect some of the sun's light back into space – another potential way of engineering the climate to counteract global warming that's been widely discussed – doesn't seem to lessen the activity of plankton in nutrient-poor stretches of ocean.
The model did show that solar shading changes the distribution of this growth through the water column, with more algae counter-intuitively growing in shallow water layers. But the total productivity throughout the water column stays about the same. This makes sense if you consider the natural seasonal cycle seen in the sub-tropical gyres; photosynthetic phytoplankton production is highest just after the mid-winter solstice when sunlight is least, and lowest in summer when sunlight is highest. The model also showed that the changes to plankton activity were reversed as soon as sunlight returned to normal levels.
The result is important because it suggests that one possible problem with these schemes isn't too serious. Some experts were concerned that shading the Earth would reduce the amount of sunlight illuminating ocean ecosystems and slow down phytoplankton growth. So there would be less sunlight heating the planet, but also less carbon being removed from the atmosphere because of reduced plankton growth.
The worry was that the harm from the second factor could limit or even outweigh the benefits of the first. The new study suggests this isn't the case - if we decide to do this kind of solar shading, the effect on plankton growth needn't be a major concern.
Aiken says more research is needed - ideally, an expedition in which scientists add nutrients to a low-nutrient area of ocean and monitor the effects on plankton activity for a long period – in particular how much more carbon is absorbed, and how much of this ends up on the seabed. This could be done fairly close to the UK, perhaps in the continental shelf seas to the southwest.
In summer these have a similar stratified physical structure to that seen all year round in the subtropical gyres, with the water column separating into distinct layers. The fertilisation could be done by pumping nutrient-rich water from deeper layers to the surface layer, powered by solar panels attached to surface buoys. Careful management and monitoring would be needed to ensure there were no adverse effects.
This study has wider significance for the global oceans as the planet's climate changes. The subtropical gyres - around 50 per cent of Earth's surface area - are all warming and expanding and the concentration of chlorophyll is changing, but with regional variations.
The research team included scientists from PML, Hokkaido University in Japan, the Commonwealth Scientific and Industrial Research Organisation in Australia and the UK National Centre for Earth Observation.
Impacts of light shading and nutrient enrichment geo-engineering approaches on the productivity of a stratified, oligotrophic ocean ecosystem. Nick J. Hardman-Mountford, Luca Polimene, Takafumi Hirata, Robert J. W. Brewin and Jim Aiken. J. R. Soc. Interface 6 December 2013 vol. 10 no. 89 20130701. DOI: 10.1098/rsif.2013.0701
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