Location, location, location

Algae coated ice off the Antarctic coast

Algae coated ice off the Antarctic coast

19 January 2015 by Josie Robinson

If we could trap the CO2 we produce in the deepest depths of the ocean, where should we do it? Maybe not where scientists first thought, says Josie Robinson. She explores the theory behind ocean iron fertilisation in the Southern Ocean, and describes her work on testing it.

The idea of scientists manipulating nature to avoid climate change evokes images of Dr Evil toying with volcanoes. Yet the concept - known as geoengineering - is a very real one. It's controversial, but it may be a necessary last resort if we don't lower our CO2 emissions.

If we reach a fatal tipping point with climate change - a serious possibility - we would look to scientists for help, like in the film The Day After Tomorrow. So although geoengineering is far from ideal - it'd be better not to emit the CO2 in the first place - scientists are a well-prepared bunch, and we research every option. Proposed geoengineering methods range from orbiting space mirrors to simply pumping CO2 into the ground. The one I focus on is ocean iron fertilisation (OIF) which aims to make the oceans absorb more CO2, removing it from the atmosphere before it can affect the climate.

You might remember from school biology that plants take up CO2 during photosynthesis so they can grow. In the oceans this job is done by tiny plants, known as phytoplankton or algae - like the green sludge that grows in lakes. When the phytoplankton die, they slowly sink down into the depths. Most of the dead plant matter decomposes as it sinks, releasing the carbon that was absorbed during photosynthesis into the water; from there it ultimately escapes back into the atmosphere. But if the phytoplankton sink deep enough before being broken down, the sheer volume of the ocean could keep the carbon confined to the deep dense water - perhaps for centuries.

This happens naturally across the world's oceans, enabling the sea to sponge up a third of human CO2 emissions and reducing our impact on the climate. It doesn't happen in the Southern Ocean, though. The water here contains lots of nutrients, but it lacks iron, which phytoplankton need to grow. So an oceanographer got the idea of fertilising the Southern Ocean with the missing nutrient iron to allow algae to grow there as well, soaking up more CO2 into the ocean.

Just add iron?

That's the theory, but would it actually work? In field studies when scientists have gone to the Southern Ocean and thrown iron into the sea, they did indeed cause algal blooms. We've also looked at places around islands in the Southern Ocean where iron fertilisation happens naturally as iron is released from the mud; there, we do see more carbon sinking to the deep.

Now if the first stage of OIF is trapping CO2 in plant form and the second is transporting the carbon to the deep ocean when the phytoplankton die, then what's the third and final stage? This is where I come in. The third stage is long-term carbon sequestration in the deep ocean. To sequester something means to keep it out of reach in a safe place; the goal is to keep CO2 out of the atmosphere by trapping it deep under water for a long time, at least a century.

I assume that steps one and two have been successful, and we've managed to get a lot of carbon into the deep ocean - actually quite a big assumption and a hot research topic in itself. So what happens next? By using a computer model which replicates the ocean's circulation, we can predict what might happen to the carbon over 100 years. To do this I put markers, representing the carbon, across the Southern Ocean at a depth of 1,000m within the ocean circulation model. The markers have no weight so neither sink nor float; instead they ride the ocean currents, just like CO2 molecules would. I tracked the markers within the simulated ocean for 100 years to see where they'd end up and, crucially, if the carbon would stay away from the atmosphere - the whole purpose of OIF.

Ocean fertilisation

Ocean fertilisation

Having analysed 25,000 markers, I can tell you that a lot of them (66%) did find a way out of the deep ocean and back into contact with the atmosphere, taking on average 38 years. We expected some of the carbon would leak out over 100 years, but two thirds is quite a lot. Not only that, but it escaped very quickly!

With over half the carbon escaping back into the atmosphere, you may be beginning to wonder what the point is, but remember this is only one experiment and only a computer simulation of the ocean. The real ocean is such a complex system that recreating its behaviour in a computer model is immensely difficult, a bit of a black art of oceanography. We can never fully rely on simulations - although the level of accuracy in ours is impressive - but we can use them to provide us with potential scenarios, whose likelihood we can then check with further research.

So how did this Houdini carbon manage to escape in our experiment? The key factor is the ocean circulation. The Southern Ocean is like the M25; it connects all major oceans - not just horizontally, but also vertically. Here, deep water rises up to the surface on a massive scale. This is due to the trade winds, which push water near the surface away from Antarctica so that water from the deep rises to fill the gap. In our experiment this was the carbon's main escape route from the deep ocean.

The bottom line is that location is crucial. We were half right to think the Southern Ocean is a good place for OIF; it has an abundance of all essential nutrients except iron. Yet our research suggests that although there are some areas that are suitable for geoengineering, ultimately it's a bad location because of its highly dynamic circulation. Even carbon that did stay in deep water for the full century was spread all over the world by the circulation, making it almost impossible to monitor the sequestered carbon - this would be essential in a carbon credit market where someone would be paying to have carbon locked up on their behalf to offset the pollution they were causing.

All this suggests OIF alone is probably not the miracle cure for all our climate-change problems. But it may have a role to play alongside other geoengineering methods. There are places in the world's oceans that do seem to be suitable for OIF, so the research must continue. Of course, the best course of action is to cut our carbon emissions, and leave geoengineering to movie directors.

Josie Robinson is a PhD student at the University of Southampton and National Oceanography Centre.