PV Gap

Brian Said:

Are all the socially conscious liberals going to boycott chocolate this Valentine's Day?

We Answered:

You must be kidding. You can't even get them to give up their SUVs.

Anita Said:

Do you think photosynthesis can be used to remove CO2 from the atmosphere?

We Answered:

Do I think photosynthesis can be used to remove CO2 from the atmosphere? In a word – yes.

In a lot more words…


Using Photosynthesis
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There are some proposals for combating climate change that are looking at mimicking nature. The forerunner in this area is the use of phytoplankton. These minute marine plants exist in vast numbers in some parts of the oceans, they're invisible to the naked eye but can be seen from space as blue-green blooms in the oceans. They need nutrient rich water in which to survive and reproduce, the thinking is that by enriching parts of the oceans that are currently devoid of certain nutrients, their growth can be encouraged.

Trials have already been conducted which involved seeding a section of ocean with iron filings, these trails met with some success. A similar proposal advocates the use or urea to enrich areas of ocean lacking in nitrogen.

Because phytoplankton are plants they photosynthesise and in doing so they remove carbon dioxide from the atmosphere. When the plants die they sink to the ocean floor taking the sequestered carbon with them.

The use of algae has also been trialed as this follows the same principle.

Planting trees is another way in which carbon dioxide can be removed from the atmosphere


Planting Trees
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The two primary factors to consider are the types of trees and where they will be planted. Ideally the trees should produce a harvest of some kind in tropical regions.

Trees are like all plants in that they photosynthesise. Using the energy from sunlight they convert carbon dioxide and water into sugar and oxygen. The sugar is produced through the process of carbon fixation and the resulting carbohydrate is what makes up the majority of a plants mass; typically 60 to 70% cellular matter with most of the remainder being water.

One problem with trees is that if a tree is grown and it subsequently degrades then the stored carbon is re-released back into the atmosphere and there has been no long-term benefit. However, if the tree produces a yield of nuts, fruit, berries, sap, resin etc and this is eaten or used rather than being allowed to rot, then there will be a net reduction in carbon dioxide levels.

A typical fruit tree produces a net reduction of between 20kg and 40kg per annum - depending on the type of fruit and where the tree is. Over an average 90 year lifespan this means that one fruit tree will remove 2 or 3 tonnes of CO2 from the atmosphere.

Other ways in which trees can act as carbon sinks is by increasing the number of trees. Provided the number increases there will be more CO2 being stored than is being released when old trees die off. An alternative is to store the timber from the trees by using it in construction, furniture making etc; this way the carbon remains locked in the timber rather than being released when the timber rots away.


Artificial Trees
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Schemes that are designed to modify the climate are known as geoengineering or climate engineering and one such scheme involves the use of synthetic trees.

It's so called because it copies a real tree by removing CO2 from the atmosphere and vaguely resembles a tree in appearance. The device consists of a large open-slatted 'screen', exposed to the atmosphere and mounted atop a column.

Sodium hydroxide within the 'screen' would sequester CO2 from the atmosphere. There are a couple of ways of doing this and both involve a bit of chemistry. One involves the use of atomised sodium hydroxide to react with the CO2 and produce sodium bicarbonate. Calcium oxide would be added and the NaOH recovered for reuse, the residual calcium carbonate would be heated and the CaO recovered for reuse leaving behind CO2 for disposal underground.

A similar scheme using NaOH has been proposed by Prof. Klaus Lackner, it employs a different chemical process (described in the link below) but comes with the advantage that the byproduct is synthetic diesel or gasoline which can be sold to offset the running costs.

The synthetic tree approach has several advantages. Unlike some other geoengineering proposals it doesn't interfere with any natural mechanism or process, it simply removes what we're adding to the atmosphere. It's fully controllable and can be stopped, started, increased or decreased with ease. It's an efficient system with each 'tree' having the capacity to remove 90,000 tons of CO2 from the atmosphere each year (roughly the equivalent emissions from a town of 15,000 people). By comparison to some other schemes it's relatively cheap, as far as I'm aware there's no accurate costings available but it's likely to be in the order of $10 per person per year.

One big advantage is that it's simply an adaptation of existing technology that has been used successfully for many years around the world. Effectively it's taking the emission scrubbers away from factories and smoke-stacks and placing them in the open atmosphere.

Of course, geo-engineering schemes, as good or bad as they may be, don't address the underlying cause of global warming - they're basically treating the symptoms without treating the cause.

This white paper contains a lot more (sometimes technical) information about the synthetic trees option http://www.netl.doe.gov/publications/pro…