Carbon sequestering

Carbon sequestering is the drawing of carbon dioxide from the atmosphere and storing it in a carbon sink that is either natural or man made.

The most common is biosequestration where plants capture carbon as carbon dioxide and through photosynthesis convert CO2 to carbohydrates in plant tissues that, in turn, die and decompose into soil carbon.

There is also a priming effect of vegetation cover and plant growth on biological activity in soil that can promote further carbon sequestration.

In many places it is also possible to manage plants to sequester more carbon into vegetation and soils than would happen normally — we do this routinely and call it agriculture.

Carbon sequestering is also used to describe the engineering solution of carbon capture and storage [CCS], where carbon is stripped from the emissions stacks of coal-fired power stations and stored underground as liquid CO2. Only this is not strictly sequestration because the CO2 was released in the generation of electricity from coal power in the first place.

The most effective carbon sinks that we can manipulate to increase carbon sequestration are

• vegetation
• soil
• phytoplankton in the oceans

And it is revegetation of degraded land and the improvement of soil carbon that has the biggest potential to help with our desire to balance the levels of greenhouse gases in the atmosphere.

How much biosequestration?

In Australia at least 300,000 km2 of agricultural land is considered degraded.

This is 68% of all the arable land in the country and this is a typical percentage for the more arid regions of the world.

Professor Ratten Lal, Director of the Carbon Management and Sequestration Centre, Ohio State University estimates that

"the carbon sink capacity of the world's agricultural and degraded soils is 50 to 66% of the historic carbon loss of 42 to 78 gigatons of carbon".

In other words we could put back over half of the carbon we have let loose from land clearing for agriculture — and that is about the same amount as emissions from transport.

Not only is this a huge opportunity but adding carbon to soil almost always increases agricultural productivity as well, a win-win.

So if emissions reduction efforts for fossil fuel use are successful and an equivalent effort put into land management that leads to sequestration then carbon neutrality is achievable.

But then so is the earthquake proofing of major cities such as Los Angeles and Istanbul, only we have not quite got around to shoring up all the buildings in those places.

It is also possible to artificially enhance carbon sequestering by phytoplankton in the oceans. However such large scale manipulations are expensive and could lead to ocean acidification.

the conundrum

One in seven of the people in the world go to bed hungry more often than not. 

Some one billion people spread over 100 countries experience food security risk from variable and uncertain agricultural production. Often made worse by drought and poor land management.

According to United Nations Convention to Combat Desertification [UNCCD], over 250 million people are directly affected by land degradation.

Even without the climate benefits, it makes huge sense to sequester carbon into degraded soils around the world because it will reduce this food security risk by helping to rehabilitate degraded land.

So many benefits come about from improving soil carbon that it is amazing that the topic remains a quirky one out on the peripheries of agricultural science. Its a bit like lawyers not bothering with case law anymore and consigning it to the archives. 

Instead what we mine soil for money or subsistence.  We have to reverse this notion if sequestration is to be sufficiently widespread to make a difference to the climate.

Unfortunately the real issue is really that despite major benefits to production and the atmosphere, revegetation and rehabilitation of degraded land is often too costly.