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Carbon Brief Staff

02.08.2012 | 2:12pm
ScienceCarbon uptake has doubled over last 50 years – but where is it going?
SCIENCE | August 2. 2012. 14:12
Carbon uptake has doubled over last 50 years – but where is it going?

The amount of carbon taken up by land and ocean carbon sinks has nearly doubled over the last 50 years, according to new research, published in the journal Nature. But where is the carbon going?

The amount of carbon dioxide going into the atmosphere as a result of human activities has been fairly steadily increasing over recent decades. Carbon dioxide emissions from cement and burning fossil fuels, for example, are estimated to have increased by nearly 30 per cent between 2000 and 2009.

But not all of the carbon dioxide humans emit remains in the atmosphere. Scientists have identified a number of natural ‘carbon sinks’ – forests and oceans, for example – which are able to re-absorb some of it. Different sinks can lock away carbon for anything from years to thousands of years.  

In order to work out how manmade carbon dioxide emissions might affect climate, we need to know just how much carbon these sinks are taking in, and how this might change in the future. That’s what the new paper is about.

Researchers have calculated how much carbon is being removed from the atmosphere and stored in carbon sinks. They did this by comparing how much carbon dioxide is going into the atmosphere from human activities such as fossil fuel burning and land use changes (purple area in the graph below) to the measured levels of carbon dioxide in the atmosphere (blue area in the graph below). The difference between the two is represented by the red area in the graph below.

Carbon sink graph NOAA

Source: NOAA.

They conclude that around half of all manmade carbon dioxide emissions are taken up by carbon sinks on the land and in the oceans, and that global carbon uptake almost doubled between 1960 and 2010 – from roughly 2.4 billion tonnes of carbon per year to around 5.0 billion tonnes of carbon per year.

Carbon sinks and the atmosphere are naturally roughly in balance – that is why when we increase levels of carbon dioxide in the atmosphere, the sinks absorb more carbon.

Where is all the carbon dioxide going?

That’s the important question coming out of this research, as Ingeborg Levin, Heidelberg University, points out in a  comment article accompanying the research paper. Heidelberg says  it’s important to work out precisely where the carbon is going because:

“It makes a big difference whether the extra carbon emitted is stored in reservoirs such as the deep oceans, where it could stay for hundreds or thousands of years, or whether it is taken up by the growth of new forests, where it would stay for only a few years or decades before being returned to the atmosphere […] Another equally important reason is the need to understand the processes responsible for carbon uptake, because this knowledge will allow reliable predictions to be made of future atmospheric CO2 abundance.”

But, as yet, it’s not entirely clear where the carbon is going, because there isn’t yet a detailed analysis of every sink’s carbon capacity on every relevant timescale.

Will sinks absorb carbon indefinitely?

Previous research has suggested that carbon uptake might be slowing down as we continue to release carbon dioxide into the atmosphere and the Earth warms, because the effects of climate change might be impacting how much carbon the sinks can absorb. The new research shows that this isn’t the case yet, although scientists still expect this sink slowdown to happen in the future. Pieter Tans, a climate researcher with NOAA’s Earth System Research Laboratory, explains:

“Globally, these carbon dioxide ‘sinks’ have roughly kept pace with emissions from human activities, continuing to draw about half of the emitted CO2 back out of the atmosphere. However, we do not expect this to continue indefinitely […] The uptake of carbon dioxide by the oceans and by ecosystems is expected to slow down gradually”.

Climate models indicate that that capacity of some sinks may decline as the Earth warms. Ocean pH, for example, is changing as the oceans take up more carbon dioxide; the carbon dioxide dissolved in seawater lowers the seawater pH (making it more acidic and less alkaline – known as ‘ocean acidification’). Ocean acidification lowers the amount of carbon dioxide the oceans can contain.

Changes to carbon sinks can have a significant effect on levels of atmospheric carbon dioxide, and therefore affect future climate change. Research like this – which looks at how carbon sinks are changing as we change the planet’s climate – is an important part of getting a full picture of the consequences of increased carbon emissions.


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