Where does all the carbon go?
22 June 2007
In a climate change U-turn, scientists have discovered that Northern forests soak up just under half the carbon previously thought. So do global warming predictions - and remedies - need a re-think?Antenna investigates...
This research was published in the journal Science on 22 June 2007.
Global estimates suggest we emit roughly 26 billion tonnes of carbon dioxide a year. The big climate question has been exactly where this goes. Now an international team of researchers has the answer... and it isn't what they were expecting.
Lead researcher Britt Stephens from the National Centre for Atmospheric Research in the US explains:
'Around 40% of CO2 emissions hang around in the atmosphere adding to global warming but there's been this mystery as to where the rest is ending up. We've known for a while that roughly 30% get's taken up by land plants - mainly forests, but we didn't know where and why until now.'
Britt Stephens, National Center for Atmospheric Research, Colarado, USA
Image: Carlye Calvin
Current carbon cycle models predict that most CO2 taken up on land is soaked up by forests in the Northern hemisphere. But it turns out that the evidence on the ground doesn't match up...
The air samples were collected by aircraft every few weeks for up to 27 years.
Image: NOAA Global Monitoring Division
Britt and his team analysed CO2 levels from 12 different locations around the world, bringing together decades' worth of data for the first time. |
These glass flasks were used to collect the air samples.
Image: NOAA Global Monitoring Division
At each location small planes were flown to up to 6km. A system of pumps collected the air at different altitudes every few hundred metres and stored the samples in glass flasks to be analysed for CO2 levels back in the lab. |
What did they find?
The team discovered that about 40% of the CO2 that was thought to be taken up by Northern forests was actually being soaked up by tropical forests instead. Almost 4 billion tonnes of CO2 a year in total.
Image: Gary Anthes, University Corporation for Atmospheric Research
'For a long time people thought that plants in the North were taking up most of this CO2. But people looking for evidence of that carbon sink haven't found it.' explains Britt. 'One of our conclusions is that the reason they can't find it is because it's not actually there.'
Although tropical forests are still emitting a small amount of CO2 overall, this is because enormous amounts of carbon are being released by deforestation. The areas of intact forest are actually really efficient CO2 scrubbers.
Tropical deforestation releases hundreds of millions of tonnes of CO2 into the atmosphere every year.
Carbon cycling expert Chris Jones thinks these findings could be really significant. 'Understanding the role of natural ecosystems in carbon cycling is very important. These results help reduce the uncertainties about the function of Northern forests and could have a real impact on future climate change predictions.
'It also highlights that if left alone, tropical forest can absorb more CO2 than we thought. So preserving these forests could play a part in slowing global warming.'
Chris Jones, carbon cycling expert, Met Office Hadley Centre for Climate Change.
Image: Chris Jones
The findings could also affect companies and individuals relying on Northern forests to help them go 'carbon neutral'.
'There's a huge effort from companies trying to make a business of planting forests and preventing deforestation in the US and worldwide,' says Britt. 'But our results imply that countries in the North aren't offsetting as much CO2 as hoped through these natural processes.'
'My own view is that using Northern forests to off-set emissions is not going to be as valuable as people currently think.'
But Britt thinks that even investing in tropical forests won't be enough to stop global warming now. 'Trees are a relatively short-term CO2 store. In the long-term we really need to work out how to get the carbon into long-term sinks in the oceans or deep underground, or better still, how not to emit it in the first place,' he says.
Image: climateprediction.net