A new study published in Nature today has found a large carbon sink in the tropical forests of Africa.Each year humans emit around 9 billion tonnes of carbon, but only 4 billion tonnes remain in the atmosphere. Like a giant game of hide and seek, scientists have been looking for 20 years, trying to find where that missing 5 billion tonnes of carbon goes.
It is well known that approximately half of that missing carbon goes into the oceans, and half somewhere on land.
This new research by Dr Simon Lewis, of Leeds University, has shown that around 5% of our total emissions are being absorbed by African forests. When this new data is combined with other data from Amazonian and Asian forests, scientists think they have now found at least half the total land sink.
Dr Simon Lewis is online to answer some of our questions about the research.
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8 comments:
Dr Lewis,
How did you measure the CO2 absorbed by the African forests?
Are forests in other parts of the world absorbing lots of CO2 too, or is there something special about African forests?
I worked with a team of scientists from many African countries and elsewhere to go deep into remote rainforests and measure the sizes of lots of trees, over 70,000 in total!
We randomly chose an area of forest, and measured the diameters of all the trees bigger than 10 cm diameter in that area. For each tree we noted its diameter, location, and which species it belonged to. From this data we used an equation to calculate the amount of above-ground biomass - the weight of the wood, leaves and branches - in each tree. Each tree is about 50% carbon, so adding together the biomass of each tree gives us a measure of the carbon stocks at the start of the study for each area.
We then returned years later to re-measure the surviving trees and note the newly recruited trees, to allow us to again calculate the amount of carbon stored. We then compared the two times that we have estimates of carbon storage for, and see if that particular area is increasing in carbon storage, decreasing or staying the same.
We repeated this for 79 areas of forest in 10 African countries, from Liberia in West Africa, to Cameroon in Central Africa, to Tanzania in East Africa. On average, the 79 areas were increasing in carbon storage, showing that they are net absorbers of carbon dioxide from the atmosphere, what we call a ‘carbon sink’.
In terms of increasing carbon storage, there appears to be nothing special about African forests (I do think they’re special in other ways though).
Comparable data from South America show a similar result on a per area basis. Amazonia forests increased their carbon stocks by 0.6 tonnes of carbon per hectare (100 m x 100 m) per year over recent decades, according to a 2004 study I was part of. This is an identical figure to the new African study.
Of course, South America has a little over half the world’s tropical forests, so the total carbon sink is larger there, with Africa being only 30 per cent of the world’s tropical forest, so the sink is proportionately smaller in total.
Overall, combining African, Central and South American and Asian forest data, on average, each hectare of forest has increased by 0.5 tonnes of carbon per year. Together these forests are absorbing a massive 4.8 billion tonnes of carbon dioxide from the atmosphere. For comparison, human activity (fossil fuel use and land use change, mostly tropical deforestation) releases 32 billion tonnes of carbon dioxide to the atmosphere each year.
Your research says that forests only account for about half of the "missing" land carbon. Do scientists have an idea of where the rest is and are they looking for it?
Our new results on the changes in the carbon stocks of tropical forests shows that across the tropics 1.3 billion tonnes of carbon per year extra is being stored in tropical forest trees. So, we now have a good idea about where half the ‘land sink’ carbon is going.
The other half may be going into soils, and increasing carbon storage in mature forests in temperate and boreal forests (mostly in Russia and Canada), alongside forest regeneration in abandoned areas where crops were once grown (in the tropics and in the Europe and North America). However, it is difficult to say exactly how much is going into the different components of different ecosystems without exhaustive long-term monitoring programmes. This is why monitoring the environment is essential to our understanding of global environmental problems and hence our ability to respond to them.
I'd like to say thank you to Dr Lewis for his time over the past few days. Due to his busy schedule Dr Lewis won't be answering any more questions, but please feel free to continue to comment on this post.
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