Sounding like the sort of mystery Sherlock Holmes might resort to solving during a slow week for the detective industry, this puzzle actually has far greater importance for humanity than might first meet the eye.
Humans are doing their utmost to pollute, spewing out an estimated 8 billion tonnes of carbon each year. However, on average only 3.2 billion tonnes of this accumulates in the atmosphere. Good news for us, but where does the rest of it go? The oceans are absorbing around 2.2 billion tonnes (creating a whole new problem of ocean acidification, but that’s another story….), and terrestrial sinks, predominantly forests, account for the rest. So what exactly is all the fuss about?
Well the problem lies with the fact that scientists weren’t sure which trees were taking up all this carbon. Conventional carbon models predicted that over 90% of the 2.6 billion tonnes of terrestrially sequestered carbon were being taken up by forests in the mid to high latitudes, but no evidence could be found to support this level of net carbon uptake.
(http://www.fao.org/forestry/32194/en/)
In 2007 a group of scientists led by Britton Stephens in Colorado published a paper in Science which revealed that most conventional atmospheric carbon models do not properly reflect vertical mixing through the atmosphere and this was producing misleading results. According to their findings, tropical ecosystems were in fact much stronger carbon sinks than had been previously thought.
HOWEVER, another twist came along in 2011 when Pan et al asserted that despite tropical forests having the highest carbon uptake rates, due to high levels of deforestation and the associated emissions, tropical forests were rendered effectively carbon neutral. In other words deforestation in the tropics is especially destructive since the potential carbon sequestration is much higher than elsewhere. In terms of the global carbon cycle the tropics are no longer acting as a sink for carbon, and the "missing carbon sink" may reside in boreal and temperate forests after all…
There is still debate about this, but the bottom line is that forests around the globe are currently saving our skins by slowing the rate of atmospheric change. So how about the fact we’re cutting down 13 million hectares of forests a year? Shooting ourselves in the foot doesn't even cover it.
Nice post, Jess. Dr Watson would be proud...
ReplyDeleteSurely it's hardly surprising that these two studies concluded different things. Pan et al. don't really have any data for the boreal forests in North American and highly uncertain data for the carbon stocks and fluxes in Asia. Based on the picture accompanying your post, it seems these areas a vast and would greatly impact global carbon sequestration.
Additionally, Pan's paper uses 'Tropical gross deforestation emission', which seems like a slightly ambiguous term. What does that actually encompass? The paper mentions logging. They're not counting the reduction in carbon stocks as emissions are they?
Hello David,
ReplyDeleteThanks for your comment. Firstly you’re right that there are gaps in Pan et al’s data, and the uncertainties associated with their estimates are consequently large. This is partly due to a scarcity in long-term, pan-forest data-sets, although progress is being made to remedy this eg http://www.rainfor.org/. However I think it is still a useful study, highlighting, if nothing else, the need for more information and also the extent to which deforestation is upsetting the global carbon balance. I recommend taking a quick look at Peter Reich’s review of their paper http://www.sysecol2.ethz.ch/Refs/EntClim/R/Re183.pdf which I think gives a fair appraisal of the limitations.
Secondly, the phrase “gross tropical deforestation emissions” is used when talking about land-use changes in the tropics. They define it themselves (in the footer of Table 1) as “the total C emissions from tropical deforestation and logging, not counting the uptake of C in tropical regrowth forests”. This is balanced against the carbon sequestered in areas where forest is regrowing to calculate the net carbon balance.
I hope this helps! Jess
Nice post Jess, congrats ! I just want to add some information about this topic. Based on the figure showed on your post, rainforests play one of the most important roles in Tropical Forests and deforestation of rain forests is a common issue in south america, specially for the change of land use. These activities can change the hidrologycal cycle and therefore limiting the regeneration of these forests without taking into account the destruction of the huge biodiversity in these areas. I would say, rain forests is a key point when it comes to terrestrial Carbon sinks.
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