A little over half of the carbon which enters the atmosphere each year as a result of human activities does not accumulate there. Instead, it gets taken up into one or more "sinks" which accumulate carbon. 0bservations of the rate at which C02 exchanged into ocean waters around the world suggest that of the roughly 8.5 billion extra tonnes of carbon that entered the atmosphere each year in the 1990s, 2.5 billion was being taken up into the oceans. Calculations of the rate of regrowth of young forests in the northern hemisphere suggest that 1 billion tonnes were taken up into this reservoir. Yet only 3 billion tonnes ended up as C02 in the atmosphere in an average year. This leaves a gap of 2 billion tonnes, a "missing" sink which is swallowing up carbon beyond that accounted for by calculations of the ocean and forest sinks.
So the balance sheet according to a summary by Skee Houghton at Woods Hole is something like this:
On the positive side:
Deforestation gives off around 2.2 billion tonnes C each year. Fossil fuel burning produces around 6.3 billion tonnes of C each year.
Yet, only 3.2 billion tonnes C accumulates as CO2 in the atmosphere. On the negative side:
About 2.4 billion tonnes C are taken up by the oceans each year. About 1 billion tonnes C are taken up into forests.
This leaves a "missing" sink of about 2 billion tonnes.
The missing sink is not quite as mysterious as it might sound; no-one believes this carbon is leaving for outer space, or being sucked deep into the earth's interior, so it must be going somewhere within either the oceans or land ecosystems. Probably, it is a result of some simple, basic errors in calculation of uptake by familiar processes in the oceans or forests. However, it might be the result of other poorly understood mechanisms, such as a direct CO2 fertilization effect making existing forests grow faster than before and accumulate carbon (see Chapter 8). It is important to try to understand the missing sink in order to predict how it might behave in future. For instance, perhaps before long it will begin to saturate and stop taking up CO2, or even go into reverse and start releasing carbon? For this reason there have been many studies in recent years which aim to narrow the uncertainties. These include eddy flux studies (next section).
An important clue to the missing sink comes from detailed measurement of average CO2 concentrations around the world. The CO2 concentration over the mid-latitudes of the northern hemisphere (Europe, Russia, China, North America and the northern Atlantic) is fairly low. This "dip" in CO2 over the northern mid-latitudes suggests that this is where the missing carbon in the calculations is going. It looks like there is an especially strong absorption of CO2 going on over the eastern part of the USA and Canada. Even though these regions are actually big sources of greenhouse gas due to fuel-burning, the amount of CO2 coming off them should be substantially larger than it actually is, showing that some is going missing. Most scientists who work on this subject think that the extra CO2 is going into maturing forests in the temperate regions, and that they are just taking it up faster than anyone had expected. It could be though that part of the unaccounted-for carbon is being absorbed in the North Atlantic, and that previous studies of CO2 uptake in that region underestimated how fast it is dissolving in the ocean water. However, recent detailed studies of CO2 concentration in the atmosphere—using aircraft taking measurements around the world at different heights above the ground—suggest that a lot of the CO2 is actually being taken up into the remaining areas of tropical forests. This might be because they are recovering from past logging, increasing their carbon storage as the trees grow bigger. It might also be that the estimates of the release of carbon from tropical forest clearance are a bit too high, tending to widen a "gap" (compared with C02 accumulation in the atmosphere) that really was not so large in the first place.
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