Naked Science Forum
Life Sciences => The Environment => Topic started by: pirunner on 07/10/2007 03:40:08
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It is quite obvious that when trees loose their leaves in the winter their photosynthesis rate (and therefore CO2 absorption)slows dramatically (or even ceases). Does this have any measurable effect on the CO2 or O2 percentage in the air? Granted, it is always winter somewhere, so there should not really be any worldwide change. Furthermore, the majority of the world's oxygen is produced in the rainforests, where there is essential no winter anyway.
But, where winter does exist, is there a measurable change, or does the worldwide level sufficiently diffuse to cancel everything out? Do summer regions have a higher oxygen percentage and winter regions a lower?
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I would question a number of assumptions in the above.
Most critically, I do not believe most CO2 is absorbed in the rainforests. There is even doubt as to whether rainforests are a net consumer of CO2. Mature forests generally do not consume more carbon than they release (i.e. while trees may absorb CO2, as the trees decay, whether through microbial action, or through animal ingestion, they will release this carbon back into the atmosphere). Young forests, where trees are growing faster than they are decaying, will have a nett absorption of carbon, but rainforests cannot generally be seen as young forests.
In order to remove carbon from the atmosphere for the long term, carbon has to somehow be removed from being accessible to animals, either by being laid down as sediment on the ocean floors, or by being incorporated into peat bogs, or some similar process.
It should also be remembered that photosynthesis happens in many organisms, from grasses, to mosses and algae, not only through trees.
In general, it is inevitable (whether or not trees lose their leaves) that photosynthesis should reduce during the winter months, simply because there is less sunlight through the winter months. The question is as to where greater photosynthesis is happening, in the southern hemisphere (with its large oceans, and more marine algae), or in the northern hemisphere (much greater land masses), so which winter is the more significant (that of the north or that of the south)? Then you have the additional issue that as the oceans get colder in winter, they will dissolve more CO2, and as they get warmer in the summer they will release some of this CO2 back into the atmosphere (this is also made more difficult to judge because of the complexities of the ocean currents).
Additional factors that you have to take into account is the increase in forest fires in summer (and peat fires), and seasonal changes in human activity. Both of these factors are primarily terrestrial, and so will have a greater dependence on the seasons in the northern hemisphere, where there are greater land masses.
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I think two things need to be clarified, those being absorption and storage of co2.
Rainforests, and large woodlands are good co2 sinks (storage), the problem is that they do not absorb too much new co2. They have all but reached their storage capacity.
This is why there are campaigns to save the rainforests, not because of what co2 they can absorb. but the levels they will release if felled.
With reguard to absorption, the grasses are the best at doing the job. again there is a but. that being grasses die seasonally and release the co2 they have stored, although during the time they are "alive" they do convert co2 in to usable carbon compounds.
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The simple answer is yes, there is a measurable seasonal change to overall CO2 levels.
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The simple answer is yes, there is a measurable seasonal change to overall CO2 levels.
Of itself, this does not surprise me - what I am more dubious about is whether this is simply down to leaf fall from trees, since there are are such a wide variety of seasonal effects that should affect CO2.
One question that would be interesting is whether the differences are global, hemispherical, or continental in nature (and even what the correlation between season and CO2 is - simply saying there are seasonal changes does not tell us whether it rises (due to reduced photosynthesis) in winter, or whether it rises during the summer (due to greater CO2 production by both humans and animals, and CO2 coming out of solution in the warmer ocean currents)?