"I read somewhere on the net an estimate that give 20% of the rise in co2 to respiration, and only 7% to fossil fuels"
It depends on what you are trying to say. If you mean % of CO2 flux, 7% is about right. If you mean 7% of the change, you are way lowball.
However, if you mean planetary respiration, and not anthropogenic, we do produce only a small amount of the CO2 flux..
The magnitudes of C fluxes are as follows (all in 1015 g of C per year):
· Ocean uptake = 1.7 (x 1015 g C / yr)
· Photosynthesis = 111
· Respiration = 110
· Fossil fuels = 6.3
· Biomass burning = 1.6
Now, that doesn't seem important,except that, without our input,there was a homeostasis. So we have taken a balanced system and have overwhelmed it with an a extra 7 GT of carbon a year.
 You can get a bigger picture here..http://www.cet.edu/ete/modules/carbon/efcarbon.html
We are all familiar with how the atmosphere and vegetation exchange carbon. Plants absorb CO2 from the atmosphere during photosynthesis, also called primary production, and release CO2 back in to the atmosphere during respiration. Another major exchange of CO2 occurs between the oceans and the atmosphere. The dissolved CO2 in the oceans is used by marine biota in photosynthesis.
Two other important processes are fossil fuel burning and changing land use. In fossil fuel burning, coal, oil, natural gas, and gasoline are consumed by industry, power plants, and automobiles. Notice that the arrow goes only one way: from industry to the atmosphere. Changing land use is a broad term which encompasses a host of essentially human activities. They include agriculture, deforestation, and reforestation.
The adjacent diagram shows the carbon cycle with the mass of carbon, in gigatons of carbon (Gt C), in each sink and for each process, if known. The amount of carbon being exchanged in each process determines whether the specific sink is growing or shrinking. For instance, the ocean absorbs 2.5 Gt C more from the atmosphere than it gives off to the atmosphere. All other things being equal, the ocean sink is growing at a rate of 2.5 Gt C per year and the atmospheric sink is decreasing at an equal rate. But other things are not equal. Fossil fuel burning is increasing the atmosphere's store of carbon by 6.1 Gt C each year, and the atmosphere is also interacting with vegetation and soil. Furthermore, there is changing land use.
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