What are the major sources of atmospheric carbon dioxide?

I read that oxygen breathing creatures who exhale carbon dioxide are the major source. By my calculations mammals contribute four-sevenths of the co2 and fossil fuels contribute about three sevenths. If the population growth is as great as predicted over the next fifty years we could still be in trouble even if there was NO fossil fuel source.

Answer:
Surprisingly, it makes a difference whether the CO2 is natural or man made.

There are a great many natural sources and sinks for carbon dioxide. But the present global warming is (mostly) the result of man made CO2 from burning fossil fuels.

There is a natural "carbon cycle" that recycles CO2. But it's a delicate balance and we're messing it up.

Look at this graph.

http://scrippsco2.ucsd.edu/graphics_gall...

The little squiggles are nature doing its' thing. CO2 falls a bit during summer when plants are active, and rises during the winter. The huge increase is us, burning fossil fuels. The scientists can actually show that the increased CO2 in the air comes from burning fossil fuels by using "isotopic ratios" to identify that CO2. The natural carbon cycle buried carbon in fossil fuels over a very long time, little bit by little bit. We dig them up and burn them, real fast. That's a problem.

Man is upsetting the balance of nature. We need to fix that.

More here:

http://environment.newscientist.com/chan...
The major cause of atmospheric carbon dioxide is ocean water.
Carbon dioxide isn't really that great of a problem as it is metabolized by vegetation, however another greenhouse gas is carbon monoxide, this cannot be metabolized by vegetation, does not break down in the atmosphere and is a very toxic gas.
There is a natural carbon cycle and then there are human emissions of CO2 which are relatively small but throw the natural cycle out of balance and thus increase global warming. Breathing is part of the carbon cycle, so it doesn't impact global warming.

You can see a breakdown of human greenhouse gas (GHG) emissions in the USA and the world as a whole in the links below.
Expiration does indeed produce carbon dioxide - the average human produces about 800g of CO2 per day and as a whole the global population produces a little under two billion tons per year.

What's important to be aware of is something called the 'Carbon Cycle'. This is the process by which nature releases and absorbs carbon dioxide, primarily through the oceans, soils and biomass. The carbon cycle is capable of recycling all the carbon dioxide poroduced via respiration and all other natural sources combined and still have a surplus capacity of 3 billion tons per year.

If un-natural sources of carbon dioxide totalled less than 3 billion tons a year then the natural carbon cycle would be able to accommodate them, unfortunately last year alone we produced 29 billion tons of CO2 leading to a surplus of 26 bn tons, much of which is just hanging around in the atmosphere contributing to global warming.

It's predicted that population growth will level out at around 11 billion people, this is 4.5 billion more than the current world population. An increase in population on this scale would produce an extra 1.3 billion tons of CO2 each year through respiration, an amount within the processing capabilities of the natural carbon cycle.

The problem isn't so much the number of people on the planet but the emissions that result from their demand for energy, fuel, agricultural products etc.

If everyone lived in a cave without any modern conveneinces and grew their own food then the planet could house about 20 billion people.
There are two cycles of CO2.

There is the natural background CO2 levels which can be seen in the picture I link to below. Animal exhalation, volcanism, plant growth and decay are all a part of the natural cycle.

The anthropogenic (human) CO2 has been added to the atmosphere via the removal from burial and burning of fossil carbon. This carbon has not been in the earth system for millions of years and is now being put into the atmosphere.
Can you name me something that does not emit carbon dioxide?
Humans are responsible for about 3% of the global CO2 emissions. Biggest sources are the oceans and decaying vegetation.

CO2 in turn is responsible for about 3% of the greenhouse effect.

Man contributes to about 0.117% of the greenhouse effect through our CO2 emissions.
Cows. So if you want to help global warming do in a vegetarian.

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