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All IPCC definitions taken from Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Annex I, Glossary, pp. 941-954. Cambridge University Press.

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How do human CO2 emissions compare to natural CO2 emissions?

What the science says...

Select a level... Basic Intermediate

The natural cycle adds and removes CO2 to keep a balance; humans add extra CO2 without removing any.

Climate Myth...

Human CO2 is a tiny % of CO2 emissions

“The oceans contain 37,400 billion tons (GT) of suspended carbon, land biomass has 2000-3000 GT. The atpmosphere contains 720 billion tons of CO2 and humans contribute only 6 GT additional load on this balance. The oceans, land and atpmosphere exchange CO2 continuously so the additional load by humans is incredibly small. A small shift in the balance between oceans and air would cause a CO2 much more severe rise than anything we could produce.” (Jeff Id)

Before the industrial revolution, the CO2 content in the air remained quite steady for thousands of years. Natural CO2 is not static, however. It is generated by natural processes, and absorbed by others.

As you can see in Figure 1, natural land and ocean carbon remains roughly in balance and have done so for a long time – and we know this because we can measure historic levels of CO2 in the atmosphere both directly (in ice cores) and indirectly (through proxies).

Figure 1: Global carbon cycle. Numbers represent flux of carbon dioxide in gigatons (Source: Figure 7.3, IPCC AR4).

But consider what happens when more CO2 is released from outside of the natural carbon cycle – by burning fossil fuels. Although our output of 29 gigatons of CO2 is tiny compared to the 750 gigatons moving through the carbon cycle each year, it adds up because the land and ocean cannot absorb all of the extra CO2. About 60% of this additional CO2 is absorbed. The rest remains in the atmosphere, and as a consequence, atmospheric CO2 is at its highest level in 15 to 20 million years (Tripati et al. 2009). (A natural change of 100ppm normally takes 5,000 to 20,000 years. The recent increase of 100ppm has taken just 120 years). [Paragraph updated July 2022, to correct information on % of additional CO2 that is absorbed.]

Human CO2 emissions upset the natural balance of the carbon cycle. Man-made CO2 in the atmosphere has increased by a third since the pre-industrial era, creating an artificial forcing of global temperatures which is warming the planet. While fossil-fuel derived CO2 is a very small component of the global carbon cycle, the extra CO2 is cumulative because the natural carbon exchange cannot absorb all the additional CO2.

The level of atmospheric CO2 is building up, the additional CO2 is being produced by burning fossil fuels, and that build up is accelerating.

Basic rebuttal written by GPWayne

Update July 2015:

Here is the relevant lecture-video from Denial101x - Making Sense of Climate Science Denial

Last updated on 5 July 2015 by gpwayne. View Archives

Printable Version  |  Offline PDF Version  |  Link to this page

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Further reading

Both graphs from this page are taken from Chapter 2 of the IPCC AR4 report.

Real Climate goes in-depth into the science and history of C13/C12 measurements.

The World Resources Institute have posted a useful resource: the World GHG Emissions Flow Chart, a visual summary of what's contributing to manmade CO2 (eg - electricity, cars, planes, deforestation, etc).

UPDATE: Human CO2 emissions in 2008, from fossil fuel burning and cement production, was around 32 gigatoones of CO2 (UEA).


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Comments 376 to 380 out of 380:

  1. Rob #373, 374.

    I apparently wasn't clear in my second paragraph. I'm referring to our current practice where there is always enough fossil or nuclear power available when solar or wind isn't available. This comes at no extra investment; it's sunk cost.

    That scenario is the absolute best economic case for the use of renewables based on today's cost. We're eliminating the CO2 emmissions that would otherwise come from the replaced gas and it's costing us $7.61/Mwhr.

    Try some other scenarios. I don't think you'll find any more favorable situation. Remember to account for the battery storage when needed.

  2. Doug @376... This is starting to veer onto a new topic that might be better for a different thread, but what you're doing is promoting the canard about intermittency of wind and solar. 

    The fact is, we are not replacing existing FF facilities until the end of their useful life. Once a facility is built it will continue operation until retirement. New facilities are built to address increased energy needs and to replace retiring facilities. Because of the LCOE most new installed facilities are now wind and solar. 

    As the cost of grid level storage falls below the cost of peaker plants, those facilties will are become economically unviable to build. Existing peaker plants will continue to operate until their useful life expires and grid storage will replace them.

    You say, "Remember to account for the battery storage when needed" but that is part of the canard about intermittency. Each of these are independent elements of the grid system. Each supply power when available (yes, even FF sources often unexpectedly go down as well). 

    With the falling cost of grid level storage what many new wind and solar facilities are looking at is co-location as opposed to grid level arbitrage. That storage cost could like fall to half of the current price in the coming decade, and once that happens there is no way for any mix of FF to compete in the market. It would just be a matter of allowing existing FF facilities to live out their useful lives to then be replaced with renewable sources.

  3. Rob @ 377

    Intermittency of renewables is not a "canard" (unfounded rumor or story), it's an absolute fact that has to be dealt with.

    re "we are not replacing existing FF facilities until the end of their useful life. Once a facility is built it will continue operation until retirement"

    I totally agree. And that's all we can do until the intermittancy issue is solved. But cutting the storage cost of $124.84/Mwhr in half is not enough.

    I think another strategy worth considering is to forget the battery storage approach for now. Find the optimum mix of renewables that results in an effective capacity factor of 50% (wind and solar downtimes don't totally overlap). Then build renewables up to half our electricity and use FF for the other half. It will take decades to get to that point. By then the storage cost issue may be resolved.

    You're right though. This this is probably an issue for another thread.

  4. Doug... Note when you read the the LCOE reports they use the term "resource-constrained." All sources are intermittent. Wind and solars are merely not "dispatchable" in the same manner. 

    Once again, use of the term "intermittent" is a canard because it doesn't fully describe the situation.

    I've read estimates are that renewables (wind, water, solar, geothermal) in conjunction with about 10% penetration of storage could supply all energy needs. You don't need 50% penetration for storage with integrated grids due to the fact other renewable resources are dispatachable (water, geothermal). 

    You say, "...cutting the storage cost of $124.84/Mwhr in half is not enough" but I would suggest that is a baseless assertion when already peaker plants functionally perform the same task and are a critical part of the energy mix at virtually the same levelized cost factor.


    [BL] Rob has duplicated this comment on the other thread he mentions in the next comment. Please make any follow-up comments on the other thread.


  5. Moved the conversation to a more relevant thread here.

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