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I am reading an article on deforestation entitled Tropical Deforestation and Carbon Emissions from PADDD. Part of the article reads:

If future deforestation rates follow recent trends, PADDDed forests would emit approximately 6.0–10.9 million Mg C by 2100.

What is the meaning of Mg C? Is it milligrams of carbon?

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    $\begingroup$ According to the NIST, the sentence would be better written as "If future deforestation rates follow recent trends, the mass of carbon emitted by PADDDed forests by 2100 would be 10.9 Tg." Clearer, IMO. $\endgroup$ – Steve Emmerson Nov 3 '15 at 21:14
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    $\begingroup$ @SteveEmmerson yes I agree Tg is a much clearer (and more conventional too?) way of saying that! $\endgroup$ – farrenthorpe Nov 3 '15 at 22:35
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Mg C is a tonne of Carbon.

An Mg - a megagramme - is a tonne. It's a silly way of writing a tonne for a general readership, but it's the strict SI equivalent of a tonne. So an "Mg C" is a tonne of Carbon - the standard unit when it comes to talking about pricing Carbon.

Normally you wouldn't see anyone writing about a million Mg of anything; (in this case, of carbon). You'd either write about a Teragramme, or a million tonnes, but not a million Mg.

However, if you draft a paper referring to millions of tonnes, and then the editor or a peer reviewer comes back and says that the tonne is not a strict SI unit but the Mg is, then someone in the chain of authors and editors may just do a search and replace on all the "tonnes" to replace them with "Mg". And that's one way that you might end up with lots of references to "million Mg C", instead of the more usual "million tonnes of carbon"

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When discussing about the carbon cycle in scientific papers, the gram is usually the reference unit used. Depending on what the particular topic is, and especially its scale, an appropriate prefix will be appended to the gram unit.

For instance, when the target is hemispheric or global, petagrams (Pg) is often used to discuss carbon exchanges between large reservoirs. Therefore when discussing about a smaller unit (as in your case, a single forest), it seem to be appropriate to scale down the unit to megagrams (Mg). Here is the current reference table displaying common prefixes for SI units.

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  • $\begingroup$ user4738 if the answer helped you, maybe you can select it as the accepted answer. Cheers! $\endgroup$ – kwinkunks Nov 3 '15 at 21:05

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