What is the man-made share of greenhouse emission gases?

Man-made greenhouse gases account for about 3.2 percent of the total (see Table 1). Even if man-made greenhouse gases were cut by 50 percent (sending us back to pre-industrial revolution levels), that would be just 1.6 percent. On top of that, the sun's energy output is more important than greenhouse gases. If greenhouse gases make up 25 percent of the causes, man's share of the effect falls to 0.8 percent and a 50 percent cut in that reduces the impact to just 0.4 percent. Of course, there is also the issue of temperature changes driving changes in carbon dioxide and not the other way around, but the main point is already clear.

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Anonymous Anonymous said...

There doesn't se4em to be a link in there anywhere...what's this about?

3/20/2007 11:00 AM  
Blogger John Lott said...

Please click on the 3.2 percent in the first sentence.

3/20/2007 5:00 PM  
Blogger dWj said...

Your source indicates that 25% of the change in greenhouse gases has been anthropogenic. It's not clear to me from my reading of it, but it looks like the "natural" sources include second-order effects -- what you mention as warming causing increases in gases. This seems disingenuous; the relevant question seems to be, "If we remove x% of anthropogenic global warming gases, how much effect will that have on the warming of the earth above pre-industrial levels?" In particular, were it the case that the higher-order effect led to a 4 times multiplier effect, that man-made contributions were 5% of current amounts, and that 20% of current amounts were new -- that total levels had increased by 25% from the baseline -- I would attribute all gas effects to humans. That the new addition is much smaller than a stable baseline is, in fact, irrelevant.

If second-order effects are imputed to "ultimate" sources, I think you're largely distributing the actual changes to anthropogenic gases and to solar cycles, with roughly equal amounts to each of them.

3/23/2007 1:50 AM  

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