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This video presents this graph and claims that doubling the CO2 levels would only increase the global warming by 1%, whatever that means. Now it seems like the graph contains real data, so what is up with the claim? My only guess would be that the heating from CO2 at that point would cause water to evaporate and increase the warming by a significant amount.

  • The broad idea that the impact of higher CO2 is logarithmic isn't the issue (it is). The issue is the detail about the degree of warming coming from that increase which depends on far more than just the CO2 concentration. And where we actually are on the curve (which this claim may not have got right). – matt_black Mar 10 '19 at 13:00
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    I double-taked (3m mark) at "3% times 4% - or 12 thousandths of a percent" - with an overlay showing the numbers in a confusing manner to hide the order of magnitude error. – Oddthinking Mar 10 '19 at 15:41
  • (Looks at the X axis) (Laughs hysterically) We're at 411 ppm. And climbing... – DevSolar Mar 11 '19 at 8:00
  • FWIW, the stuff about anthropognic CO2 only being 3% is also profoundly misleading, we are responsible for 100% of the post-industrial increase in atmospheric CO2 - see my paper citeseerx.ist.psu.edu/viewdoc/… . It is risible that the Heartland Institute promulgate this Gish gallop of such feeble canards. – Dikran Marsupial Mar 12 '19 at 15:13
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    @DunkeyKing IIRC the direct warming effect from increased CO2 is about 1C per doubling of CO2, however there are feedback mechansims that amplify this warming. One of these is water vapour feedback - a warmer atmosphere can hold more water vapour, but that water vapour is also a greenhouse gas and will warm the atmosphere further. These feedback mechanisms are included in the climatologists estimate of climate sensitivity (the amount of eventual warming per doubling). The IPCC estimate is about 3C plus or minus 1.5C per doubling. Hope this helps. – Dikran Marsupial Mar 12 '19 at 15:42