Comments 1 to 4:

1. swampfoxh at 07:42 AM on 6 March, 2020

   Can anyone refer me to the source of the idea that it takes thousands of years for the planet to reach equilibrium from the relatively modest rising "rate" of greenhouse gas levels and that the planet is still warming from the effects of 260ppm about 1,500 years ago?  (not in just a few decades).  I have been told that a study by Goreau in 1990 and another by Rohling in 2009 addresses this "stretched" delayed effect?  

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2. MA Rodger at 20:31 PM on 6 March, 2020

   swampfoxh @1,

   Your question doesn't 'ring any bells' with me and the authors you mention don't seem to lead anywhere that I can see.

   The 1,500 year timescale is occasionally mentioned as the time it takes the ocean waters to re-appear at the surface and so reach CO2-quilibrium with the atmosphere. That is part of what Goreau (1990) 'Balancing Atmospheric Carbon Dioxide' addresses but it is not the same as timescales for temperature equilibrium under otherwise-fixed CO2 levels.

   Rohling is the lead author of Rohling et al (2009) 'Antarctic temperature and global sea level closelycoupled over the past five glacial cycles' and this does consider multi-millennial equilibrium timescales but this concerns sea level. There is a connection in that the melt-out of, say, Greenland would both impact sea level and temperature as the albedo change constitutes a slow climate feedback (see this SkS post). But over such multi-millennial timescales with Milankovitch cycles in operation, it would require more than "modest" climate forcings to be significant.

   More directly addressing your question, the ice cores do show a small increase in CO2 levels over the last 8,000 years. This would provide roughly a 0.3Wm^-2 climate forcing which (slow and fast feedbacks so perhaps a sensitivity of 6ºC) could have given a total temperature rise of +0.5ºC. Yet spread over eight milennia, any residual effect today would be now miniscule.

   

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3. nigelj at 06:30 AM on 7 March, 2020

   MAR, "More directly addressing your question, the ice cores do show a small increase in CO2 levels over the last 8,000 years..."

   The Marcott study here shows global temperatures falling over about the last 3500 years until about 1900. So perhaps the milankovitch cycle cancelled out the slow low level rise of CO2 concentrations over the same period? 

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4. Daniel Bailey at 06:46 AM on 7 March, 2020

   This graphic shows both global temperatures and CO2 levels over the past 10,000 years:

   

   IIRC, Ruddiman's hypothesis helps account for some of that discrepancy that you note.

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