Sunday, October 14, 2018

The effects of Sun and CO2 over the Holocene temperature

I have been debating climate change intensively on Twitter for the past three weeks. 

It has been a bit pointless most of the time, because most of the contrarians are simply there to gainsay any and every point made by defenders of the science of climatology, but it is useful to know what the main talking points are at the moment.

Some of them repeatedly post misleading graphs, and are totally immune to reason when they misunderstand the science. 

They often use insulting and emotional language, and the most common argument they use is a cherry-picked factoid which is supposed "bang the last nail in the coffin of global warming".

However, one graph posted by a contrarian was interesting, and pushed me to look more closely at Milankovitch cycles. This post comes with a health warning: I am an amateur and an autodidact, so I very much stand to be corrected by solar and climatology scientists.

It shows the temperatures in Greenland over the last 10,000 years (the Holocene period), set alongside CO2 levels over the same time span. Now Greenland is in the Northern Hemisphere, and the CO2 readings are gained from the Vostok Ice core in Antarctica, but CO2 is a well-mixed gas, so that should not, on the face of it,  have any effect on the situation, although the actual process is very complex, involving changes in the Atlantic Meridional Overturning Circulation.  

However, in essence, the Greenland temperatures show the general shape of inter-glacial periods - warm beginnings followed by a gentle cooling. The graph stops before we reach the modern temperature spike. The interesting thing is that CO2 levels are dropping during the warm early Holocene, and rise when the temperature is dropping. The implication of the person that posted the graph is that this contradicts the idea that CO2 has a warming effect.

The variations of CO2 in the graph are slight: they amount to only 15ppm over 10,000 years. 
We on the other hand have produced a 120 ppm change in only 170 years. 

The answer to this observation is to look for what the Sun has been doing during the Holocene. The red line shows the variation in the solar energy (insolation) reaching Earth. Note that temperature and CO2 stays up for a while after the insolation declines.

Here is a longer look back at the relationship of Milankovitch cycles and T:

It is reasonably clear that Milankovitch sun cycles do have a relationship with Earth temperature, although relationship is not uncomplicated . Note that Milankovitch cycles mainly affect the region of the globe that receives most insolation; the actual amount varies, but not by so much. Which is why the AMOC is involved, and why also outgassing of CO2 resulting from initial warming from solar cycles is necessary to achieve the large (6-8*C) temperature swings that happen in the Ice Age cycles.

The take-home lesson is that, in contrast to the claims and beliefs of the contrarians, climatology takes account not just of CO2, but of all known significant factors.

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