[yongjik]

https://www.realclimate.org/index.php/archives/2004/12/how-d...

> CO2 produced from burning fossil fuels or burning forests has quite a different isotopic composition from CO2 in the atmosphere. This is because plants have a preference for the lighter isotopes (12C vs. 13C); thus they have lower 13C/12C ratios. Since fossil fuels are ultimately derived from ancient plants, plants and fossil fuels all have roughly the same 13C/12C ratio – about 2% lower than that of the atmosphere. As CO2 from these materials is released into, and mixes with, the atmosphere, the average 13C/12C ratio of the atmosphere decreases.

...

> What is found is at no time in the last 10,000 years are the 13C/12C ratios in the atmosphere as low as they are today. Furthermore, the 13C/12C ratios begin to decline dramatically just as the CO2 starts to increase — around 1850 AD. This is exactly what we expect if the increased CO2 is in fact due to fossil fuel burning.

In other words, all CO2 with biological source (including fossil fuels) comes pre-tagged, and the pattern we see in the atmosphere exactly matches the assumption that they're from fossil fuels.

So, unless your acquaintances theorize that it's the global temperature that drives mankind's demand for fossil fuels, there's really no ground in doubting the causal direction.

It's too bad they're doubting quietly. If they actually voiced their doubts in the form of an honest question, they would have learned about C13 ratio by now.

[guymcgwire]

I think the case has been made quite convincingly that humans have dramatically increased atmospheric CO2. I don't personally know anyone who questions this either (but I don't know any oil lobbyists, to be fair). In fact, as someone who worked in climate advocacy for many years, I think that we proved this link quite well, almost to a fault. I say a fault, because so many resources have gone into proving that humans have caused the increase in atmospheric CO2 that we paid less attention to making a convincing case that the rise in global temperatures can only be explained by the reflected heat caused by that atmospheric CO2. (Just consider your knee-jerk reaction to my comment where you automatically assumed that this was a question about atmospheric CO2 without considering the possibility that it might be something/anything else).

When you look at the graphs for atmospheric CO2 and man-made carbon emissions laid on top of each other, it looks perfect. This is the money shot. But, when you lay the 150 year temperature data on top of it, it suddenly doesn't look so good (ignoring the normal 11 year solar fluctuations, just looking at the general trend line). There's a 70 year stretch when temperatures are flat to declining as atmospheric CO2 climbs by almost 20% (in advocacy training we'd be encouraged to skip past this: zoom out and highlight that they are _directionally_ similar and definitely never chart in percent change, of course).

Personally, I still ride with Team Atmospheric Carbon. But I also don't think there's anything unhealthy with good-faith questioning of the new dogma. The Earth's magnetic field has weakened by about 5% over the past 100 years. Global temperatures have risen by about 5.5% over the past 100 years. The magnetosphere protects the Earth's atmosphere from charged solar particles which would otherwise strip off parts of the upper atmosphere, allowing more UV radiation to hit Earth. Is that a coincidence? Maybe. Is increased UV-B penetration just as likely to cause warming as increased reflected infrared radiation? Yeah? So I keep an open mind.