Fluxes in the carbon cycle mostly (which contribute over a magnitude more of the CO2 than people) - the same thing that has caused it to increase and decrease in a periodice fashion every 100,000 years. Interestingly we happened to reach local maximums of atmoshperic CO2 100,000 years ago, 200,000 years ago, 300,000 years ago, and 400,000 years ago. Looking at the historical trends it would be very surprising if we were not in a high period of CO2. The ice core samples clearly show the periodic nature of the carbon cycle, but many scientists have noted problems in deducing the actual concentration of atmospheric CO2 from these samples. Dr. Jaworski's (a Polish scientist who has studied glaciers for over 40 years) comments below:
But Dr. Jaworowski says that the ice core-based CO2 estimates are unreliable.
First, ice core-based CO2 estimates vary even more than the 19th century direct measurements, generally ranging from 160 ppm to about 700 ppm with some readings as high as 2,450 ppm. But because the higher estimates are politically incorrect – that is, they don’t support the notion of manmade global warming – Dr. Jaworowski says they haven’t been mentioned in the published scientific literature since the mid-1980s when global warming fever began to spread.
The official “rationale” for ignoring the higher ice core readings is that they supposedly have been “contaminated” by the contemporary atmosphere – but it’s an excuse that actually undermines the validity of all ice cored-based measurements. Ice core data do get contaminated, according to Dr. Jaworowski, but in the opposite direction.
In order for ice core data to be considered reliable, the ice matrix must be a closed system – that is, once air is trapped in ice it should remain unchanged. But Dr. Jaworowski says that glaciers aren’t closed systems. Liquid water is present even in the coldest
Antarctic ice (-73 degrees Centigrade).
“More than 20 physico-chemical processes, mostly related to the presence of liquid water, contribute to the alteration of the original chemical composition of the air inclusion in polar ice,” Dr. Jaworowski told Senators.
The act of drilling for ice core samples further alters the composition of the trapped air. As deep ice is compressed, trapped air bubbles turn into tiny crystals. Drilling decompresses ice cores – causing cracks in the ice and decomposition of the crystals into gases which differentially escape at varying pressures and depths – leading to a net depletion of CO2 in the air trapped in the ice cores, according to Dr. Jaworowski.
“This is why the records of carbon dioxide… in deep polar ice show values lower than in the contemporary atmosphere, even for epochs when the global surface temperature was higher than now,” Dr. Jaworowski testified.
If pre-industrial CO2 levels are in fact closer to the directly measured 19th century average of 335 ppm versus the questionably estimated 280 ppm, then human activity would be correlated with a much smaller increase in atmospheric CO2 levels – which only adds to the confusion over global warming.
Mean global temperature appears to have warmed by about one degree Fahrenheit during the 20th Century. About half that warming occurred prior to 1940, while most of the century’s manmade greenhouse gas emissions occurred after 1940. The global cooling that occurred from 1940 to 1970 – which led some worriers to sound alarms during the mid-1970s about a looming ice age – actually occurred simultaneously with increasing manmade greenhouse gas emissions.