Geezer - I am not a climate scientist. This is my understanding of the CO2 contribution, but given the chaotic complexity of the climate will be subject to revision.
The continual presentation of additional carbon dioxide from human activities such as burning of fossil fuels, deforestation etc - carbon dioxide that had effectively been locked away into one of the natural carbon sinks - is the issue. The Global dynamic CO2 equilibrium is being effected by this - A system that was broadly in balance, over geological time lines, now has to adapt to that extra contribution. Much of that extra CO2 remains in the atmosphere for significant periods of time.Water Vapour is the most significant greenhouse warming agent.The amount of water vapour in the atmosphere is increased with increasing temperature, a positive feedback loop. The water vapour cycle and balances ( rain, snow, oceans, lakes, evaporation etc) is short lived and cycles rapidly.
CO2 is also a greenhouse gas.Increases in levels of atmospheric CO2 will cause an increase in global temperature. This, in turn, will cause more water vapour to be aborbed, which in turn will further increase the temperature.
CO2 cycles much more slowly, and the equilibrium between CO2 in the atmosphere and that in the "carbon sinks" reacts much more slowly, in geological terms - So a rapid increase in CO2 levels cannot be dealt with rapidly by the natural equilibrium mechanisms.The consequence of this is an inexorable increase in the CO2 in the atmosphere, which in turn means an inexorable increase in the greenhouse gas effect, which inexorably raises the atmospheric temperature, which inexorably increases the water vapour in the atmosphere, which inexorably raises global atmospheric temperature.
Isotopic measurements of the 12C/13C ratio of samples from tree rings and ice cores show a significant increase of Carbon isotopes from fossil stores over the last 150 years - this can only be from anthropogenic activity, industrialisation,etc.This is recycling carbon that had already been absorbed out of the atmosphere.Deforestation does the same kind of thing.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 might be expected if the increased CO2 is in fact due to fossil fuel burning.
All Volcanic activity by the way produces only a small fraction of CO2 compared to that produced by human activity. Anthropogenic CO2 contribution to total global CO2 is indeed small compared to the volumes attributed to all of the various natural sources - but those natural sources have already established, over geological time,a dynamic equilibrium with the various carbon sinks. Of the extra CO2 contributed by man, that equilibrium can accomodate around 30-40% in the carbon sinks over geological time, but the rest is retained in the atmosphere, cumulatively and inexorably contributing to increasing temperature, which in turn will effect the amount of water vapour absorbed, which in turn further contributes to increases in the temperature.
Global temperatures are rising, decade on decade.Observational,recorded data from 1000s of measurement points confirms this.No other model or combination of climate /solar cycles currently advanced as an alternative to Co2 enhanced global warming can explain the observational changes in the climate.
The current atmospheric CO2 level is around 380 -390ppm, and has been shown to be increasing at a rate of nearly 2ppm / year in the decade between 2000 and 2010.Some 30 years earlier, the measured rate of change was around 1ppm/year - so the rate of increase in CO2 levels is accelerating.These measurements already take into account the seasonal fluctuations in atmospheric CO2 due to growing seasons etc. The CO2 level in the atmosphere is the highest for around 600,000 years.