Final answer:
The average concentration of carbon dioxide between 1000 and 1500 was around 280 parts per million (ppm), as derived from ice core samples and historical atmospheric data. This level was part of a natural cyclical range of CO2 concentrations that did not significantly exceed 300 ppm until the advent of industrialization.
Step-by-step explanation:
The average concentration of carbon dioxide in the years 1000 to 1500 was approximately 280 parts per million (ppm). This period is considered part of the pre-industrial era when human activities had not yet caused significant changes to atmospheric concentrations of greenhouse gases. Information from ice cores and historical data show that these levels were relatively stable during that time frame. The atmosphere's carbon dioxide levels have been found to naturally cycle between 180 and 300 ppm over historical periods, with higher concentrations correlating with warmer global temperatures.
It is interesting to note that, prior to the industrial revolution, the CO2 concentration in the Earth’s atmosphere had been through natural cycles but remained within a certain range. During periods known as interglacial periods, which are warmer periods between ice ages, the CO2 levels reached the higher end of this range around 280 ppm, remaining steady for at least a thousand years before the fossil fuel age.