Final answer:
The length of time for CO2 to go from 300 to 400 ppm cannot be accurately estimated based on the provided information.
Step-by-step explanation:
To estimate the length of time it took for CO2 to go from 300 ppm to 400 ppm, we can refer to the provided information.
The graphs and data indicate that atmospheric CO2 levels have been typically between 200-300 ppm over the past 700,000 years.
However, in the past 50 years, there has been a steep and unprecedented increase in CO2 levels. Without specific data points between 300 ppm and 400 ppm, it is difficult to estimate the exact time it took for this increase.