This line graph has as its horizontal axis the years from 900 to 2000. The vertical axis represents carbon dioxide concentrations in parts per million by volume, with values ranging from 260 to 380. The early data was obtained by ice cores while the more recent values were obtained through direct measurements. The general trend shows a rise in carbon dioxide concentrations over time, with quite a sharp increase beginning in the mid 1800s. Thus, whereas the concentrations from 900 to the early 1800s stay around the 280 parts per million level, after the early 1800s they begin to increase, reaching around 340 parts per million by the late 1900s.