Two line graphs are shown, one representing the emission rate of carbon per year under three scenarios, while the other shows the resulting concentration of carbon dioxide as a result of these scenarios. In the emission graph, the vertical axis shows the emission rate in billion metric tonnes of carbon per year, with values ranging from 0 to 25. The concentration graph has a vertical axis representing the concentration of carbon dioxide, with values ranging from 300 to 800 parts per million by volume. The graphs have a horizontal axis that shows years: in both cases, the range is from 2000 to 2150. The approximate values for the year 2000 are seven billion tonnes of emissions and 350 parts per million by volume of carbon dioxide. Two scenarios, IS92a and IS92b, show the CO2 emission rate goes up to 20 and 18 billion tonnes of carbon per year, respectively, by the year 2100, , and the resulting concentration of CO2 levels going up to around 700 parts per million by volume by that date. The third scenario, S550, shows the CO2 emission rate gradually going down from seven billion tonnes in 2000 to less than five billion tonnes by 2150, with the result that the resulting concentration of CO2 level is stabilized at approximately 550 parts per million by volume. (The significance of “550” is that it is twice the CO2 level of pre-industrial times).