A country's annual rate of carbon dioxide (CO2) emissions is plotted over a 20-year period. For the entire period shown, the emission rate, while fluctuating, remains positive (greater than zero). From year 10 to year 15, the graph shows that the rate of emissions is steadily declining, but it does not reach zero. Based on this information, what is happening to the total cumulative amount of CO2 in the atmosphere resulting from this country's activity during the period from year 10 to year 15?