What is one possible cause of the increase in carbon dioxide in the atmosphere since the mid-19th century?
When hydrocarbons are burned with sufficient oxygen they produce carbon dixide and water. In the mid nineteenth century western Europe and the United States began the so called "industrial revolution".
As the need for more goods developed, a greater emphasis was placed on replacing people and animal power with other, more reliable and stronger sources of power. The main source which evolved was the combustion of fossil fuels. Initially this was in the form of coal and peat, but eventually included oil and gas products. Fossil fules are for the most part hydrocarbons. Consequently as the industrial revolution flourished we see an ever growing use and dependency on the burning of fossil fuels creating more carbon dioxide than can be naturally recycled in the carbon dioxide-oxygen cycle. Therefore we see an overall increase in atmospheric carbon dioxide.