The exponential increase in CO2 concentrations around the 1860s is primarily attributed to the combustion of fossil fuels by humans. This period marks the beginning of the Industrial Revolution, during which there was a significant increase in the use of coal, oil, and natural gas for energy and transportation. The burning of these fossil fuels releases large amounts of carbon dioxide into the atmosphere, contributing to the observed rise in CO2 levels.
While deforestation also contributes to increased CO2 concentrations by reducing the number of trees available to absorb CO2, the most significant factor for the rapid increase since the 1860s has been fossil fuel combustion. Other factors like glaciers melting and increased agricultural practices have their own impacts but are not the primary drivers of the observed rise in CO2 during that period.