Original sentence: This increase in atmospheric CO2 is considered to be largely due to the combustion of fossil fuels, and has been accelerating in recent years, that some people, including some scientists, say is due to increased fossil fuel combustion and the release of frozen CO2 from melting ice caps and permafrost
Edited sentence: This increase in atmospheric CO2 is considered to be largely due to the combustion of fossil fuels, and has been accelerating in recent years, most likely due to increased fossil fuel combustion and the release of frozen CO2 from melting ice caps and permafrost