Most scientists believe that the increasing levels of carbon dioxide in the atmosphere come from the interplay of multiple factors. Loss of plant cover on the land areas is the most visible issue, but is generally considered to be the smallest of the problems, and the easiest to address.
Another issue is human-caused changes to the oceans. Keep in mind that the Earth is a water planet; there is twice as much sea surface as dry land surface. The plants of the sea, particularly the tiny floating phytoplankton, perform about 70% of the Earth's photosynthesis. Hence the sea is responsible for producing most of our oxygen, and trapping most of the carbon dioxide. Humans have been using the oceans as a sewer for many centuries, and as our population and technology levels rise, the oceans have begun to show negative effects from all the accumulated pollution. As the phytoplankton die off, the ocean absorbs less carbon dioxide. This effect seems to have begun, and no one knows how bad it will get or how quickly it will proceed.
The third issue, which is generally considered to be the biggest carbon dioxide contributor, is the burning of fossil fuels. Scientists believe that in the past the Earth was much warmer, with higher levels of carbon dioxide in the atmosphere. When the plants and animals that eventually became coal and petroleum were buried in the earth, that removed some of the carbon dioxide from global circulation. Now we are digging and drilling into the earth to find those fossil fuels, and burning them, which releases that long-stored carbon dioxide into the atmosphere once again. In just two centuries we have burned up fossil fuels that took millennia to create. The fear is that, if we put that ancient carbon dioxide back into the air, we will return the planet's climate to the way it was when the fossil fuels began to be created.