The following factors will affect the current oxygen and carbon dioxide levels.
Burning fossil fuels increases the output of CO2 outside of organisms that perform cellular respiration-CO2 is a green house gas and is a cause of global warming. burning fossil fuels will increase CO2 in the atmosphere which cannot be balanced by the natural sinks like forests and oceans.
Deforestation reduces the number of photosynthetic organisms able to produce oxygen and consume carbon dioxide. Deforestation causes habitat loss and hence a large amount of tree cover is lost which will reduce the amount of oxygen generated and CO2 absorbed.
When land is deforested for agriculture, the photosynthetic productivity of crops is not equivalent to the forests being replaced. This is so because forests have large trees and plants which have a greater photosynthesis capacity however new crops which are small and are removed annually are not able to compensate for the loss of large forest trees.
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