Deforestation is only a small, yet not negligible contribution to global warming. The major cause to global warming is the fossil fuel burning.
Fossil which is largely composed of carbon, has been intensively used over the last 200 years until today in many of our today's life appliances, with the heaviest application being industrializations. Residential, transportation, commercial, and agricultural are also the major contributions to fossil fuel burning.
The earth has a natural carbon cycle where the carbon dioxide concentration level is naturally maintained. These processes include:
Organism respiration will release carbon dioxide to the atmosphere which will increase its concentration.
Plants photosynthesis will convert the carbon dioxide back into carbon compound and reduce its concentration.
The carbon is reduced when carbon dioxide gas dissolved in the ocean and carbon from lands and rivers is flowing into the ocean.
Plant and animal corpse will be slowly buried on lands and ocean and will eventually turn to fossil (limestone, coal, gas, oil).
Release of carbon from the ocean directly to the atmosphere in the form of carbon dioxide gas.
Release of carbon from the eruptions of volcanoes.
Burning of fossils and deforestation has a big impact on this natural carbon cycle where enormous amount of carbon has been released to the atmosphere because of fossil fuel burnings. The slow natural carbon removal process cannot keep pace with the rapid rate at which the carbon is emitted to the atmosphere by burning fossil fuels, and reduction of photosynthesis due to deforestation have even worsen the situation where less and less carbon dioxide is converted, have together increased the concentration of carbon dioxide in the atmosphere by a large scale over the last few hundred years.
Dioxide concentration during the past 650,000 years have never exceeded 300ppmv, is now increased to well over 350ppmv due to fossil fuel burnings and deforestation in just merely 200 years.
Measurement of carbon dioxide concentration has shown a rapid unusual increase.
The increase of carbon dioxide concentration in the atmosphere is no more natural, at a pace that the natural carbon removal process has no way of keeping up, and this high level of carbon dioxide concentration in the atmosphere has trapped and held more and more heat, keeping the earth warmer than it's before.
Global temperature had rose, due to the increase in carbon dioxide concentration in the atmosphere.