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Fossil fuel use has driven the recent jump in atmospheric CO2

From 1000 A.D. to about 1750 AD, carbon dioxide levels in the atmosphere hovered between 275 and 285 parts per million (ppm), and then began to increase. Initially, the increase was largely due to the burning of coal, which was the primary energy source for the Industrial Revolution, and whose exhaust products when burned include CO2. Since then, the other major fossil fuels, oil and natural gas, have also become sources of growth in CO2 levels.  The latest IPCC report presents statistics over the years since 1970, which are indicative of the historical proportion that fossil fuel burning occupies in the sources of CO2.  The percentage of emissions from solid, liquid and gas fuels represents about a 70% fraction of CO2 emissions and has seen its share increasing during this period.1

But other factors contribute as well. For example, the widespread cutting down of forests can add CO2 to the atmosphere if the trees are burned; like fossil fuels, they release this greenhouse gas as well. If the trees are left to rot, that too releases CO2, albeit more slowly. And because living trees absorb CO2 in the process of photosynthesis, the cutting of forests eliminates a source of CO2 removal, so the gas builds up more quickly than it otherwise might. The same estimates from the IPCC quantify deforestation and land-use change emissions as about 22% of CO2 emissions.

Some manufacturing processes add CO2 to the atmosphere as well. The manufacture of cement is one; it does not just require energy, which often comes from fossil-fuels, but the chemical reactions involved in its manufacture release large amounts of the gas as well. All in all cement production has occupied a 3% share of CO2 emissions.

All this said, fossil fuel burning remains the predominant source of the historical increase in atmospheric CO2 concentrations which added about 100 ppm (36%) over the last 250 years to the CO2 levels of the pre-industrial era.2

References
  1. Barker T., I. Bashmakov, L. Bernstein, J. E. Bogner, P. R. Bosch, R. Dave, O. R. Davidson, B. S. Fisher, S. Gupta, K. Halsnæs, G.J. Heij, S. Kahn Ribeiro, S. Kobayashi, M. D. Levine, D. L. Martino, O. Masera, B. Metz, L. A. Meyer, G.-J. Nabuurs, A. Najam, N. Nakicenovic, H. -H. Rogner, J. Roy, J. Sathaye, R. Schock, P. Shukla, R. E. H. Sims, P. Smith, D. A. Tirpak, D. Urge-Vorsatz, D. Zhou: Technical Summary. (PDF) In: Climate Change 2007: Mitigation. Contribution of Working Group III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [B. Metz, O. R. Davidson, P. R. Bosch, R. Dave, L. A. Meyer (eds)], Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.
  2. Forster, P., V. Ramaswamy, P. Artaxo, T. Berntsen, R. Betts, D.W. Fahey, J. Haywood, J. Lean, D.C. Lowe, G. Myhre, J. Nganga, R. Prinn, G. Raga, M. Schulz and R. Van Dorland: Changes in Atmospheric Constituents and in Radiative Forcing. (PDF) In: Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M.Tignor and H.L. Miller (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.

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