With the mixed publicity in the media related to climate change, the scientifically credible, robust facts are often overshadowed by mixed, contradicting, or plainly wrong stories. This paper, therefore, dispels myths and lays the foundation behind the scientific evidence pertaining to the on-going changes in our climate and to our planet.
Atmospheric carbon dioxide is the most important of the radiative forcing components driving the ongoing change in climate (IPCC, 2007). The global increases in atmospheric carbon dioxide concentration are driven primarily by carbon dioxide release during fossil fuel combustion and land use change. On average, 40% of the carbon dioxide released by fossil fuel combustion stays in the atmosphere and the remainder is removed from the atmosphere by Earth's oceanic and terrestrial biosphere. Life on Earth, on land and in the oceans, offsets the impact of human modification of the global carbon cycle. Quantifying and projecting the removal of atmospheric carbon dioxide is critical to understanding how the Earth's climate will change and evolve over the next years, decades, and centuries. The Fourth Assessment Report (AR4) of Working Group 1 of the Intergovernmental Panel on Climate Change (IPCC, 2007) was the first to address the "Earth System" by including a coupled carbon cycle and climate.
Here are a few facts excerpted from the recent Fourth Assessment Report to help frame the importance of the global carbon cycle and the rise in atmospheric carbon dioxide concentration (from frequently asked question 7.1, IPCC, 2007):
"The concentration of carbon dioxide is now 379 parts per million, very likely (>90% probability) much higher than any time at least 650 thousand years, during which atmospheric carbon dioxide remained between 180 and 330 parts per million. The current rate of increase in atmospheric carbon dioxide is very likely at least seven times faster than at any time during the two thousand years before the Industrial Era. Finally, the recent rate of change is dramatic and unprecedented; increases in atmospheric carbon dioxide never exceeded 30 ppm in 1000 years- yet now atmospheric carbon dioxide concentrations have risen by 30 ppm in the last 17 years."
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The natural carbon cycle is characterized by photosynthesis, respiration, decay, and sea-surface gas exchange lead to massive exchanges, sources and sinks, of carbon dioxide between the land and atmosphere (estimated at ~120 Gt-C per year) and the ocean and atmosphere (estimated at ~70 Gt-C per year) (Figure 1). The natural global carbon cycle under current climatic conditions is generally stable. In fact, the natural sinks for carbon with high fluctuations from year to year produce a small net uptake of carbon dioxide of approximately 3.3 Gt-C per year over the last 15 years, partially offsetting the human-caused emissions (Denman et al, 2007) (Figure 16). The ocean sink for C[O.sub.2] uptake is slightly smaller (2.2 Gt C per year) and better characterized than the land-based sink. Had these sinks not existed, atmospheric concentrations of C[O.sub.2] would have increased even more dramatically.
The rise in atmospheric dioxide concentration is driven by emissions of carbon dioxide from fossil fuel combustion and cement manufacturing is responsible for more than 75% of the increase in atmospheric carbon dioxide concentration. The remainder of the increase comes from land-use changes dominated by deforestation and associated biomass burning with contributions from changing agricultural practices. All these processes are caused by human activity. The natural carbon cycle cannot explain the observed atmospheric increase of 3.2-4.1 Gt-C in the form of carbon dioxide, per year over the last 25 years (one Gt-C equals [10.sup.15] grams of carbon, that is, 1 billion metric tons).
Further evidence of the human fingerprint on the carbon cycle comes from examining the character of carbon dioxide in the atmosphere, in particular the ratio of its heavy to light carbon atoms, which has changed in a way that can be attributed to addition of fossil fuel carbon. …