.about 25% since the beginning of the 19th century. This change is larger than any natural fluctuation that has occurred since the retreat of the glaciers 11,000 years ago and is almost certainly attributable to human-induced causes, principally the burning of fossil fuels and defor estation. To understand why the human influence is so noticeable, it is useful to compare the sizes of the various carbon reservoirs and the rates at which carbon cycles in both the natural and perturbed systems.