About 3 billion tons are released from deforestation, and an unknown additional quantity comes from the accelerated decay of organic matter. Scientists have been monitoring carbon dioxide levels in the atmosphere since 1958. Since then, Keeling has measured a steady rise in carbon dioxide levels in Earth's atmosphere. These levels can be compared with the concentrations found in bubbles of air trapped in glacial ice up to 160,000 years ago. For example, in 1990, atmospheric carbon dioxide levels were about 25 percent higher than those measured in air trapped in ice that formed in 1880.
Experts estimate that deforestation has released a somewhat lesser amount.
The effect of adding carbon dioxide and other heat-trapping gases to the atmosphere will be to make Earth warmer than it would have been otherwise. But many other uncertainties surround the issue of global warming. Chief among them is when the heat-trapping gases, among the various factors that affect the temperature of the Earth, will dominate and cause a rapid warming. Most scientists say that the rising levels of carbon dioxide have already caused an increase in Earth's average surface temperature. To establish such a link, the scientists compare the rising carbon dioxide levels with temperature measurements taken during the past 100 years.