How do we know that most global warming is attributable to human activities rather than natural causes?
The present atmospheric concentration of carbon dioxide has not been exceeded for the past 420,000 years, and possibly not for 20 million years. Ice core records that go back 420,000 years show that carbon dioxide levels in the atmosphere varied between 180 and 280 parts per million (ppm) due to glacial cycles. For the past 10,000 years global atmospheric carbon dioxide has been quite stable at between 260 and 280 ppm, and level at about 280 ppm from 1000 to 250 years ago. However, since the beginning of the Industrial Revolution, some 250 years ago, the concentrations of greenhouse gases in the atmosphere have increased dramatically. Human activities, such as burning fossil fuels (coal, oil and gas), land clearing and agricultural practices have increased carbon dioxide by more than a third (to about 380 ppm), nitrous oxide levels by about 17 per cent and methane concentrations have more than doubled. The current rate of increase in carbon dioxide is unlikely to have been experienced
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