Question 4:
How do we know that most recent 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
650,000 years, and possibly not for 20 million years. Ice core records 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 concentrations
by more than a third (to approximately 380 ppm), nitrous oxide levels by about 19 per cent and
methane concentrations have more than doubled. The rate of increase in carbon dioxide during the
industrial era is very likely to have been unprecedented in more than 10,000 years.
The observed changes in climate, especially temperature increases since about 1970, cannot be
explained solely by natural causes such as solar activity. Reconstructions of climate data for the
past 1000 years indicate that this recent warming is unusual and is unlikely to have resulted from
natural causes alone.
Scientists use computer models to simulate past and future climate variations. Simulations of the
20th century have been driven by observed changes in various factors that affect climate. When only
natural factors, such as volcanic aerosols and solar activity, are included in the models, the
simulations do not explain the observed warming in the second half of the century. The warming in
the second half of the century can only be explained if human-induced changes in greenhouse
gases are included in the models.
Global atmospheric concentrations of three greenhouse gases
| Carbon Dioxide | Methane |
|---|
 |  |
| Nitrous Oxide |
|---|
 |
