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Why the Climate is Changing
Evidence shows that the Earth's climate has changed significantly over its long history. Many of the changes are cyclical, ranging from events like the El Niño/La Niña phenomenon recurring every 3 to 8 years, to glacial periods lasting 100,000 years. Scientists have identified causes for this natural climate variability, including long-scale changes in solar energy output and plate tectonic shifts, volcanic eruptions and meteor impacts, and seasonal changes in ice and snow cover (affecting the reflectivity of the earth's surface).
Since the Industrial Revolution, human activities - in particular, the burning of fossil fuels and land clearing - have been increasing the amount of greenhouse gases in the atmosphere. For example, carbon dioxide and methane are now higher than at any other time during the past 650,000 years (see figure below). Today, scientists attribute most of the gradual warming over the last half-century to these human influences.
The latest report from the Intergovernmental Panel on Climate Change (IPCC Fourth Assessment) is “unequivocal:” the Earth’s climate is warming based on evidence of increasing global air and ocean temperatures, widespread melting of snow and ice, and rising average sea levels. The report also confirms that the current atmospheric concentrations of carbon dioxide and methane far exceed their natural range over the last 650,000 years, as determined from ice cores.
Other key findings include:
- Eleven of the last 12 years rank among the hottest on record (since 1850).
- The intensity of tropical cyclones (hurricanes) in the North Atlantic has increased over the past 30 years.
- Droughts have become longer and more intense, affecting larger areas since the 1970s, especially in the tropics and subtropics.
- Mountain glaciers and snow cover have declined worldwide.
- According to satellite data since 1978, the extent of summer Arctic sea ice has shrunk by more than 20 per cent.
- Sea levels have risen with an expanding ocean (from heating), melting glaciers, and losses from the Greenland and Antarctic ice sheets