3. Greenhouse emissions: a global view

There is new and stronger evidence that most of the warming observed over the last 50 years is attributable to human activities. It is unlikely that the warming is to be entirely natural.

Reconstructions of climate data from the last 1000 years also indicate that this 20th century warming was unusual and unlikely to be the response to natural forcing alone. Volcanic eruptions and variations in solar irradiance do not explain the warming in the latter half of the 20th century, but they may have contributed to the observed warming in the first half.

Data from United Nations Environment Programme/GRID–Arendal: Vital Climate Graphics.
Image by Simon Gilmore

Since pre industrial times, the atmospheric concentration of greenhouse gases has grown significantly. Carbon dioxide concentration has increased by about 31%, methane concentration by about 150%, and nitrous oxide concentration by about 16%.

The present level of carbon dioxide concentration (around 375 parts per million) is the highest for 420,000 years, and probably the highest for the past 20 million years.

Source: Watson, Robert T. and the Core Writing Team (editors). 2001. Climate Change 2001: Synthesis Report. A contribution of Working Groups I, II and III to the Third Assessment Report of the Intergovernmental Panel on Climate Change. UK: Cambridge University Press.