Only about a third
| This graph shows the change of Earth's global temperature over the past 400 years. The recent measurements are shown as red, and the estimated temperatures from the past are shown in blue. On the same plot is the green curve, the estimated solar energy input to the surface (reconstructed from several kinds of data). Measured over centuries, the largest changes are due to variations in the solar input ("solar forcing"). However, in the 1800's three dips in temperature are seen which are not associated with the Sun. These are caused by dust from volcanoes shielding out sunlight, resulting in a cooler Earth ("volcanic forcing"). In the last hundred years we observe a rise in temperature and a rise in solar energy. However, the rise in temperature is considerably larger than what would be predicted from the change in solar energy alone. Roughly 1/3 of the recent temperature increase is from the Sun; the rest is presumably from man-made sources such as increased carbon dioxide and methane ("anthropogenic forcing"). |
Credit: Judith Lean, Naval Research Laboratory
For more info: http://www.gcrio.org/CONSEQUENCES/winter96/sunclimate.html